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Low-temperature thermal reduction of graphene oxide films in ambient atmosphere: Infra-red spectroscopic studies and gas sensing applications
Low-temperature thermal reduction of graphene oxide films in ambient atmosphere: Infra-red spectroscopic studies and gas sensing applications E. Tegou, G. Pseiropoulos, M.K. Filippidou, S. Chatzandroulis PII: DOI: Reference:
S0167-9317(16)30142-3 doi: 10.1016/j.mee.2016.03.030 MEE 10213
To appear in: Received date: Revised date: Accepted date:
15 October 2015 10 March 2016 15 March 2016
Please cite this article as: E. Tegou, G. Pseiropoulos, M.K. Filippidou, S. Chatzandroulis, Low-temperature thermal reduction of graphene oxide films in ambient atmosphere: Infra-red spectroscopic studies and gas sensing applications, (2016), doi: 10.1016/j.mee.2016.03.030
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ACCEPTED MANUSCRIPT Low-temperature thermal reduction of graphene oxide films in ambient atmosphere: infrared spectroscopic studies and gas sensing applications E. Tegou, G. Pseiropoulos, M.K. Filippidou, S. Chatzandroulis
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Institute of Nanoscience and Nanotechnology, NCSR “Demokritos”, Athens, Greece
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e-mail: [email protected]
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Abstract
Low-temperature (≤ 300C) thermal treatment of graphene oxide (GO) films in ambient air is examined. In particular, the role of low to moderate heating temperatures, to the evolution of the
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original functional groups anchored on the GO skeleton, is closely investigated by Fourier transform infra red (FT-IR) spectroscopy. The study shows that, contrary to vacuum or inert
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ambient heating, heating under ambient atmosphere triggers concomitant reduction and oxidation reactions. Hydroxyl and epoxy groups are progressively eliminated, but at the same time newly formed carbonyls appear due to oxidation. Electrical measurements indicate that despite the
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presence of oxygen containing groups in the restored graphene sp2 network, the conductivity
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enhances. The process, therefore, lends itself to the production of conductive reduced GO with increased functionalities suitable for application in gas sensor fabrication. The concept is evaluated
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with a humidity sensor where thermally reduced GO is prepared at different reduction temperatures. The evaluation unveils that a critical reduction temperature exists where sensor sensitivity is optimized.
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Keywords: graphene oxide, thermal reduction, oxidation, gas sensors, infra-red spectroscopy, electrical conductivity
1. INTRODUCTION Graphene is a two-dimensional monolayer of carbon atoms tightly packed into a flat hexagonal structure, similar to a honeycomb lattice possessing innovative mechanical, electrical, thermal, and optical properties. [1] Current research on graphene materials is an exciting field that interfaces chemistry, physics, materials science and engineering. As graphene materials and their derivatives, maintain the unique graphene’s properties in bulk, they have a wide range of applications in various fields such as batteries, [2, 3] organic photovoltaics, [4-10] transistors, [11, 12] biology, [13] hydrogen storage, [14] as field-emission cathodes [15], as well as for the mass production of solution processable chemically exfoliated graphene oxide. [16, 17] In particular, graphene’s twodimensional nature and high electron mobility translate into high sensitivity to surface chemical
ACCEPTED MANUSCRIPT interactions thereby making it an ideal platform for gas sensors. [18] Nevertheless, graphene is difficult to produce and process on a large scale. Among other approaches, one of the promising mass production routes of graphene is through reduction of graphene oxide (GO).[19] GO,[20, 21]
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prepared by the oxidation of graphite and the exfoliation of the generated graphite oxide, can be
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viewed as a graphene layer asymmetrically decorated with oxygen-containing functional groups on the basal plane and the edges. Despite extensive research, GO remains an elusive material with
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hardly obtainable direct structure information. [22] According to the widely accepted LerfKlinowski model, [23] GO contains two kinds of regions: aromatic regions with flat unoxidized benzene rings and wrinkled regions with alicyclic six-membered rings bearing C=C, hydroxyl, and
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ether groups, while GO sheets terminate with hydroxyl and carboxyl groups. In other words, GO is
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an insulating and disordered analogue of the highly conducting crystalline graphene.
