Studies of Reduced Graphene Oxide and Graphite Oxide in the Aspect of Their Possible Application in Gas Sensors

sensors

Article Studies of Reduced Graphene Oxide and Graphite Oxide in the Aspect of Their Possible Application in Gas Sensors

Sabina Drewniak 1,*, Roksana Muzyka 2, Agnieszka Stolarczyk 3, Tadeusz Pustelny 1, Michalina Kotyczka-Mora ´nska 2 and Maciej Setkiewicz 1

Received: 25 November 2015; Accepted: 8 January 2016; Published: 15 January 2016 Academic Editor: Gregory Schneider

1 Department of Optoelectronics, Silesian University of Technology, 2 Akademicka Str., Gliwice 44-100, Poland; [email protected] (T.P.); [email protected] (M.S.) 2 Institute for Chemical Processing of Coal, 1 Zamkowa Str., Zabrze 41-803, Poland; [email protected] (R.M.); [email protected] (M.K.-M.) 3 Department of Physical Chemistry and Technology of Polymers, Silesian University of Technology, 9 Strzody Str., Gliwice 44-100, Poland; [email protected] * Correspondence: [email protected]; Tel.: +48-32-237-1263

Abstract: The paper presents the results of investigations on resistance structures based on graphite oxide (GRO) and graphene oxide (rGO). The subject matter of the investigations was thaw the sensitivity of the tested structures was affected by hydrogen, nitrogen dioxide and carbon dioxide. The experiments were performed at a temperature range from 30 ˝C to 150 ˝C in two carrier gases: nitrogen and synthetic air. The measurements were also aimed at characterization of the graphite oxide and graphene oxide. In our measurements we used (among others) techniques such as: Atomic Force Microscopy (AFM); Scanning Electron Microscopy (SEM); Raman Spectroscopy (RS); Fourier Transform Infrared Spectroscopy (FT-IR) and X-ray Photoelectron Microscopy (XPS). The data resulting from the characterizations of graphite oxide and graphene oxide have made it possible to interpret the obtained results from the point of view of physicochemical changes occurring in these structures.

Keywords: graphite oxide; graphene oxide; gas sensor; explosive detection

1. Introduction The development of industry, agriculture, medicine and public safety requires the concomitant development of suitable control and monitoring systems [1]. This is connected with the need to provide selective and sensitive sensors [2] as the worldwide progress of civilization and technology demands better and better sensor solutions. This can be achieved by modification of already existing solutions or finding new ones, which involve the necessity of investigating the modification of already existing possibilities and endeavoring to find new ones which might detect changes in the natural work environment. Based on research conducted all over the world, it can be stated that materials like ZnO [3], TiO2 [4] and WO3 [5] can serve as excellent sensing layers. Also hybrid materials of conductive polymers with two-dimensional nanofilters ought to be taken into account (e.g., graphene oxide-poly (3-hexylthiophene) nanocomposites) [6]. It is to be stressed that the use of carbon materials just for decorating for instance with palladium [7,8] or for use in hybrids (and also in pristine form) are becoming more and more interesting, not only in the sensor domain The range of potential applications of carbon structures is rather wide and information provided in the respective literature indicates that carbon nanomaterials may be potentially used, among others, as selective molecular membranes [9].

Sensors 2016, 16, 103; doi:10.3390/s16010103 www.mdpi.com/journal/sensors Sensors 2016, 16, 103 2 of 16

The applications of graphene, graphene oxide and reduced graphene oxide in the production of sensors is also a very promising ﬁeld [10,11]. The techniques applied for the preparation of graphite oxide [12] and its further reduction [13] to reduced graphene oxide affect the properties of these materials, due to the presence of additional functional groups. Graphite oxide contains, among others, many hydroxyl, carboxyl [14] and epoxy [15] groups. The amount of these groups on graphene sheets directly inﬂuences the electric properties of the material. An increase in the amount of functional groups reduces the electric conductivity, which in turn would result in an increase in the electric mobility [13,15]. Thus, graphite oxide and graphene oxide can be used in a resistance sensor making it possible to detect selected gaseous atmospheres. In this paper, the authors present the results of experiments aimed at investigating the responses of resistance structures based on graphite oxide and graphene oxide exposed to different atmospheres. We also present the results of characterization of the layers comprising, among others, an analysis of the composition and topography of the surfaces. The aim of our investigations was to understand the physicochemical phenomena occurring during contact of the sensor structures with the given gaseous atmospheres. Our investigations were focused on the detection of three dangerous gases: nitrogen dioxide, hydrogen and carbon dioxide. Gases from the NOx group pollute the natural environment, leading to the formation of smog and affecting the lungs [16]. Hydrogen is harmful because it may already cause explosions at a concentration of 4% in air [17]. Carbon dioxide belongs to the so-called thermal gases (greenhouse gases) [18]. Both hydrogen and carbon dioxide are colorless, non-aromatic and tasteless gases [13,17] and that is why they should be detected.

2. Experimental Sction

2.1. Preparation of Graphite Oxide and Graphene Oxide

daf Commercial natural graphite powder (90 µm, La-58 nm, Lc-30 nm, d002-0.336 nm and C -99.5%), supplied by Graphit Kropfmühl AG (Hauzenberg, Germany) was ground and sieved to a particle size <20 µm and oxidized according to the methodology described in Table1.

Table 1. Reaction conditions for the preparation of GRO.

Sample Reactants Reaction Time Reference Graphite (1 g); H SO (30 mL); Graphite oxide 2 4 2 h [19] NaNO3 (3 g); KMnO4 (3 g)

Concentrated H2SO4 (95%–97%) was used as an acid. KMnO4 and NaNO3 were slowly added to keep the temperature in the 10–15 ˝C range. After their addition, the reaction temperature was maintained between 10 and 20 ˝C. After each reaction, 120 mL of milli-Q water and 50 mL of 3% H2O2 were slowly added. The mixture was stirred for 30 min and then centrifuged, the supernatant being decanted off. The graphite oxide was washed with milli-Q water and centrifuged repeatedly until a neutral pH was achieved. Graphite oxide was finally dried overnight under vacuum at 50 ˝C and stored in the presence of P2O5 as desiccant. Graphene oxide was obtained from graphite oxide by thermal treatment at 900 ˝C in a vertical furnace, under a nitrogen flow of 50 mL/min. The residence time at the final temperature was 5 min. Figure1 shows the preparation of graphite oxide and graphene oxide. Sensors 2016, 16, 103 33 ofof 1616

Figure 1. Preparation of graphite oxide and graphene oxide. Figure 1. Preparation of graphite oxide and graphene oxide.

2.2. Applied Measurement Methods Both graphite graphite oxide oxide (GRO) (GRO) and and graphene graphene oxide oxide (rGO (rGO)) possess possess unique unique properties properties that differ that differfrom thosefrom thoseof pristine of pristine graphite graphite because because of the structural of the structural changes changesarising due arising to the due introduction to the introduction of oxygen 2 functionalitiesof oxygen functionalities into the sp2 intobonded the carbonsp bonded network. carbon We network. used a combination We used a of combination several measurement of several methodsmeasurement to reveal methods the tostructural reveal the evolution structural from evolution pristine from graphite pristine to graphite graphite to graphiteoxide and oxide next and to graphenenext to graphene oxide (upon oxide thermally (upon thermally reduction reduction and exfolia andtion). exfoliation). The further The furthersections sections discuss discussthe results the obtainedresults obtained using these using methods. these methods. The topography ofof thethe surfacesurface of of graphite graphite oxide oxide and and graphene graphene oxide oxide was was investigated investigated by by means means of ofScanning Scanning Electron Electron Microscopy Microscopy (SEM) (SEM) and and Atomic Atom Forceic Force Microscopy Microscopy (AFM). (AFM). For measurements For measurements using usingScanning Scanning Electron Electron Microscopy, Microscopy, an INSPECT an INSPECT S50 (FEI, S50 (FEI, Hillsboro, Hillsboro, Oregon, Oregon, OH, USA)OH, USA) instrument instrument was wasused. used. The measurementThe measurement parameters parame were:ters HVwere: = 5 HV kV, bias= 5 =kV, 0, spotbias ==3.0 0, andspot HV = 3.0 = 2 and kV, biasHV == 14002 kV, V, biasspot = = 1400 3.5 for V, graphitespot = 3.5 oxide for graphite and graphene oxide oxide,and graphene respectively. oxide, In respectively. order to illustrate In order both to materials,illustrate bothan Everhart-Thornley materials, an Everhart-Thornley Detector (ETD) Detector was used. (ETD) The was measurements used. The measurements were made under were high made vacuum under ´5 highconditions vacuum (1.19 conditionsˆ 10 mbar). (1.19 × The 10− microscope5 mbar). The magnification microscope magnif was ˆ1000ication and wasˆ5000. ×1000 Additionally, and ×5000. Additionally,the investigations the byinvestigations means of scanning by means electron of scanning microscopy electron were extendedmicroscopy by applyingwere extended an energy by applyingdispersive an spectrometer energy dispersive X-ray spectrometer (EDX) detector. X-ray This (EDX) detector detector. allows This for detector qualitative allows and for quantitative qualitative andanalysis quantitative of the sample analysis composition. of the sample The composit measurementsion. The were measurements examined by were using examined a Nova by NanoSEM using a Nova450 microscope NanoSEM (FEI,450 microscope Hillsboro, Oregon,(FEI, Hillsboro, OH, USA) Oregon, operating OH, underUSA) operating low vacuum under (0.3 low mbar) vacuum at an (0.3accelerating mbar) at voltagean accelerating of 10–15 voltage kV and of using 10–15 a large-fieldkV and using secondary a large-field electron secondary detector. electron detector. The results obtained by applying SEM microsco microscopypy were supplemented by the results from AFM microscopy. Investigations using this technique were performed on an N_TEGRA Prima platform (NT-MDT, Moscow,Moscow, Russia)Russia) usingusing thethe intermittentintermittent contactcontact mode.mode. The images were obtained at a resonance frequency equal to 136.281 kHz, both for graphite oxide and graphene oxide. The HA_NC tip was used in thosethose measurements.measurements. Moreover, Moreover, the Raman Spectroscopy (RS), Fourier Transform Infrared Spectroscopy (FT-IR), X-ray Diffraction (XRD) and X-ray Photoelectron Microscopy (XPS) (XPS) were also applied for the characterization of the layers. The Raman spectra were obtained from the N_TEGRA Spectra platform (NT-MDT). (NT-MDT). In the measurements, the the wavelength wavelength was was equal equal to to 532 532 nm. nm. ´1 The spectra were recorded withwith aa resolutionresolution ofof 2.892.89 cmcm−1. . FT-IR measurements were made with a Tensor 27 spectrometer (Bruker, Ettlingen, Germany). Germany). ´1 Absorbance spectra ofof discsdiscs withwith aa KBr/sampleKBr/sample ratio of 500:1 were collected atat aa resolutionresolution ofof 44 cmcm−1.. XRD measurements werewere taken taken with with an an X’Pert X’Pert PRO PRO PW PW 3040/60 3040/60 by by PANalytical PANalytical B.V. B.V. (Quebec, (Quebec, QC, QC,Canada) Canada) diffractometer. diffractometer. Samples Samples of ground of ground graphite, graphite, graphite graphite oxide and oxide thermally and thermally reduced graphenereduced grapheneoxide were oxide deposited were ontodeposited glass andonto analyzed glass and by analyzed using Co by Ka1 using radiation Co Ka1 with radiation a voltage with of a 40 voltage kV and of a 40current kV and of 30a current mA. of 30 mA. The elements elements present present in in graphite graphite oxide oxide and and graphe graphenene oxide oxide as well as well as their as theirchemical chemical state were state wereidenti identifiedfied by XPS by XPSanalysis. analysis. X-ray X-ray Photoelectron Photoelectron Microscopy Microscopy was was carried carried out out using using a aPHI PHI 5000 VersaProbe—ScanningVersaProbe - Scanning ESCA ESCA Microprobe™ Microprobe™ (ULVAC(ULVAC-PHI,-PHI, Chigasaki, Japan/Chanhassen,Japan/ Chanhassen, MN, MN, USA) USA) and a a monochrome monochrome Al Al K Kαα sourcesource (1486.6 (1486.6 eV). eV). The The charging charging of ofthe the sample sample was was corrected corrected applying applying the C1sthe C1speak peak at 284.5 at 284.5 eV eVas asan aninternal internal standard. standard. Curve Curve fitting fitting of of the the spectra spectra was was performed performed using using a Gaussian-Lorentzian peakpeak shape shape after after conducting conducting a Shirley a Shirley background background correction. correction. The high-resolution The high- resolutionC1s signal wasC1s signal deconvoluted was deconvoluted into five individual into five peaksindividual ascribed peaks to graphiticascribed to carbon graphitic (284.5 carbon eV), carbon (284.5 3 eV),atoms carbon with spatomshybridization with sp3 hybridization (285.4 eV), hydroxyl(285.4 eV), and hydroxyl epoxy groupsand epoxy (286.5 groups eV), carbonyl(286.5 eV), or carbonyl quinone orgroups quinone (287.6 groups eV), carboxyl (287.6 eV), groups carboxyl (288.9 groups eV) and (288.9 a satellite eV) and peak a correspondingsatellite peak corresponding to the π–π* transition to the πin– theπ* transition aromatic systemsin the aromatic (290.4 eV). systems The O1s (290.4 excitation eV). The was O1s resolved excitation into was two resolved peaks corresponding into two peaks to corresponding to C=O groups such as ketone and carbonyl (531.7 eV), hydroxyl and epoxy

Sensors 2016, 16, 103 44 ofof 1616 functionalities (533.4 eV) and a carboxyl group (535.3 eV). Curve fittings were performed using an C=O groups such as ketone and carbonyl (531.7 eV), hydroxyl and epoxy functionalities (533.4 eV) iterative least squares algorithm (CasaXPS software) with a Gaussian-Lorentzian (70/30) peak shape and a carboxyl group (535.3 eV). Curve fittings were performed using an iterative least squares and Shirley background removal. The resulting spectra represent the binding energies of pyridinic algorithm (CasaXPS software) with a Gaussian-Lorentzian (70/30) peak shape and Shirley background (398.5 ± 0.2 eV), amine (399.4 ± 0.2 eV), hydroxypyridinic (400.5 ± 0.2 eV), quaternary-N (401.2 ± 0.2 removal. The resulting spectra represent the binding energies of pyridinic (398.5 ˘ 0.2 eV), amine eV) and N-oxide (402.9 ± 0.2 eV) groups. (399.4 ˘ 0.2 eV), hydroxypyridinic (400.5 ˘ 0.2 eV), quaternary-N (401.2 ˘ 0.2 eV) and N-oxide The elementary composition of graphite oxide and graphene oxide (carbon, oxygen, hydrogen, (402.9 ˘ 0.2 eV) groups. nitrogen and sulphur content) was determined directly using a Vario Macro Cube automatic The elementary composition of graphite oxide and graphene oxide (carbon, oxygen, hydrogen, elementary analyser (Elementar Analysysteme GmbH Company, Hanau, Germany). nitrogen and sulphur content) was determined directly using a Vario Macro Cube automatic elementary The properties of graphite oxide and graphene oxide as a function of temperature were analyser (Elementar Analysysteme GmbH Company, Hanau, Germany). investigated by the following techniques: Thermogravimetric (TG), Differential Thermogravimetric The properties of graphite oxide and graphene oxide as a function of temperature were (DTG) and Differential Scanning Calorimetry (DSC). TG, DTG and DSC curves were obtained using investigated by the following techniques: Thermogravimetric (TG), Differential Thermogravimetric a TG-STA209LUXX unit (Netzsch Group, Selb, Germany). The samples were tested in the range from (DTG) and Differential Scanning Calorimetry (DSC). TG, DTG and DSC curves were obtained using a 40 to 1000 °C with a heating rate of 10 K/min and a flow of argon kept at 25 mL/min. TG-STA209LUXX unit (Netzsch Group, Selb, Germany). The samples were tested in the range from The resistance of sensor samples as a function of temperature was tested in a measuring system 40 to 1000 ˝C with a heating rate of 10 K/min and a flow of argon kept at 25 mL/min. designed and realized by the staff of the Department of Optoelectronics of the Silesian University of The resistance of sensor samples as a function of temperature was tested in a measuring system Technology (Gliwice, Poland). This system allows presetting and control of the temperature of designed and realized by the staff of the Department of Optoelectronics of the Silesian University of samples. The measurements were made within the range of temperature from 30 °C to 150 °C. The Technology (Gliwice, Poland). This system allows presetting and control of the temperature of samples. same system made it possible to measure the resistance at various degrees of temperature in various The measurements were made within the range of temperature from 30 ˝C to 150 ˝C. The same system gaseous atmospheres. It can preset and control the composition of the gaseous atmosphere over the made it possible to measure the resistance at various degrees of temperature in various gaseous samples (the gas flow: 100 mL/min and 500 mL/min in the undertaken experiments). The atmospheres. It can preset and control the composition of the gaseous atmosphere over the samples measurements were taken in the atmospheres of nitrogen and synthetic air, which were used as (the gas flow: 100 mL/min and 500 mL/min in the undertaken experiments). The measurements were carrier gases, as well as in the atmosphere of hydrogen, nitrogen dioxide and carbon dioxide at taken in the atmospheres of nitrogen and synthetic air, which were used as carrier gases, as well as in various concentrations in the carrier gases. the atmosphere of hydrogen, nitrogen dioxide and carbon dioxide at various concentrations in the 2.3.carrier Description gases. of a Sensor Structure 2.3. DescriptionThe tested ofsensor a Sensor structures Structure (Figure 2) differed from each other merely in the sensitivity of the layers–the sensitive layers were deposited on the same substrates (the so-called “basic substrates”). The tested sensor structures (Figure2) differed from each other merely in the sensitivity of the In order to get basic substrates, gold comb electrodes were deposited on silicon substrates (with an layers–the sensitive layers were deposited on the same substrates (the so-called “basic substrates”). oxidized surface). A layer of chromium was sputtered to improve the adhesion of gold to the In order to get basic substrates, gold comb electrodes were deposited on silicon substrates (with an substrate. oxidized surface). A layer of chromium was sputtered to improve the adhesion of gold to the substrate.

Figure 2. Photos of sensor structures obtained using Scanning Electron Micr Microscopyoscopy (magnification: (magniﬁcation: ×1000):ˆ1000): (a (a) )structure structure with with graphite graphite oxide, oxide; ( (bb)) structure structure with with graphene graphene oxide. oxide.

To obtain a 1.2% (wt.%) solution the appropriate amounts of graphite oxide and graphene oxide (sensitive layers) were mixed with ethanol (ethyl alcohol absolut 99.8% pure, Avantor Performance Materials S.A, Gliwice, Poland) and then transferre transferredd to an ultrasonic bath (Transsonic T310, Clamlab Ltd., Cambridge, UK). After 24 h ultrasonication ultrasonication a homogeneous homogeneous dispersion was obtained, which was deposited on the basic structures. After that the basic structures were placed in an vacuum dryer (Memmert VO400, Schwabach, Germany) and dried at 22 °C under nitrogen atmosphere for