GO reduction can be effected by various external stimuli such as heating, [24] chemical treatment, [25], and laser [26] or gamma ray irradiation.[27] It yields reduced GO (rGO) that resembles
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pristine graphene structure and is electrically conductive (due to restoration of graphene sp2
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network). Depending on reduction extent, a partly restored sp2 lattice may be generated while also retaining some oxygen-bearing groups. In this way, rGO can find applications as a graphene
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alternative and provide ad hoc functionalized forms of graphene, for instance in order to optimize rGO response to gas vapors.
Graphene-based gas sensors can detect vapor adsorption down to the single-molecule level, [28]
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with the mechanism stated to be charge transfer induced by adsorption/desorption of molecules (which act as electron donors or acceptors) on the graphene surface, leading to changes in graphene conductance. In recent years, research has aimed to identify and optimize the factors affecting the analytical performance of rGO gas sensors, namely, sensitivity, selectivity and stability. Such factors include voltage activation, [29] film thickness, [30] e-beam radiation, [31, 32] chemical functionalization, [33, 34] nanoparticle doping, [35-37] the type of chemical reduction,[38] and the extent of reduction. [39-40]
In this work, aided by Fourier-transform infra red (FT-IR) spectroscopy, we investigate in a systematic way the extent of GO thermal reduction at low to moderate heating temperatures (90C300C), for gas sensing applications. The reduction is conducted under ambient air conditions, i.e. without high vacuum or any special atmosphere involved. Our findings prove that both oxidation and reduction take place concurrently: hydroxyl and epoxy groups are progressively eliminated, but
ACCEPTED MANUSCRIPT at the same time newly formed oxidation groups appear. The final product is a partly restored graphene sp2 network bearing oxygen containing groups and it is electrically conductive. Furthermore, a humidity sensor based on a mildly thermally treated rGO is fabricated and evaluated
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for different reduction temperatures.
2. EXPERIMENTAL
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2.1 Material
A commercial “Single Layer Graphene Oxide Ethanol Dispersion” was purchased from ACS
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Material® and used as is.
2.2 Characterization of rGO
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GO dispersion was spin-coated on 4” silicon wafer substrates to create uniform films of approx. 500 nm thickness. Then the films were heated on a hot plate at 90C, 120C, 150C, 180C, 200C, and 300C, for 1 h. Film thickness was measured before and after each heating step by a profilometer
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(XP-2, Ambios Technology). The morphology of thermally treated GO films was characterized by
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field emission scanning electron microscopy (FESEM, JSM -7401f, JEOL). Thermal gravimetric analysis (TGA) was performed on a Mettler Toledo TGA/SDTA851e thermogravimetric /
mL/min air flow.
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differential thermal analyzer. GO thermal behavior was examined at 1C/min heating rate and 50
Chemical structure characterization of the films was performed by a FT-IR spectrophotometer
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(Spectrum One, Perkin Elmer). All spectra were recorded from 4000 to 400 cm-1 in transmittance mode with a resolution of 4 cm-1 at 128 scans. Clean silicon wafers were used as background because of their transparency at the mid-IR range. In order to quantify the spectroscopic observations, the transmission measurements were converted into corresponding absorbance data and a polynomial baseline was subtracted from all raw spectra. By dividing by the film thickness of each sample, the spectra were normalized. Finally, peak areas were calculated between relative baseline points.
For the electrical characterization of GO, the dispersion was drop-casted using a micropipette between two gold electrodes, patterned on insulating silicon oxide over silicon substrate with standard lithographic techniques. Then, the samples were heated. Electrical transport measurements of thermally treated GO drops were carried out at room temperature using a two-point probe
ACCEPTED MANUSCRIPT technique (Prober Karl-Suss Micromanipulator 7000 LTE equipped with HP4140B pAmeter/DC voltage source). GO electrical resistance was calculated from the slope of the I-V curves, and then
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converted to conductivity (σ).