Sensors 2016, 16, 103 5 of 16

Sensors 2016, 16, 103 5 of 16 deposited on the basic structures. After that the basic structures were placed in an vacuum dryer (Memmert VO400, Schwabach, Germany) and dried at 22 ˝C under nitrogen atmosphere for 24 h. 24 h. Next, the dried structures were annealed with temperature ramp 2 °C/min to the final Next, the dried structures were annealed with temperature ramp 2 ˝C/min to the ﬁnal temperature temperature of 100 °C at which were maintained for additional 3 h. Afterwards the temperature was of 100 ˝C at which were maintained for additional 3 h. Afterwards the temperature was increased increased up to 150 °C using the same ramping program and kept for 2 h under reduced pressure (20 up to 150 ˝C using the same ramping program and kept for 2 h under reduced pressure (20 mbar) mbar) to removal of any residual ethanol from the prepared structures. The whole annealing process to removal of any residual ethanol from the prepared structures. The whole annealing process were were conducted in an inert atmosphere of nitrogen to avoid oxidation of the samples. conducted in an inert atmosphere of nitrogen to avoid oxidation of the samples. The surface of the sensors structures was about 1 cm2. The application of the sensitive layer on The surface of the sensors structures was about 1 cm2. The application of the sensitive layer on such large surfaces caused that the obtained results were repetitive for different groups of sensors such large surfaces caused that the obtained results were repetitive for different groups of sensors (the resistance of the structure with graphite oxide was approximately equal to 48 Ω, while the (the resistance of the structure with graphite oxide was approximately equal to 48 Ω, while the resistance of the structure with graphene oxide was approximately equal to 5.8 Ω. resistance of the structure with graphene oxide was approximately equal to 5.8 Ω. As it is shown in SEM images (Figure 2), the thickness of the GRO and rGO is relatively large As it is shown in SEM images (Figure2), the thickness of the GRO and rGO is relatively large compared to many structures presented in the literature [20–22]. We made numerous attempts to compared to many structures presented in the literature [20–22]. We made numerous attempts to exfoliate layers. The resulting layers were delaminated but their properties were not stable (after exfoliate layers. The resulting layers were delaminated but their properties were not stable (after some some time, the layers agglomerated again). We wanted to make the layers stable over time and time, the layers agglomerated again). We wanted to make the layers stable over time and thermally thermally stable. The structures with such layers are presented in Figure 2. stable. The structures with such layers are presented in Figure2. 3. Results and Discussion 3. Results and Discussion

3.1. Characterization of the Structures—Topography andand CompositionComposition ofof GraphiteGraphite OxideOxide andand GrapheneGraphene Figure3 3 shows shows thethe photosphotos obtainedobtained byby usingusing ScanningScanning ElectronElectron MicroscopyMicroscopy (magniﬁcation(magnification ofof 5000 times), concerning both graphite oxideoxide andand graphene oxide.oxide. The size of the grains of both oxides is similar,similar, from from only only a fewa few to ato score a score of micrometers, of micrometers, although although the surface the surface of graphite of graphite oxide is distinctlyoxide is moredistinctly developed. more developed. The structure The ofstructure graphite of oxide graphite is smoother oxide is thansmoother thatof than graphene that of oxide;graphene moreover, oxide; itmoreover, displays ait paralleldisplays arrangement a parallel arrangem of the respectiveent of the layersrespective (Figure layers3a). (Figure 3a).

Figure 3. Photos of sensor structures obtained by usin usingg Scanning Electron Micr Microscopyoscopy (magnification (magniﬁcation of ˆ×5000):5000): ( (aa)) structure structure with with graphite graphite oxide, oxide; ( (bb)) structure structure with with graphene graphene oxide. oxide.

The results of tests carried out when applying Atomic Force Microscopy conﬁrmconfirm the difference in the development development of of these these structures. structures. The The topograp topographyhy (2 × (2 2 ˆµm)2 µ ofm 2the) of surface the surface of graphene of graphene oxide oxideis presented is presented in Figure in Figure4a. The4 a.topography The topography of graphite of graphite oxide changes oxide changeswithin a larger within range a larger (from range 0 to (from170 nm) 0 tothan 170 that nm) of thangraphene that ofoxide graphene (from 0 oxide to 150 (from nm). Analyzing 0 to 150 nm). the cross-sections Analyzing the (Figures cross-sections 4b and (Figures5b) across4b the and surface5b) across (marked the surface by the (markedblue lines by in the Figures blue lines 4a and in Figures5a), we4 seea and that5a), both we seematerials, that both the materials,changes in the height changes amount in height to several amount nano tometers several along nanometers the cross-section along the of cross-section 250 nm. of 250 nm. The value of of the the RMS RMS coefficient coefﬁcient for for graphite graphite oxide oxide amounts amounts to toRMS RMS = 15.98 = 15.98 nm, nm, whereas whereas for forgraphene graphene oxide oxide to RMS to RMS= 28.51 = nm, 28.51 which nm, whichproves provesthe better the development better development of the surface of the of surface graphene of grapheneoxide. oxide.

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(a) (b)

Figure 4.4. TopographyTopography of of the the surface of graphite oxide: ( (aa)) picture picture obtained obtained by by using AFM, AFM; ((b)) cross-sectionscross-sections ofof thethe markedmarked area.area.

(a) (b) Figure 5. Topography of the surface of graphene oxide: (a) picture obtained by using AFM, Figure 5.5. TopographyTopography of of the the surface of graphene oxide: ( (aa)) picture picture obtained obtained by by using AFM, AFM; (b) cross-sections of the marked area. ((b)) cross-sectionscross-sections ofof thethe markedmarked area.area.

The crystalline structure of synthesized graphite oxide and graphene oxide, as well as that of the The crystalline structure of synthesized graphite oxide and graphene oxide, as well as that of parent graphite, were analysed by XRD. As shown in Figure 6, the XRD pattern of the graphite shows the parent graphite, were analysed by XRD. As shown in Figure6, the XRD pattern of the graphite a diffraction peak at 2Θ = 30.87°, corresponding˝ to its interlayer spacing (d002) of 0.3363 nm. The shows a diffraction peak at 2Θ = 30.87 , corresponding to its interlayer spacing (d002) of 0.3363 nm. intensity of this peak sharply decreases for GRO, and a new 001 peak appears at 2Θ = 13.18°. The d001 The intensity of this peak sharply decreases for GRO, and a new 001 peak appears at 2Θ = 13.18˝. The interlayer distance calculated for this sample is 0.7795 nm, which is in agreement with its high degree d interlayer distance calculated for this sample is 0.7795 nm, which is in agreement with its high of001 oxidation [23,24]. The variation in the interlayer spacing of GRO results from the variation in the degree of oxidation [23,24]. The variation in the interlayer spacing of GRO results from the variation degree of oxidation on graphite and is proportional to the content of oxygen. Apart from the in the degree of oxidation on graphite and is proportional to the content of oxygen. Apart from the characteristic sharp diffraction peak at 2Θ approximately 13°, displays mildly oxidised RGO exhibits characteristic sharp diffraction peak at 2Θ approximately 13˝, displays mildly oxidised RGO exhibits another weak peak at 2Θ = 30°. In the case of thermal reduction of GRO, the characteristic peak of another weak peak at 2Θ = 30˝. In the case of thermal reduction of GRO, the characteristic peak of GRO at 2Θ = 13.18° has been reported to disappear and a new broad peak appears at 2Θ = 30.50°. The GRO at 2Θ = 13.18˝ has been reported to disappear and a new broad peak appears at 2Θ = 30.50˝. The decrease in the interlayer spacing between the thermally reduced graphene oxide sheets is attributed decrease in the interlayer spacing between the thermally reduced graphene oxide sheets is attributed to the removal of considerable oxygen functionalities from the GRO sheet during the reduction to the removal of considerable oxygen functionalities from the GRO sheet during the reduction process. process. The interlayer spacing d002 amounts to 0.3393 nm. The interlayer spacing d amounts to 0.3393 nm. The oxygen speciation002 was studied by means of XPS measurements. Figure 7a shows the C1s The oxygen speciation was studied by means of XPS measurements. Figure7a shows the C1s core-level XPS spectra of graphite and graphene oxides. The spectra of GRO show an intense peak at core-level XPS spectra of graphite and graphene oxides. The spectra of GRO show an intense peak at 284.5 eV, attributed to carbon with sp2 and sp3 hybridization accompanied by some shoulders at 284.5 eV, attributed to carbon with sp2 and sp3 hybridization accompanied by some shoulders at higher higher binding energies due to the presence of oxygen linkages [25,26]. The samples also show a binding energies due to the presence of oxygen linkages [25,26]. The samples also show a second second intense peak at 286.5 eV, ascribed to hydroxyl and epoxy groups, and less intense intense peak at 286.5 eV, ascribed to hydroxyl and epoxy groups, and less intense contributions of contributions of carbonyl/quinone (287.6 eV) and carboxyl (288.9 eV) groups. The oxygen speciation carbonyl/quinone (287.6 eV) and carboxyl (288.9 eV) groups. The oxygen speciation in these samples in these samples is in agreement with the model of Lerf-Klinowski concerning highly oxidized is in agreement with the model of Lerf-Klinowski concerning highly oxidized graphitic structures [27]. graphitic structures [27]. The spectrum of rGO shows an intense peak at 284.5 eV attributed to carbon

Sensors 2016, 16, 103 7 of 16

SensorsSensors 2016 2016, 16, 16, 103, 103 7 7of of 16 16 The spectrum of rGO shows an intense peak at 284.5 eV attributed to carbon with sp2 hybridization. Thewithwith C1s sp sp spectrum2 2hybridization. hybridization. consists The The of C1s aC1s peak spectrum spectrum at 285.4, consists consists 286.2, of 287.0,of a a peak peak 287.9, at at 285.4, 288.9,285.4, 286.2, 286.2, and 290.4287.0, 287.0, eV 287.9, 287.9, assigned 288.9, 288.9, toand and C–C, C–O,290.4290.4 C=O, eV eV assigned assigned O–C–O, to O=C–OHto C–C, C–C, C– C–O, andO, C=O, C=O,π– πO–C–O, O–C–O,* transition O=C–OH O=C–OH in aromatic and and π π––π systemsπ* *transition transition [28 ].in in aromatic aromatic systems systems [28]. [28].

˝ FigureFigureFigure 6. 6. 6.X-ray X-ray X-ray patternspatterns patterns for for the the parent parent graphite, graphite,graphite, GRO GRO and and rGO rGO rGO treated treated treated at at at900 900 900 °C. °C. C.

(a()a ) (b(b) ) Figure 7. Deconvolution of (a) C1s and (b) O1s core-level XPS spectra of the GRO and rGO. FigureFigure 7. 7.Deconvolution Deconvolution of of ( a()a) C1s C1s and and ( (bb)) O1sO1s core-levelcore-level XPS spectra spectra of of the the GRO GRO and and rGO. rGO.

TheThe O1s O1s core-level core-level spectra spectra for for GRO, GRO, shown shown in in Figu Figurere 7b 7b , ,were were deconvoluted deconvoluted into into four four peak; peak; to to quinonequinoneThe O1s (530.3 (530.3 core-level eV), eV), oxygen oxygen spectra double double for-bonded GRO,-bonded shown to to carbon carbon in Figure (531.7 (531.77 beV), eV), , were oxygen oxygen deconvoluted single-bonded single-bonded into to to four carbon carbon peak; to(532.5 quinone(532.5 eV) eV) (530.3and and carboxylic carboxylic eV), oxygen group group double-bonded (535.3 (535.3 eV). eV). The The to O1 carbonO1s score-level core-level (531.7 spectra eV),spectra oxygen for for rGO, rGO, single-bonded shown shown in in the the to same same carbon (532.5diagram,diagram, eV) andwere were carboxylic deconvoluted deconvoluted group into into (535.3 four four peaks, eV).peaks, The ascr ascr O1sibedibed core-levelto to quinone quinone spectra (530.3 (530.3 eV), eV), for oxygen rGO,oxygen shown double-bonded double-bonded in the same diagram,toto carbon carbon were (531.7 (531.7 deconvoluted eV), eV), oxygen oxygen into si single-bonded fourngle-bonded peaks, ascribedto to carbon carbon to (533,4 quinone(533,4 eV eV) (530.3 )and and carboxylic eV),carboxylic oxygen group group double-bonded (535.3 (535.3 eV) eV) to carbon[28,29].[28,29]. (531.7 The The resultseV), results oxygen of of the the single-bondeddeconv deconvolutionolution of toof the carbonthe C1s C1s and (533.4and O1s O1s eV) XPS XPS and spectra spectra carboxylic are are gathered gathered group (535.3in in Table Table eV) 2. 2. [ 28,29]. The results of the deconvolution of the C1s and O1s XPS spectra are gathered in Table2. TableTable 2. 2. Elemental Elemental composition composition of of GRO GRO and and rGO rGO determined determined by by XPS. XPS. Table 2. Elemental composition of GRO and rGO determined by XPS. ElementalElemental Composition C1s Deconvolution O1s Deconvolution Sample Composition C1s Deconvolution O1s Deconvolution Sample Elemental(%) (%) C1s Deconvolution O1s Deconvolution CodeCode Composition (%) Sample QuinoneQuinone CC OO CCspsp2 2 CCspsp3 3 C–OC–O C=OC=O O–C–O O–C–O C(O)OH C(O)OH C–OC–O C=OC=O C(O)OHC(O)OH Code groupsQuinone CO Csp2 Csp3 C–O C=O O–C–O C(O)OH C–O C=O C(O)OH groups 83.83. Groups GROGRO 69.0 69.0 31.031.0 45.445.4 7.07.0 38.538.5 6.26.2 - - 2.8 2.8 9.29.2 2.72.7 5.1 5.1 GRO 69.0 31.0 45.4 7.0 38.5 6.2 - 2.80 083.0 9.2 2.7 5.1 rGO 94.5 5.5 70.5 11.0 6.7 3.3 1.7 2.658.58. 58.5 20.4 8.5 12.7 rGOrGO 94.594.5 5.55.5 70.570.5 11.011.0 6.76.7 3.33.3 1.7 1.7 2.6 2.6 20.420.4 8.58.5 12.7 12.7 5 5 The FT-IR spectra of the samples of graphite and graphene oxides are presented in Figure8. TheThe FT-IR FT-IR spectra spectra of of the the samples samples of of graphite graphite and and graphene graphene oxides oxides are are presented presented in in Figure Figure 8. 8. In In In all the spectra, the oxidation is conﬁrmed by the presence of several bands attributed to oxygen allall the the spectra, spectra, the the oxidation oxidation is is confirmed confirmed by by th thee presence presence of of several several bands bands attributed attributed to to oxygen oxygen functionalization.functionalization.functionalization. AsAs As evidentevident evident in in Figure Figure 88, ,8, therethere there are are groups groups of ofof oxidized oxidizedoxidized graphite graphite FT-IR FT-IR FT-IR absorbance absorbance absorbance ´1 peaks.peaks.peaks. The The The most most most intense intense intense peak peak peak ( i.e.( i.e.(i.e.,, the,the the widest)widest) widest) occursoccurs occurs in in in the the the range rangerange of ofof 3320–3430 3320–34303320–3430 cm cm cm−1− 1and andand is is attributed isattributed attributed

Sensors 2016, 16, 103 8 of 16

SensorsSensors 2016 2016, 16, ,16 103, 103 8 of8 16of 16 to the C–OH stretching vibrations of a hydroxyl group [30–34]. A second, lower-intensity peak is to tothe the C–OH C–OH stretching stretching vibrations vibrations of of aa hydroxylhydroxyl group [30–34].[30–34]. AA second, second, lower-intensity lower-intensity peak peak is is ´1−1 locatedlocatedlocated at at approximately atapproximately approximately 1620 1620 1620 cm cm cm−1 and and is is attributed attributed toto to thethe the C=CC=C C=C sk sk skeletaleletaleletal vibration vibration vibration of of non-oxidized ofnon-oxidized non-oxidized graphitegraphitegraphite [31 [31,33,34]., 33[31,33,34].,34]. AA thirdAthird third peakpeak peak withwith with aa largelarge intens intensity,ity, comparablecomparable comparable to to tothat that that of of the of the thesecond second second peak, peak, peak, is is is located at approximately 1050 cm´1−1 and is attributed to the alkoxy C–O stretching vibration [30–32,34]. locatedlocated at at approximately approximately 1050 1050 cm cm−1 and is is attributed attributed to to the the alkoxy alkoxy C–O C–O stretching stretching vibration vibration [30–32,34]. [30–32,34 ].

FigureFigure 8. 8.FT-IR FT-IR spectra spectrafor for graphitegraphite oxide oxide and and graphene graphene oxide. oxide. Figure 8. FT-IR spectra for graphite oxide and graphene oxide. The next peak with the lowest intensity is located at approximately 1730 cm−1 and is attributed The next peak with the lowest intensity is located at approximately 1730 cm´1 and is attributed to Thethe C=O next stretching peak with vibrations the lowest of aintensity carbonyl is group located (at theat approximately edges of planes 1730of the cm graphite−1 and layer)is attributed [30– to the32,34,35]. C=O stretchingTwo weak vibrationspeaks located of aat carbonylapproximately group 2970 (at and the 2940 edges cm of−1 are planes characteristic of the graphitefor to the C=O stretching vibrations of a carbonyl group (at the edges of planes of the graphite´1 layer) [30– layer)asymmetric[30–32,34, and35]. symmetricTwo weak stretching peaks located vibrations at approximately of the C-H bond 2970 in CH and2 and 2940 CH3 cm groupsare [31,34]. characteristic The 32,34,35]. Two weak peaks located at approximately 2970 and 2940 cm−1 are characteristic for for asymmetricnext group of and the symmetric peaks contains stretching peaks which vibrations can be ofobserved the C-H also bond in the in spectra CH2 and of graphene CH3 groups oxide. [31 ,34]. asymmetric and symmetric stretching vibrations of the C-H bond in CH2 and CH3 groups [31,34]. The The most intense (i.e., the widest) peak is located at approximately 1220 cm−1 and is attributed to the Thenext next group group of the of thepeaks peaks contains contains peaks peaks which which can be can observed be observed also in alsothe spectra in the of spectra graphene of graphene oxide. stretching vibration of the epoxy group C–O–C [30–32,34,36]. A second, lower-intensity´1 peak is oxide. The most intense (i.e., the widest) peak is located at approximately 1220−1 cm and is attributed Thelocated most intenseat approximately (i.e., the widest) 1560 cm peak−1 and is islocated attributed at approximately to the C=C skeletal 1220 cmvibration and isof attributed the graphene to the tostretching the stretching vibration vibration of the ofepoxy the epoxygroup groupC–O–C C–O–C[30–32,34,36]. [30–32 A,34 second,,36]. A lo second,wer-intensity lower-intensity peak is planes [31,33,34,36]. A third peak with reduced´1 intensity compared to that of graphite oxides, at peak is located at approximately 1560−1 cm and is attributed to the C=C skeletal vibration of the locatedapproximately at approximately 3440 cm− 11560, is attributed cm and to isC–OH attributed stretching to the vibr C=Cations skeletal of a hydroxyl vibration group of the [30–34,36]. graphene grapheneplanesThe intensity[31,33,34,36]. planes of [31 this,33 A ,peak 34third,36 is]. peakclosely A third with related peakreduced to with the intensity oxygen reduced contentcompared intensity in the to compared testedthat of samples graphite to that after oxides, of high- graphite at ´1 oxides,approximatelytemperature at approximately 3440treatment. cm−1, 3440isFinally, attributed cm a fourth, to is C–OH attributedpeak withstretching the to C–OHmost vibr reducedations stretching of intensity, a hydroxyl vibrations visible group also of [30–34,36]. a in hydroxyl the groupThespectrum intensity [30–34, 36of of ].graphene this The peak intensity oxide, is closely (l ofocated this related peakat approximately to is the closely oxygen related 1710 content cm to−1 )in theis the attributed oxygen tested contentsamplesto the stretching inafter the high- tested samplestemperaturevibration after of high-temperaturetreatment. the carbonyl Finally, group treatment. aC=O fourth [30–32,34–36]. peak Finally, with athe fourth most peakreduced with intensity, the most visible reduced also intensity,in the visiblespectrum alsoThe inof Raman thegraphene spectrum spectra oxide, of of graphite graphene (located oxide at oxide, approximatelyand (locatedgraphene atoxide 1710 approximately are cm shown−1) is attributed in1710 Figure cm 9.to´ The1 )the is two attributedstretching most to thevibration stretchingintensive of peaksthe vibration carbonyl occur of in thegroup thecarbonyl ranges C=O 1300–1400[30–32,34–36]. group C=O cm−1 [(D)30– and32,34 1550–1600–36]. cm−1 (G) [37]. The ratio of the TheintensitiesThe Raman Raman of spectrathe spectra D and of of G graphite graphite peaks for oxideoxide graphite andand oxide graphene is equal oxide oxide to are0.90, are shown shownwhereas in in Figurefor Figure graphene 9.9 The. The oxidetwo two most it most equals 0.98. The growth of this ratio suggests that −the1 amount of defects increased.−1 intensiveintensive peaks peaks occur occur in in the the rangesranges 1300–1400 cm cm´ (D)1 (D) and and 1550–1600 1550–1600 cm cm (G)´1 [37].(G) The [37]. ratio The of ratio the of theintensities intensities of of the the D D and and G G peaks peaks for for graphite graphite oxide oxide is isequal equal to to0.90, 0.90, whereas whereas for for graphene graphene oxide oxide it it equalsequals 0.98. 0.98. The The growth growth of of this this ratio ratio suggestssuggests thatthat the amount of of defects defects increased. increased.