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2.3 Sensor fabrication
For the evaluation of sensor efficiency, GO was drop-casted between electrodes and heated at
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120C, 150C, 180C, 200C, and 300C, for 1 h. Different concentrations of analyte vapors were introduced in a small volume (~7 cm3) chamber where relative humidity and temperature were controlled to within 0.1% and 0.1oC, respectively. The device response was investigated by
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measuring electrical resistance changes, when the analyte molecules were adsorbed on the sensing
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element surface.
3. RESULTS AND DISCUSSION 3.1 rGO characterization
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For all samples heated up to 200C, a rough wrinkled sheet-like surface with no obvious
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morphology change was observed (Figure 1), while at 300C surface morphology appeared less wrinkled but “blistered”. During thermal treatment, volatile products (e.g. residual H2O, CO2, CO
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or other small gas molecules) were formed and trapped within GO layers due to the limited nature of diffusion; therefore they created the observed “blisters”. We noticed that upon heating at higher
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temperatures (500C) these molecules were released by tearing the surface.
200°C, 1 h
300°C, 1 h
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(c) Figure 1. (a) and (b) FESEM pictures of rGO films heated at 200°C and 300C for 1 h. At 200°C a rough wrinkled sheet-like morphology is observed while at 300°C the surface is less wrinkled but “blistered”. (c) A typical TGA thermogram of GO showing a significant mass loss due to the
ACCEPTED MANUSCRIPT removal of oxygen containing groups from the basal plane. Remaining film thickness of GO after each thermal step is also presented in the same plot.
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TGA diagram gave additional information about the thermal stability of oxygen-containing groups
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attached to GO. The thickness of GO films remaining after each thermal step was also compared to the TGA results. Both curves presented in Figure 1(c), displayed three discrete stages. Firstly, a
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slight mass loss (~6%) occurred up to 120C, primarily attributed to the elimination of physisorbed and interlamellar water molecules. Secondly, the thermal decomposition of covalently bonded oxygen caused an additional significant mass loss (~31%), occurring up to 200C. Finally, a 12%
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mass loss observed up to 300C was probably due to the removal of more stable oxygen containing functional groups. These results are in good agreement to previous thermograms published in
of GO films by hydroiodic acid. [43]
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literature. [41, 42] Furthermore, 50% film shrinkage has also been reported as a result of reduction
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To probe the course of reactions occurring upon heating in ambient air, we employed FT-IR
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spectroscopy as a monitoring tool of the evolution of GO functional groups. Figure 2a depicts all FT-IR spectra as heating temperature increases. We observe that a critical transition temperature
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exists at 150C where chemical changes have almost completed. Above this critical temperature, the spectral changes are not that pronounced. In particular, the O-H bond stretch vibration (broad band, ranging from approx. 3700 cm-1 to 2600 cm-1), decreases progressively until 150C and then
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it almost disappears. The same observation applies for the weak peak at 856 cm -1, commonly used for the classification of epoxides (off-plane three-member epoxy rings), which decrease concomitantly to the hydroxyl groups. On the other hand, prominent sp2 graphitic domains are constantly generated in GO as proven by the 1578 cm-1 peak increase, [41, 42, ,44, 45] attributed to C=C stretch vibration of non-substituted aromatic bonds. At the same time, the stretch vibrations of the C=O bond (1738 cm-1) increases.
The spectral changes in the C=O region were used to probe the oxidation reaction, as presented in detail in Figure 2b. As the temperature increases, we observe three qualitative characteristics: a) a peak widening, b) a peak shift to higher wavenumbers (it moves from 1725 cm-1 to 1744 cm-1), and c) a new peak appearing at 1837 cm-1 (for temperatures higher than 150C). These observations indicate both intra- or/and inter- molecular anhydride and ester formation. In general, anhydrides are formed between two adjacent carboxyl groups and cause two well-defined absorbance peaks at
ACCEPTED MANUSCRIPT ~1775 cm-1 and at ~1835 cm-1 (the band at 1775 cm-1, overlapped by the initial C=O stretching, causes the peak shift to higher wavenumbers). In addition, newly formed esters between a hydroxyl and a neighboring carboxyl group, may also cause the carbonyl peak shift to higher wavenumbers
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(in general, ester carbonyl bonds stretch at higher wavenumbers than acids, aldehydes and ketones).