Figure 9. Raman spectra of graphite oxide and graphene oxide.

FigureFigure 9. 9.Raman Raman spectraspectra of graphite oxide oxide and and graphene graphene oxide. oxide.

Sensors 2016, 16, 103 9 of 16 Sensors 2016, 16, 103 9 of 16

TheThe presencepresence of of additional additional elements elements in the in analyzed the analyzed structures structures was also conﬁrmedwas also byconfirmed application by ofapplication other methods of other of measurements,methods of measurements, for example Scanningfor example Electron Scanning Microscopy Electron with Microscopy the EDX with detector. the FigureEDX detector. 10a shows Figure highly-wrinkled 10a shows highly-wrinkled graphitic layers graphi provingtic layers that there proving is a distortionthat there is in a the distortion graphene in layersthe graphene due to thelayers linkage due ofto thethe residuallinkage oxygenof the residual after thermal oxygen reduction, after thermal while reduction, large nanosheet while sizeslarge arenanosheet preserved. sizes are preserved.

FigureFigure 10.10. SEM image of (a)) graphenegraphene oxide and (b) EDX maps of the corresponding area for carbon;carbon, ((cc)) oxygenoxygen) and and ( d(d)) sulphur. sulphur.

EDXEDX mapsmaps areare shownshown inin FigureFigure 1010b–d;b–d; carbon,carbon, oxygenoxygen andand sulfursulfur areare detecteddetected onon thethe surfacesurface ofof bothboth samplessamples and and the the presence presence of sulfurof sulfur on the on surface the surface is a striking is a stri difference.king difference. The amount The ofamount elements of detectedelements by detected using various by using methods various is methods summarized is summ in Tablearized3. One in Table has to 3. be One aware has of to the be fact aware that of both the EDXfact that and both XPS detectEDX and the contentXPS detect of elements the content on the of surface,elements while on the EA surface, is a complex while analysis. EA is a complex analysis. Table 3. Elemental composition (of carbon, oxygen and sulphur) of graphite oxide and graphene oxide determinedTable 3. Elemental by XPS, EDXcomposition and EA methods.(of carbon, oxygen and sulphur) of graphite oxide and graphene oxide determined by XPS, EDX and EA methods.

CXPS,%OXPS,%CEDX,%OEDX,%SEDX,%CEA,%OEA,%SEA,% CXPS, % OXPS, % CEDX, %OEDX, %SEDX, %CEA, % OEA, % SEA, % GraphiteGraphite oxide oxide 69.0 69.0 31.0 31.0 68.9 68.9 26.70 26.70 4.40 4.40 63.80 63.80 32.29 1.62 1.62 Graphene oxide 94.5 5.5 89.5 8.50 2.00 89.80 8.53 0.95 Graphene oxide 94.5 5.5 89.5 8.50 2.00 89.80 8.53 0.95

Nevertheless,Nevertheless, the analyses are in quitequite goodgood agreement,agreement, but the EDXEDX andand EAEA datadata confirmconfirm thethe presencepresence ofof sulphursulphur in in the the samples. samples. On On the the basis basis of of the the maps, maps, it mayit may be concludedbe concluded that that the the samples samples are veryare very uniform uniform over over the entirethe entire volume. volume. 3.2. Characteristics of the Structures—The Dependence of Selected Properties on the Temperature 3.2. Characteristics of the Structures—The Dependence of Selected Properties on the Temperature The properties of graphite oxide and graphene oxide change together with the temperature, The properties of graphite oxide and graphene oxide change together with the temperature, affecting directly the properties of sensor structures based on the mentioned materials. Therefore, affecting directly the properties of sensor structures based on the mentioned materials. Therefore, measurements were accomplished using the TG/DTG, and DSC techniques and also measuring the measurements were accomplished using the TG/DTG, and DSC techniques and also measuring the resistance at various degrees of temperature. resistance at various degrees of temperature. Analysing the data from Figure 11, it can be seen that pyrolysis occurs in several steps. In the first Analysing the data from Figure 11, it can be seen that pyrolysis occurs in several steps. In the step (from 40 ˝C to 140 ˝C), the maximum DTG temperature amounts to 100 ˝C (the change in the first step (from 40 °C to 140 °C), the maximum DTG temperature amounts to 100 °C (the change in weight is then equal to 3.6%) The weight change is (in this case) associated with a loss of moisture in the weight is then equal to 3.6%) The weight change is (in this case) associated with a loss of moisture the samples. The second pyrolysis step occurs within the range of temperature from 140 ˝C to 240 ˝C in the samples. The second pyrolysis step occurs within the range of temperature from and from 240 ˝C to 1000 ˝C. In the temperature range from 140 ˝C to 240 ˝C, the maximum DTG 140 °C to 240 °C and from 240 °C to 1000 °C. In the temperature range from 140 °C to 240 °C, the temperature is equal to 200 ˝C, connected with a loss of weight of: 14.2%, while in the temperature maximum DTG temperature is equal to 200 °C, connected with a loss of weight of: 14.2%, while in range from 240 ˝C to 1000 ˝C, the maximum DTG temperature amounts to 275 ˝C with a loss of weight the temperature range from 240 °C to 1000 °C, the maximum DTG temperature amounts to 275 °C of: 19.2%. The second step of pyrolysis is associated with a disconnection of the functional groups with a loss of weight of: 19.2%. The second step of pyrolysis is associated with a disconnection of the from the surface (from the oxidized graphene planes) [38]. DSC data shown in Figure 11 reveal that functional groups from the surface (from the oxidized graphene planes) [38]. DSC data shown in for GRO an exothermic peak is observed between 140 ˝C and 240 ˝C with peak location at 200 ˝C. Figure 11 reveal that for GRO an exothermic peak is observed between 140 °C and 240 °C with peak This peak corresponds to the melting point of the material, specifically the melting of γ phase crystals. location at 200 °C. This peak corresponds to the melting point of the material, specifically the melting The melt enthalpy is ´475.3 J/g [39]. of γ phase crystals. The melt enthalpy is −475.3 J/g [39].

Sensors 2016, 16, 103 10 of 16 Sensors 2016, 16, 103 1010 ofof 1616

Figure 11. Contains onset degradation temperatures, residual material, melting points and melt Figure 11. Contains onset degradation temperatures, residual material, melting points and melt enthalpyFigure 11. from Contains TGA and onset DSC degradation analysis for temperatures, GRO. residual material, melting points and melt enthalpy from TGA and DSC analysis for GRO. enthalpy from TGA and DSC analysis for GRO. Sensor structures are also characterized by various resistance values. In the course of the oxidationSensor process, structuresstructures the areincorporated are also also characterized characterized oxygen byoverco variousby mesvarious resistance the vanresistance der values. Waals values. In bonds the courseIn andthe of ascourse the a result, oxidation of the process,oxidationinterplanar the process, distances incorporated the increase. incorporated oxygen The overcomes reduction oxygen overco theof grap vanmeshene der the Waals oxide van bonds dereliminates Waals and asbondsa considerable a result, and theas a interplanar amountresult, the of oxygeninterplanardistances and increase. distances changes The increase.the reduction impedance The of reduction graphene characteristic of oxide grap eliminatesofhene the oxide material. a eliminates considerable A reduction a considerable amount in the of oxygen amountamount and of groupsoxygenchanges containingand the impedancechanges oxygen the characteristic impedance increases the ofcharacteristic theelectronic material. conduction, of A reductionthe material. resulting in the A amount reductionin a decrease of groupsin theof resistance. amount containing of groupsoxygenIt ought containing increases to be the oxygenmentioned electronic increases that conduction, the the resistance electronic resulting ofconduction, graphite in a decrease oxide resulting of changes resistance. in a decreasein a different of resistance. way with the riseIt ought of temperature toto bebe mentionedmentioned than that thatthe the resistancethe resistance resistance of of grapheneof graphite graphite oxideoxide. oxide changes The changes conductivity in ain different a different of way a waystructure with with the containingtherise ofrise temperature of graphitetemperature than oxide thethan increase resistance the sresistance with of graphene the ofrise graphene of oxide. temperature The oxide. conductivity The(within conductivity the of a range structure offrom a containing structure50 °C to ˝ ˝ 150containinggraphite °C), both oxide graphite in increases nitrogen oxide withand increase thein synthetic rises with of temperature air.the Therise characterof (withintemperature of the these range (w changesithin from the 50 is rangelinear.C to 150 from In theC), 50 bothcase °C ofto in nitrogen,150nitrogen °C), both andthe in inrate syntheticnitrogen of changes air.and The in amounts synthetic character to air. of 37.7 these The m changescharacterΩ/°C, whereas is of linear. these Infor changes the synthetic case is of linear. nitrogen,air it In amounts the the case rate ofto ˝ ˝ nitrogen,38.6changes mΩ amounts/°C. the Inrate the to of 37.7 structurechanges mΩ/ C, amountswith whereas graphene to for 37.7 synthetic oxide,mΩ/°C, airthe itwhereas amountsresistance for to 38.6syntheticgrows mΩ with/ airC. Intheit theamounts increasing structure to withtemperature.38.6 graphenemΩ/°C. ForIn oxide, nitrogen,the thestructure resistance the rate with of grows changesgraphene with amounts theoxide, increasing tothe 7.8 resistancem temperature.Ω/°C while grows for For synthetic with nitrogen, the air increasing theit is rate equal of ˝ ˝ temperature.8.1changes mΩ/°C. amounts The For results nitrogen, to 7.8 are mΩ presentedthe/ C rate while of inchanges for Figure synthetic amounts 12. air it to is 7.8 equal mΩ 8.1/°C m whileΩ/ C. for The synthetic results areair presentedit is equal 8.1in Figure mΩ/°C. 12 The. results are presented in Figure 12.