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Figure 2. a) FT-IR spectra of GO films heated at various temperatures. Inset: Schematic
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representation of GO. b) Zoom-in of the carbonyl band after each heating step. As the temperature increases, the band gets wider and shifts from 1725 cm-1 to 1744 cm-1. The shoulder at 1835 cm-1
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denotes anhydride formation.
Finally, if we calculate the respective peak areas, we may quantify all the aforementioned qualitative spectral observations. An illustration of such quantification showing the evolution of GO
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functionalities (OH, C=O, and C=C) during thermal reduction is presented in Figure 3. Figure 3 also shows the electrical conductivity enhancement of GO proceeding with temperature. Electrical conductivity enhances with temperature predominantly as a consequence of the elimination of hydroxyl and epoxy groups along with the simultaneous gradual restoration of the graphene sp2 network. The conductivity begins to increase above 90C indicating that a graphene-like structure starts to generate, and exhibits a percolation-type behavior up to 150C. Above this critical temperature and for the tested range of temperatures no further significant conductivity increase is observed.
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Figure 3. Removal of hydroxyl functionalities, formation of carbonyls groups, and simultaneous
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restoration of the sp2 graphene network (C=C), along with electrical conductivity enhancement as a function of heating temperature. At 150°C the rGO structure has been formed: the OH groups have
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decreased, the amount of C=C bonds have increased and the electrical conductivity has enhanced
3.2 Gas sensor response
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sharply.
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In order to probe the sensor performance, five sensors were fabricated and exposed to water vapors. The deposited GO sensitive layer was reduced at 120C, 150C, 180C, 200C and 300C for 1 h. In all cases, a wide range of vapour concentrations (500 – 20,000 ppm) were introduced in the
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chamber interchanged with dry N2 intervals to allow water molecules to desorb and the sensor to recover. In Fig. 4a the normalized response ΔR/Ro(%) of the rGO sensors upon exposure to water vapors is depicted. ΔR/Ro(%) is defined as : Δ
Δ
where Ro is the initial sensor resistance prior to exposure to analyte vapors and R the sensor resistance during testing in the presence of various analyte concentrations. We observe that the rGO sensors respond with varying sensitivity depending on reduction temperature: the sensor with GO reduced at 150C shows higher response, thus higher sensitivity, comparing to the other sensors. The above are illustrated in Figure 4b.
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(a)
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Figure 4. a) Normalized response of the rGO device upon exposure to water vapors, operating at
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room temperature. The GO sensing layer was heated at 120C, 150C, 180C, 200C and 300C for 1 h. b) comparison of all rGO sensor responses at 10,000 ppm H2O: the reduced at 150C sensor
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shows the highest response.
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The above observations may be explained if we take into account the evolution of GO functionalities during heating as evidenced by the FT-IR spectra. If we correlate the sensor response
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to the data shown in Figure 3, we notice that the highest response is achieved once the rGO structure is formed for the first time, which coincides to the exact temperature when the electrical conductivity increases sharply, i.e. 150C. At this temperature, rGO has become conductive enough
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but at the same time retains a small amount of OH groups. On the other hand, the response at 300C is the lowest. As hydroxyl functionalities are eliminated at higher reduction temperatures, the rGO droplet becomes denser. In other words, the distance between the GO layers decreases.[46] Hence, comparing to the initial three-dimensional (3D) GO structure, as reduction temperature increases water molecules penetrate into the resulting inner rGO layers with accelerated difficulty. This reasoning may also explain the increased but noisy sensor response observed at 120C reduction temperature, i.e. low-conductivity structures are likely to present noisy responses while hydrophilic 3D structures interact strongly with water vapors, hence the response is enhanced. Thus, the optimal reduction temperature lies where rGO structure is conductive enough, however still hydrophilic and not too compact. Additional experiments with methanol and ethanol vapors confirmed the higher response observed at 150C. Nevertheless, the sensor response at these vapors was significantly lower (ΔR/R < 2%) than water vapors, since they interact only with the top few layers of the rGO drop. [47]
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4. CONCLUSIONS In summary, the chemical structure of GO films versus mild to moderate heating temperatures (90-
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300C) in ambient air was studied by FT-IR spectroscopy. Thermal treatment gradually removed
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GO hydroxyl and off-the plane epoxy groups, and restored C=C sp2 bonds. At the same time, atmospheric oxygen promoted GO oxidation by forming C=O containing functional groups that
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remained in the final product. All structural changes proceeded drastically up to a critical temperature (150C). In addition, the electrical conductivity enhancement exhibited a percolationtype behavior up to that critical temperature. A humidity rGO sensor was fabricated and evaluated
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for different reduction temperatures. It was revealed that the sensor sensitivity can be optimized through controlling the reduction degree by adjusting the reduction temperature. This study points
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to ways for facilitating the design of graphene materials with functional sites across a range of applications.
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ACKNOWLEDGMENTS
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This work was partially funded by the State Scholarships Foundation (IKY) (Grant number: 22056/13) under the IKY fellowships of excellence for postgraduate studies in Greece-Siemens
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Graphical abstract
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ACCEPTED MANUSCRIPT Highlights
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Mild heating of graphene oxide in ambient air renders it conductive. Heating in ambient air triggers concomitant redox reactions. The product is reduced graphene oxide with oxygen containing functionalities. A gas sensor with GO reduced at different temperatures, is fabricated. A critical temperature exists for sensor performance optimization.
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S0167-9317(16)30142-3 doi: 10.1016/j.mee.2016.03.030 MEE 10213
To appear in: Received date: Revised date: Accepted date:
15 October 2015 10 March 2016 15 March 2016
Please cite this article as: E. Tegou, G. Pseiropoulos, M.K. Filippidou, S. Chatzandroulis, Low-temperature thermal reduction of graphene oxide films in ambient atmosphere: Infra-red spectroscopic studies and gas sensing applications, (2016), doi: 10.1016/j.mee.2016.03.030
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ACCEPTED MANUSCRIPT Low-temperature thermal reduction of graphene oxide films in ambient atmosphere: infrared spectroscopic studies and gas sensing applications E. Tegou, G. Pseiropoulos, M.K. Filippidou, S. Chatzandroulis
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Institute of Nanoscience and Nanotechnology, NCSR “Demokritos”, Athens, Greece
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e-mail: [email protected]
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Abstract
Low-temperature (≤ 300C) thermal treatment of graphene oxide (GO) films in ambient air is examined. In particular, the role of low to moderate heating temperatures, to the evolution of the
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original functional groups anchored on the GO skeleton, is closely investigated by Fourier transform infra red (FT-IR) spectroscopy. The study shows that, contrary to vacuum or inert
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ambient heating, heating under ambient atmosphere triggers concomitant reduction and oxidation reactions. Hydroxyl and epoxy groups are progressively eliminated, but at the same time newly formed carbonyls appear due to oxidation. Electrical measurements indicate that despite the
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presence of oxygen containing groups in the restored graphene sp2 network, the conductivity
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enhances. The process, therefore, lends itself to the production of conductive reduced GO with increased functionalities suitable for application in gas sensor fabrication. The concept is evaluated
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with a humidity sensor where thermally reduced GO is prepared at different reduction temperatures. The evaluation unveils that a critical reduction temperature exists where sensor sensitivity is optimized.