Figure 12. Changes of the resistance of structures with graphite oxide and graphene oxide vs. Figure 12. Changes of the resistance of structures with graphite oxide and graphene oxide vs. temperature—the measurements in nitrogen and synthetic air. Figuretemperature—the 12. Changes measurements of the resistance in nitrogen of structures and synthetic with air.graphite oxide and graphene oxide vs. temperature—the measurements in nitrogen and synthetic air. The differentdifferent conductivityconductivity characteristics characteristics are are probably probably due due to the to changesthe changes occurring occurring in the atomicin the atomicbondsThe ofbonds materials.different of materials. conductivity The connection The connectioncharacteristics of oxygen of to oxyg graphiteareen probably to (duringgraphite due the (duringto formation the changesthe offormation graphite occurring of oxide) graphite in isthe at atomicoxide)the expense isbonds at the of of doubleexpense materials. bonds of double The in theconnection bonds graphite. in theof The oxyggrap effectenhite. to of Thegraphite this effect is the (during of degradation this is the the formation degradation of the π ofset graphite of and the the π oxide) is at the expense of double bonds in the graphite. The effect of this is the degradation of the π

Sensors 2016, 16, 103 11 of 16 Sensors 2016, 16, 103 11 of 16 Sensors 2016, 16, 103 11 of 16 set and the change in the mechanism of conductivity ofof thethe currentcurrent inin thethe obtainedobtained graphitegraphite oxideoxide (from electron to semiconductive). A further reduction results in a partial regeneration of the change in the mechanism of conductivity of the current in the obtained graphite oxide (from electron structure of the π bondsbonds byby removingremoving thethe oxygenoxygen fromfrom ththe structure and a return to electron to semiconductive). A further reduction results in a partial regeneration of the structure of the π bonds conductivity which is decreasing with an increase in the temperature in the obtained reduced by removing the oxygen from the structure and a return to electron conductivity which is decreasing graphene oxide. with an increase in the temperature in the obtained reduced graphene oxide. 3.3. Resistance in Varying Gaseous Atmospheres (Hydrogen, Nitrogen Dioxide and Carbon Dioxide in a 3.3.Carrier Resistance Gas) in Varying Gaseous Atmospheres (Hydrogen, Nitrogen Dioxide and Carbon Dioxide in a Carrier Gas) 3.3.1. Detection of Hydrogen 3.3.1. Detection Detection of Hydrogen Subsequent experiments were devoted to investigations concerning the sensor properties of the Subsequent experiments were devoted to investigat investigationsions concerning the sensor properties of the structures with graphite oxide and graphene oxide in varying gaseous atmospheres. As the first test, structures with with graphite graphite oxide oxide and and graphene graphene oxide oxide in invarying varying gaseous gaseous atmospheres. atmospheres. As the As first the test, ﬁrst the resistance was recorded during an alternate dosing of hydrogen with a concentration of 4% in thetest, resistance the resistance was recorded was recorded during during an alternate an alternate dosing dosing of hydrogen of hydrogen with a with concentration a concentration of 4% in of synthetic air. These investigations were executed at various temperatures within the 30 °C to 150 °C synthetic4% in synthetic air. These air. Theseinvestigations investigations were executed were executed at various at various temperatures temperatures within withinthe 30 °C the to 30 150˝C °C to range. The resistance of the graphite oxide structure did not change in spite of changes in the range.150 ˝C range.The resistance The resistance of the ofgraphite the graphite oxide oxide structure structure did didnot notchange change in inspite spite of of changes changes in in the composition of the surrounding atmosphere (an exemplary characteristic plot is shown in Figure 13) composition of the surrounding atmo atmospheresphere (an exemplary exemplary characteristic characteristic plot plot is is shown shown in in Figure Figure 13)13) whereas the resistance of the structure with graphene oxide increased distinctly during the hydrogen whereas the resistance of the structure with graphe graphenene oxide increased distinctly during the hydrogen hydrogen dosing (Figure 14). dosing (Figure 14).14).

Figure 13. The resistance of the structure with graphite oxide during dosing of a different gaseous Figure 13. The resistance of the structure with graphite oxide during dosing of a different gaseous atmosphere (hydrogen 4% in a synthetic air/synthetic air). atmosphere (hydrogen 4% in a synthetic air/synthetic air). air).

Figure 14. The resistance of the structure with graphene oxide during dosing of a varying gaseous Figure 14. The resistance of the structure with graphene oxide during dosing of a varying gaseous atmosphere (hydrogen 4% in a synthetic air/syntheticair/synthetic air). air). atmosphere (hydrogen 4% in a synthetic air/synthetic air).

Sensors 2016, 16, 103 12 of 16 Sensors 2016, 16, 103 12 of 16 SensorsThe dynamic2016, 16, 103 change of the resistance is in this case considerable, the time of reaction12 of 16of the structureThe dynamicto its contact change with of the the hydrogen resistance is is eviden in thistly case shorter considerable, than its reaction the time to of the reaction contact of with the structure to its contact with the hydrogen is evidently shorter than its reaction to the contact with syntheticThe air dynamic(the time change of detoxication). of the resistance Such is a in character this case considerable,of the changes the may time ofbe reactionexplained of theby the synthetic air (the time of detoxication). Such a character of the changes may be explained by the formationstructure of dangling to its contact bonds with inthe the hydrogen reduction is process. evidently Favorable shorter than spots its for reaction the adsorption to the contact of the with gases formation of dangling bonds in the reduction process. Favorable spots for the adsorption of the gases are createdsynthetic in airthe (the structure time of contributing detoxication). to Such electr a characteric response of thechanges; changes hydrogen may be explained adsorbing by onto the the structureare createdformation transfers in ofthe dangling structure electrons bonds contributing and in theas a reduction consequence to electr process.ic thresponsee Favorable resistance changes; spots of forthe hydrogen the structure adsorption adsorbing is increasing of the gases onto [40]. the Thisstructure arefast created adsorption transfers in the electrons of structure hydrogen and contributing fromas a consequencethe tostructur electrice responsethprovese resistance that changes; the of weak hydrogenthe structure van der adsorbing Waals is increasing ontointeractions the [40]. takeThis structureplacefast adsorption and transfers that the of electrons hydrogen and fromundergoes as a consequence the structur a physical thee proves resistance sorption. that of the structureweak va isn increasingder Waals [ 40interactions]. This take fastTheplace adsorptiondifference and that of thebetween hydrogen hydrogen the from undergoesresistance the structure duringa physical proves the that sorption.dosing the weak of nitrogen van der Waals or synthetic interactions air takeand the resistanceplaceThe difference andduring that thethe between hydrogen dosing theof undergoes hydrogen resistance a physical (4% during in sorption.a thecarrier dosing gas) of increases nitrogen (Figures or synthetic 15 and air 16).and It the is The difference between the resistance during the dosing of nitrogen or synthetic air and the causedresistance by theduring generation the dosing of the of surface hydrogen defect (4% (ins inide a carrier a given gas) particle) increases due to(Figures the rising 15 temperature.and 16). It is resistance during the dosing of hydrogen (4% in a carrier gas) increases (Figures 15 and 16). It is caused Thecaused increasing by the generation appropriate of surface the surface of nanomaterials defect (inside leads a given to an particle) increase due in the to theamount rising of temperature. unsaturated The increasingby the generation appropriate of the surface surface defectof nanomaterials (inside a given leads particle) to an increase due to the in risingthe amount temperature. of unsaturated The coordinatingincreasing spots appropriate which can surface cooperate of nanomaterials with hydrogen. leads to an increase in the amount of unsaturated coordinating spots which can cooperate with hydrogen. coordinating spots which can cooperate with hydrogen.

Figure 15. Differences (between resistance during dosing 4% in nitrogen and resistance during dosing Figure 15. Differences (between resistance during dosing 4% in nitrogen and resistance during 0%Figure hydrogen 15. Differences in nitrogen) (between in the resistance resistance during of the dosing structure 4% in withnitrogen graphite and resistance oxide as duringa function dosing of dosing 0% hydrogen in nitrogen) in the resistance of the structure with graphite oxide as a function 0% hydrogen in nitrogen) in the resistance of the structure with graphite oxide as a function of temperature.of temperature. temperature.

FigureFigure 16. Differences 16. Differences (between (between resistance resistance during during dosing 4%4%in in synthetic synthetic air air and and resistance resistance during during dosing 0% hydrogen in synthetic air) in the resistance of the structure with graphite oxide as a function dosingFigure 16.0% Differenceshydrogen in(between synthetic resistance air) in duringthe resistance dosing 4%of thein syntheticstructure air with and graphite resistance oxide during as of temperature. adosing function 0% of hydrogen temperature. in synthetic air) in the resistance of the structure with graphite oxide as a function of temperature. 3.3.2. Detection of Nitrogen Dioxide 3.3.2. Detection of Nitrogen Dioxide The sensor structure with graphene oxide was characterized by changes of the resistance during the dosingThe sensor of nitrogen structure oxide. with Ingraphene such a oxidecase, bowasth characterizedduring alterdosing by changes nitrogen of the oxide resistance with various during the dosing of nitrogen oxide. In such a case, both during alterdosing nitrogen oxide with various

Sensors 2016, 16, 103 13 of 16

3.3.2. Detection of Nitrogen Dioxide

SensorsSensorsThe 2016 2016 sensor, ,16 16, ,103 103 structure with graphene oxide was characterized by changes of the resistance1313 of of 16 16 during the dosing of nitrogen oxide. In such a case, both during alterdosing nitrogen oxide with variousconcentrationsconcentrations concentrations inin thethe nitrogen,nitrogen, in the nitrogen, andand nitrogennitrogen and nitrogenoxideoxide inin synthetic oxidesynthetic in air, syntheticair, thethe resistanceresistance air, the ofof resistance thethe structurestructure of the dropped in the case of dosing nitrogen dioxide. The structure with graphite oxide did not change its structuredropped dropped in the case in theof dosing case of nitrogen dosing nitrogendioxide. The dioxide. structure The with structure graphite with oxide graphite did not oxide change did its not resistanceresistance in in spite spite of of changes changes of of dosing dosing gases. gases. Exemplary Exemplary characteristics characteristics are are presented presented in in Figures Figures 17 17 change its resistance in spite of changes of dosing gases. Exemplary characteristics are presented andand 18. 18. in Figures 17 and 18.

FigureFigureFigure 17. 17. 17.Changes Changes Changes of of of the the the resistance resistance resistance ofof of the the structure structure wi withwithth graphite graphite oxide oxide oxide (dosed (dosed (dosed atmosphere: atmosphere: atmosphere: nitrogen nitrogen nitrogen dioxidedioxidedioxide with with with concentration concentration concentration of of 125 125125 ppm, ppm, 25 25 25 ppm, ppm, ppm, 5 5 ppm ppm 5 ppm and and and 500 500 500ppm, ppm, ppm, temperature temperature temperature of of the the ofsample: sample: the sample: 100 100 100°C)°C)˝C)

FigureFigureFigure 18. 18. 18.Changes Changes Changes of of of the the theresistance resistance resistance of of the the structure structurestructure with with graphene graphene oxide oxide oxide (dosed (dosed (dosed atmosphere: atmosphere: atmosphere: nitrogen nitrogen nitrogen dioxidedioxidedioxide with with with concentration concentration concentration of of 125 125125 ppm, ppm, 25 25 25 ppm, ppm, ppm, 5 5 ppm ppm 5 ppm and and and 500 500 500ppm, ppm, ppm, temperature temperature temperature of of the the ofsample: sample: the sample: 100 100 100°C).°C).˝C).

InIn contactcontact withwith thethe structure,structure, nitrogennitrogen dioxidedioxide behavesbehaves likelike aa donordonor ofof electrons,electrons, reducingreducing thethe In contact with the structure, nitrogen dioxide behaves like a donor of electrons, reducing the resistanceresistance of of the the structure structure [20,21,4 [20,21,41].1]. The The influence influence of of nitrogen nitrogen dioxid dioxidee ought ought to to be be treated treated as as a a process process resistance of the structure [20,21,41]. The inﬂuence of nitrogen dioxide ought to be treated as a process ofof physical physical sorption, sorption, in in which which the the conductivity conductivity of of the the sensor sensor is is increased increased due due to to electrostatic electrostatic effect. effect. of physical sorption, in which the conductivity of the sensor is increased due to electrostatic effect. 3.3.3.3.3.3. Detection Detection of of Carbon Carbon Dioxide Dioxide 3.3.3. Detection of Carbon Dioxide DuringDuring thethe experiments,experiments, neitheneitherr thethe structurestructure withwith graphitegraphite oxideoxide nornor thethe structurestructure withwith graphenegrapheneDuring oxide theoxide experiments, changedchanged theirtheir neither resistanceresistance the structure duedue toto theithei withrr contactcontact graphite withwith oxide carboncarbon nor thedioxidedioxide structure inin aa distinctdistinct with graphene way.way. oxideInsignificantInsignificant changed changes theirchanges resistance of of resistance resistance due towere were their recorded recorded contact (Figure (Figure with carbon 19), 19), but but dioxide most most probably, probably, in a distinct these these way. changes changes Insigniﬁcant were were changesduedue to to a ofa difference difference resistance in in werethe the thermal thermal recorded conductivity conductivity (Figure 19 of of), CO CO but22 and and most the the probably, carrier carrier gas: gas: these nitrogen nitrogen changes and and synthetic weresynthetic due air. air. to a difference in the thermal conductivity of CO2 and the carrier gas: nitrogen and synthetic air.

Sensors 2016, 16, 103 14 of 16 Sensors 2016, 16, 103 14 of 16

FigureFigure 19. 19.Changes Changes of of the the resistance resistance ofof thethe sensorsensor stru structurecture with with graphite graphite oxide oxide and and graphene graphene oxide oxide ˝ duringduring dosing dosing CO CO2 2and and N N22,, temperaturetemperature of of the the sample: sample: 100 100 °C.C.

4.4. Conclusions Conclusions The paper deals with a technique for obtaining graphite oxide by applying Hummer’s method The paper deals with a technique for obtaining graphite oxide by applying Hummer’s method and the technique of getting (on this graphite oxide) reduced graphene oxide. The paper presents the and the technique of getting (on this graphite oxide) reduced graphene oxide. The paper presents the results of investigations concerning the chemical and physical properties of the obtained materials. results of investigations concerning the chemical and physical properties of the obtained materials. The investigations performed by means of the SEM and AFM techniques have proved that the The investigations performed by means of the SEM and AFM techniques have proved that the surfaces of graphite oxide were evidently more developed than the surfaces of reduced graphene surfacesoxide. The of graphite thermal oxidereduction were involved evidently a separation more developed of numerous than thefunctional surfaces groups of reduced from graphite graphene oxide.oxide, The resulting thermal in a reduction change of involvedthe chemical a separation composition of of numerous the obtained functional material. groupsThese changes from graphite were oxide,confirmed resulting by inthree a change methods of theof measurements, chemical composition independent of the of obtained each other, material. viz. XPS, These EDX changes and AE were confirmedmethods, bywhich three confirmed methods a ofconsiderable measurements, reduction independent in the amount of each of oxygen other, inviz. theXPS, graphene EDX oxide. and AE methods,Changes which caused confirmed by the reduction a considerable were also reduction confirmed in theby Raman’s amount ofSpectrometry oxygen in theand graphene the methods oxide. ChangesTG/DTG caused as well by as theDSC. reduction were also confirmed by Raman’s Spectrometry and the methods TG/DTGInvestigations as well as DSC. on the sensitivity of the electric properties of the resistance structures based on graphiteInvestigations oxide and on reduced the sensitivity graphene of oxide the electricto chosen properties gases were of also the performed. resistance structuresGenerally, basedit may on graphitebe assumed oxide that and the reduced possibility graphene of using oxide graphite to chosen oxide gases for gas were sensors also performed.is rather limited. Generally, It seems it maythat be assumedthere are that considerable the possibility perspectives of using graphite for applying oxide forreduced gas sensors graphene is rather oxide limited. for detecting It seems NO that2 therein areatmospheres considerable of perspectives synthetic air for (and applying in the reducedatmosphe grapheneres of nitrogen, oxide for too). detecting The investigations NO2 in atmospheres have ofproved synthetic that air a resistance (and in the structure atmospheres with a layer of nitrogen, of reduced too). graphene The investigations oxide can react haveto NO proved2 already that at a a concentration of ppb in an atmosphere of carrier gas. This provides options for its practical use in resistance structure with a layer of reduced graphene oxide can react to NO2 already at a concentration ofsensing ppb in anof atmospherenitrogen dioxide of carrier obtained gas. by This mean providess of the options method for suggested its practical in this use paper. in sensing of nitrogen dioxide obtained by means of the method suggested in this paper. Acknowledgments: The work was partially sponsored by the Polish National Science Centre (NCN) within the grant 2012/07/B/ST7/01 471; the Statutory R&D Project IChPW, No 11.15.024. Acknowledgments: The work was partially sponsored by the Polish National Science Centre (NCN) within the grantAuthor 2012/07/B/ST7/01 Contributions: S. 471; Drewniak the Statutory carried Rout & the D Project measurements IChPW, using No 11.15.024. AFM, SEM with ETD detector and RS, Authorcooperated Contributions: in electricalSabina measurements, Drewniak analyzed carried out all theobta measurementsined data and wrote using AFM,this paper. SEM R. with Muzyka ETD detectorprepared and RS,GRO cooperated and rGO in samples electrical and measurements, interpreted the analyzed results obtained all obtained using data SEM and EDX, wrote FT-IR, this XRD, paper. XPS, Roksana EA, TG/DTG Muzyka preparedmethods. GRO A. Stolarczyk and rGO samples prepared and the interpreted resistance samples the results and obtained interpreted using the SEM electric EDX,al results. FT-IR, XRD,T. Pustelny XPS, EA, TG/DTGanalyzed methods. and discussed Agnieszka the data Stolarczyk and reviewed prepared the paper. the resistance M. Kotyczka-Mora samples andńska interpreted carried out thethe electricalTG, DTG results.and TadeuszFT-IR Pustelnyexperiments. analyzed M. Setkiewicz and discussed cooperated the data in electrical and reviewed measurements. the paper. Michalina Kotyczka-Morańskacarried out the TG, DTG and FT-IR experiments. Maciej Setkiewicz cooperated in electrical measurements. Conflicts of Interest: The authors declare no conflict of interest. Conflicts of Interest: The authors declare no conflict of interest. References References 1. Hu, N.; Yang, Z.; Wang, Y.; Zhang, L.; Wang, Y.; Huang, X.; Wei, L.; Zhang, Y. Ultrafast and sensitive room 1. Hu, N.; Yang, Z.; Wang, Y.; Zhang, L.; Wang, Y.; Huang, X.; Wei, L.; Zhang, Y. Ultrafast and sensitive temperature NH3 gas sensors based on chemically reduced graphene oxide. Nanotechnology 2014, 25, roomdoi:10.1088/0957-4484/25/2/025502. temperature NH3 gas sensors based on chemically reduced graphene oxide. Nanotechnology 2014, 25. [CrossRef][PubMed]

Sensors 2016, 16, 103 15 of 16

2. He, Q.; Wu, S.; Yin, Z.; Zhang, H. Graphene-based electronic sensors. Chem. Sci. 2012, 3, 1764–1772. [CrossRef] 3. Borysiewicz, M.A.; Dynowska, E.; Kolkovsky, V.; Wielgus, M.; Gołaszewska, K.; Kami´nska,E.; Ekielski, M.; Struk, P.; Pustelny, T.; Piotrowska, A. Sputter deposited ZnO porous ﬁlms for sensing applications. MRS Proc. 2013, 1494, 71–76. [CrossRef]