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Keywords: graphene oxide, thermal reduction, oxidation, gas sensors, infra-red spectroscopy, electrical conductivity
1. INTRODUCTION Graphene is a two-dimensional monolayer of carbon atoms tightly packed into a flat hexagonal structure, similar to a honeycomb lattice possessing innovative mechanical, electrical, thermal, and optical properties. [1] Current research on graphene materials is an exciting field that interfaces chemistry, physics, materials science and engineering. As graphene materials and their derivatives, maintain the unique graphene’s properties in bulk, they have a wide range of applications in various fields such as batteries, [2, 3] organic photovoltaics, [4-10] transistors, [11, 12] biology, [13] hydrogen storage, [14] as field-emission cathodes [15], as well as for the mass production of solution processable chemically exfoliated graphene oxide. [16, 17] In particular, graphene’s twodimensional nature and high electron mobility translate into high sensitivity to surface chemical
ACCEPTED MANUSCRIPT interactions thereby making it an ideal platform for gas sensors. [18] Nevertheless, graphene is difficult to produce and process on a large scale. Among other approaches, one of the promising mass production routes of graphene is through reduction of graphene oxide (GO).[19] GO,[20, 21]
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prepared by the oxidation of graphite and the exfoliation of the generated graphite oxide, can be
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viewed as a graphene layer asymmetrically decorated with oxygen-containing functional groups on the basal plane and the edges. Despite extensive research, GO remains an elusive material with
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hardly obtainable direct structure information. [22] According to the widely accepted LerfKlinowski model, [23] GO contains two kinds of regions: aromatic regions with flat unoxidized benzene rings and wrinkled regions with alicyclic six-membered rings bearing C=C, hydroxyl, and
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ether groups, while GO sheets terminate with hydroxyl and carboxyl groups. In other words, GO is
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an insulating and disordered analogue of the highly conducting crystalline graphene.
GO reduction can be effected by various external stimuli such as heating, [24] chemical treatment, [25], and laser [26] or gamma ray irradiation.[27] It yields reduced GO (rGO) that resembles
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pristine graphene structure and is electrically conductive (due to restoration of graphene sp2
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network). Depending on reduction extent, a partly restored sp2 lattice may be generated while also retaining some oxygen-bearing groups. In this way, rGO can find applications as a graphene
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alternative and provide ad hoc functionalized forms of graphene, for instance in order to optimize rGO response to gas vapors.
Graphene-based gas sensors can detect vapor adsorption down to the single-molecule level, [28]
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with the mechanism stated to be charge transfer induced by adsorption/desorption of molecules (which act as electron donors or acceptors) on the graphene surface, leading to changes in graphene conductance. In recent years, research has aimed to identify and optimize the factors affecting the analytical performance of rGO gas sensors, namely, sensitivity, selectivity and stability. Such factors include voltage activation, [29] film thickness, [30] e-beam radiation, [31, 32] chemical functionalization, [33, 34] nanoparticle doping, [35-37] the type of chemical reduction,[38] and the extent of reduction. [39-40]
In this work, aided by Fourier-transform infra red (FT-IR) spectroscopy, we investigate in a systematic way the extent of GO thermal reduction at low to moderate heating temperatures (90C300C), for gas sensing applications. The reduction is conducted under ambient air conditions, i.e. without high vacuum or any special atmosphere involved. Our findings prove that both oxidation and reduction take place concurrently: hydroxyl and epoxy groups are progressively eliminated, but
ACCEPTED MANUSCRIPT at the same time newly formed oxidation groups appear. The final product is a partly restored graphene sp2 network bearing oxygen containing groups and it is electrically conductive. Furthermore, a humidity sensor based on a mildly thermally treated rGO is fabricated and evaluated
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for different reduction temperatures.
2. EXPERIMENTAL
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2.1 Material
A commercial “Single Layer Graphene Oxide Ethanol Dispersion” was purchased from ACS
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Material® and used as is.
2.2 Characterization of rGO
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GO dispersion was spin-coated on 4” silicon wafer substrates to create uniform films of approx. 500 nm thickness. Then the films were heated on a hot plate at 90C, 120C, 150C, 180C, 200C, and 300C, for 1 h. Film thickness was measured before and after each heating step by a profilometer
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(XP-2, Ambios Technology). The morphology of thermally treated GO films was characterized by
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field emission scanning electron microscopy (FESEM, JSM -7401f, JEOL). Thermal gravimetric analysis (TGA) was performed on a Mettler Toledo TGA/SDTA851e thermogravimetric /
mL/min air flow.