4. Procek, M.; Stolarczyk, A.; Pustelny, T.; Maciak, E. A study of a QCM sensor based on TiO2 nanostructures for the detection of NO2 and explosives vapours in air. Sensors 2015, 15, 9563–9581. [CrossRef][PubMed] 5. Sadasivuni, K.K.; Kafy, A.; Kim, H.-C.; Ko, H.-U.; Mun, S.; Kim, J. Reduced graphene oxide filled cellulose films for flexible temperature sensor application. Synthetic Met. 2015, 206, 154–161. [CrossRef] 6. Sriram, P.; Nutenki, R.; Mandapati, V.R.; Karruppiah, M.; Kattimuttathu, S.I. Effect of graphene oxide size and structure on synthesis and optoelectronics properties of hybrid graphene oxide-poly(3-hexylthiophene) nanocomposites. Polym. Composit. 2015.[CrossRef] 7. Dobrzańska-Danikiewicz,A.D.; Lukowiec, D.; Pawlyta, M.; Gaweł, T.; Procek, M. Resistance changes of carbon nanotubes decorated with platinium nanoparticles in the presence of hydrogen at different and constant concentrations. Phys. Status Solidi B Basic Res. 2014, 251, 2426–2431. [CrossRef] 8. Lange, U.; Hirsch, T.; Mirsky, V.M.; Wolbeis, O.S. Hydrogen sensor based on a graphene-palladium nanocomposite. Electrochim. Acta 2011, 56, 3707–3712. [CrossRef] 9. Talyzin, A.V.; Hausmaninger, T.; You, S.; Szabó, T. The structure of graphene oxide membranes in liquid water, ethanol and water-ethanol mixtures. Nanoscale 2014, 6, 272–281. [CrossRef][PubMed] 10. Medhekar, N.V.; Ramasubramaniam, A.; Ruoff, R.S.; Shenoy, V.B. Hydrogen bond networks in graphene oxide composite paper: Structure and mechanical properties. ACS Nano 2010, 4, 2300–2306. [CrossRef] [PubMed] 11. Jaworski, S.; Sawosz, E.; Kutwin, M.; Wierzbicki, M.; Hinzmann, M.; Grodzik, M.; Winnicka, A.; Lipińska,L.; Włodyga, K.; Chwalibog, A. In vitro and in vivo effect of graphene oxide and reduced graphene oxide on glioblastoma. Int. J. Nanomed. 2015, 10, 1585–1596. 12. Talyzin, A.V.; Klechikov, A.; Korobov, M.; Rebrikova, A.T.; Avramenko, N.V.; Gholami, M.F.; Severin, N.; Rabe, J.P. Delamination of graphite oxide in a liquid upon cooling. Nanoscale 2015, 7, 12625–12630. [CrossRef] [PubMed] 13. Muhammad-Hafiz, S.; Ritikos, R.; Whitcher, T.J.; Razib, N.M.; Bien, D.C.S.; Chanlek, N.; Nakajima, H.; Saisopa, T.; Songsiriritthigul, P.; Huang, N.M.; et al. A practical carbon dioxide gas sensor using room-temperature hydrogen plasma reduced graphene oxide. Sens. Actuators B Chem. 2014, 193, 692–700. [CrossRef] 14. Zhao, F.; Cheng, H.; Zhang, Z.; Jiang, L.; Qu, L. Direct power generation from a graphene oxide film under moisure. Adv. Mater. 2015, 27, 4351–4357. [CrossRef][PubMed] 15. Eigler, S.; Dotzer, C.; Hitsch, A. Visualization od defect densities in reduced graphene oxide. Carbon 2012, 50, 3666–3673. [CrossRef] 16. Ko, G.; Kim, H.-Y.; Ahn, J.; Park, Y.-M.; Lee, K.-Y.; Kim, J. Graphene-based nitrogen dioxide gas sensors. Curr. Appl. Phys. 2010, 10, 1002–1004. [CrossRef] 17. Kaniyoor, A.; Imran Jafri, R.; Arockiadoss, T.; Ramaprabhu, S. Nanostructured Pt decorated graphene and multi-walled carbon nanotube based room temperature hydrogen gas sensor. Nanoscale 2009, 1, 382–386. [CrossRef][PubMed]

18. Naama, S.; Hadjersi, T.; Keffous, A.; Nezzal, G. CO2 gas sensor based on silicon nanowires modiﬁed with metal nanoparticles. Mater. Sci. Semicond. Process. 2015, 38, 367–372. [CrossRef] 19. Drewniak, S.; Pustelny, T.; Muzyka, R.; Stolarczykand, A.; Konieczny, G. Investigations of selected physical properties of graphite oxide and thermally exfoliated/reduced graphene oxide in the aspect of their applications in photonic gas sensors. Photonics Lett. Pol. 2015, 7, 47–49. 20. Lu, G.; Ocola, L.E.; Chen, J. Gas detection using low-temperature reduced graphene oxide sheets. Appl. Phys. Lett. 2009, 94.[CrossRef] 21. Lu, G.; Ocola, L.E.; Chen, J. Reduced graphene oxide for room-temperature gas sensors. Nanotechnology 2009, 20.[CrossRef][PubMed] 22. Robinson, J.T.; Perkins, F.K.; Snow, E.S.; Wei, Z.; mSheehan, P.E. Reduced Graphene Oxide Molecular Sensors. Nano Lett. 2008, 8, 3137–3140. [CrossRef][PubMed] Sensors 2016, 16, 103 16 of 16

23. Botas, C.; Álvarez, P.; Blanco, P.; Granda, M.; Blanco, C.; Santamaría, R.; Romasanta, L.J.; Verdejo, R.; López-Manchado, M.A.; Menéndez, R. Graphene materials with different structures prepared from the same graphite by the Hummers and Brodie methods. Carbon 2013, 65, 156–164. [CrossRef] 24. Rodriguez-Pastor, I.; Ramos-Fernandez, G.; Varela-Rizo, H.; Terrones, M.; Martin-Gullon, I. Towards the understanding of the graphene oxide structure: How to control the formation of humic- and fulvic-like oxidized debris. Carbon 2015, 84, 299–309. [CrossRef] 25. Botas, C.; Álvarez, P.; Blanco, C.; Santamaría, R.; Granda, M.; Ares, P.; Rodríguez-Reinoso, F.; Menéndez, R. The effect of the parent graphite on the structure of graphene oxide. Carbon 2012, 50, 275–282. [CrossRef] 26. Jeong, H.K.; Colakerol, L.; Jin, M.H.; Glans, P.A.; Smith, K.E.; Lee, Y.H. Unoccupied electronic states in graphite oxides. Chem. Phys. Lett. 2008, 460, 499–502. [CrossRef] 27. He, H.; Klinowski, J.; Forster, M.; Lerf, A. A new structural model for graphite oxide. Chem. Phys. Lett. 1998, 287, 53–56. [CrossRef] 28. Petit, C.; Seredych, M.; Bandosz, T.J. Revisiting the chemistry of graphite oxides and its effect on ammonia adsorption. J. Mater. Chem. 2009, 19, 9176–9185. [CrossRef] 29. Fan, X.; Yu, Ch.; Yang, J.; Ling, Z.; Qiu, J. Hydrothermal synthesis and activation of graphene-incorporated nitrogen-rich carbon composite for high-performance supercapacitors. Carbon 2014, 70, 130–141. [CrossRef] 30. Yang, X.; Ma, L.; Wang, S.; Li, Y.; Tu, Y.; Zhu, X. “Clicking” graphite oxide sheets with well-defined polystyrenes: A new Strategy to control the layer thickness. Polymer 2011, 52, 3046–3052. [CrossRef] 31. Hontoria-Lucas, C.; Lopez-Peinado, A.J.; de, J.; Lopez-Gonzalez, D.; Rojas-Cervantes, M.L.; Martin-Aranda, R.M. Study of oxygen-containing groups in a series of graphite oxides: Physical and chemical characterization. Carbon 1995, 33, 1585–1592. [CrossRef] 32. Toh, S.Y.; Loh, K.S.; Kamarudin, S.K.; Daud, W.R.W. Graphene production via electrochemical reduction of graphene oxide: Synthesis and characterisation. Chem. Eng. J. 2014, 251, 422–434. [CrossRef] 33. Wang, C.; Jin, Q.; Wang, Y.; Yin, H.; Xie, H.; Cheng, R. A green route to prepare graphite oxide-poly(acrylic acid) and -poly(acrylamide) hybrids under γ-ray irradiation. Mater. Lett. 2012, 68, 280–282. [CrossRef] 34. Kim, W.J.; Basavaraja, C.; Thinh, P.X.; Huh, D.S. Structural characterization and DC conductivity of honeycomb-patterned poly(ε-caprolactone)/gold nanoparticle-reduced graphite oxide composite films. Mater. Lett. 2013, 90, 14–18. [CrossRef] 35. Blanco, M.; Alvarez, P.; Blanco, C.; Jimenez, C.V.; Fernandez-Tornos, J.; Perez-Torrente, P.; Oro, L.A.; Menendez, R. Graphene-NHC-iridium hybrid catalysts built through –OH covalent linkage. Carbon 2015, 83, 21–31. [CrossRef] 36. Mo, Z.; Zheng, R.; Peng, H.; Liang, H.; Liao, S. Nitrogen-doped graphene prepared by a transfer doping approach for the oxygen reduction reaction application. J. Power Sources 2014, 245, 801–807. [CrossRef] 37. Drewniak, S.; Pustelny, T.; Muzyka, R.; Konieczny, G.; Kałuzyński,P.˙ The effect of oxidation and reduction processes of graphite on physicochemical properties of graphite oxide and reduced graphene oxide. Photonics Lett. Pol. 2014, 6, 130–132. [CrossRef] 38. Zhang, C.; Lv, W.; Xie, X.; Tang, D.; Liu, C.; Yang, Q.-H. Towards low temperature thermal exfoliation of graphite oxide for graphene production. Carbon 2013, 62, 11–24. [CrossRef] 39. O’Neill, A.; Bakirtzis, D.; Dixon, D. Polyamide 6/Graphene composites: The effect of in situ polymerisation on the structure and properties of graphene oxide and reduced graphene oxide. Eur. Polym. J. 2014, 59, 353–362. [CrossRef] 40. Zhang, L.-S.; Wang, W.D.; Liang, X.-Q.; Chu, W.-S.; Song, W.-G.; Wang, W.; Wu, Z.-Y. Characterization

of partially reduced graphene oxide as room temperature sensor for H2. Nanoscale 2011, 3, 2458–2460. [CrossRef][PubMed] 41. Lu, G.; Park, S.; Yu, K.; Ruoff, R.S.; Ocola, L.E.; Rosenmann, D.; Chen, J. Toward practical gas sensing with highly reduced graphene oxide: A new signal processing method to circumvent run-to-run and device-to-device variations. ACS Nano 2011, 5, 1154–1164. [CrossRef][PubMed]

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