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differential thermal analyzer. GO thermal behavior was examined at 1C/min heating rate and 50
Chemical structure characterization of the films was performed by a FT-IR spectrophotometer
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(Spectrum One, Perkin Elmer). All spectra were recorded from 4000 to 400 cm-1 in transmittance mode with a resolution of 4 cm-1 at 128 scans. Clean silicon wafers were used as background because of their transparency at the mid-IR range. In order to quantify the spectroscopic observations, the transmission measurements were converted into corresponding absorbance data and a polynomial baseline was subtracted from all raw spectra. By dividing by the film thickness of each sample, the spectra were normalized. Finally, peak areas were calculated between relative baseline points.
For the electrical characterization of GO, the dispersion was drop-casted using a micropipette between two gold electrodes, patterned on insulating silicon oxide over silicon substrate with standard lithographic techniques. Then, the samples were heated. Electrical transport measurements of thermally treated GO drops were carried out at room temperature using a two-point probe
ACCEPTED MANUSCRIPT technique (Prober Karl-Suss Micromanipulator 7000 LTE equipped with HP4140B pAmeter/DC voltage source). GO electrical resistance was calculated from the slope of the I-V curves, and then
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converted to conductivity (σ).
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2.3 Sensor fabrication
For the evaluation of sensor efficiency, GO was drop-casted between electrodes and heated at
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120C, 150C, 180C, 200C, and 300C, for 1 h. Different concentrations of analyte vapors were introduced in a small volume (~7 cm3) chamber where relative humidity and temperature were controlled to within 0.1% and 0.1oC, respectively. The device response was investigated by
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measuring electrical resistance changes, when the analyte molecules were adsorbed on the sensing
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element surface.
3. RESULTS AND DISCUSSION 3.1 rGO characterization
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For all samples heated up to 200C, a rough wrinkled sheet-like surface with no obvious
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morphology change was observed (Figure 1), while at 300C surface morphology appeared less wrinkled but “blistered”. During thermal treatment, volatile products (e.g. residual H2O, CO2, CO
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or other small gas molecules) were formed and trapped within GO layers due to the limited nature of diffusion; therefore they created the observed “blisters”. We noticed that upon heating at higher
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temperatures (500C) these molecules were released by tearing the surface.
200°C, 1 h
300°C, 1 h
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(c) Figure 1. (a) and (b) FESEM pictures of rGO films heated at 200°C and 300C for 1 h. At 200°C a rough wrinkled sheet-like morphology is observed while at 300°C the surface is less wrinkled but “blistered”. (c) A typical TGA thermogram of GO showing a significant mass loss due to the
ACCEPTED MANUSCRIPT removal of oxygen containing groups from the basal plane. Remaining film thickness of GO after each thermal step is also presented in the same plot.
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TGA diagram gave additional information about the thermal stability of oxygen-containing groups
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attached to GO. The thickness of GO films remaining after each thermal step was also compared to the TGA results. Both curves presented in Figure 1(c), displayed three discrete stages. Firstly, a
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slight mass loss (~6%) occurred up to 120C, primarily attributed to the elimination of physisorbed and interlamellar water molecules. Secondly, the thermal decomposition of covalently bonded oxygen caused an additional significant mass loss (~31%), occurring up to 200C. Finally, a 12%
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mass loss observed up to 300C was probably due to the removal of more stable oxygen containing functional groups. These results are in good agreement to previous thermograms published in
of GO films by hydroiodic acid. [43]
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literature. [41, 42] Furthermore, 50% film shrinkage has also been reported as a result of reduction
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To probe the course of reactions occurring upon heating in ambient air, we employed FT-IR
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spectroscopy as a monitoring tool of the evolution of GO functional groups. Figure 2a depicts all FT-IR spectra as heating temperature increases. We observe that a critical transition temperature
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exists at 150C where chemical changes have almost completed. Above this critical temperature, the spectral changes are not that pronounced. In particular, the O-H bond stretch vibration (broad band, ranging from approx. 3700 cm-1 to 2600 cm-1), decreases progressively until 150C and then
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it almost disappears. The same observation applies for the weak peak at 856 cm -1, commonly used for the classification of epoxides (off-plane three-member epoxy rings), which decrease concomitantly to the hydroxyl groups. On the other hand, prominent sp2 graphitic domains are constantly generated in GO as proven by the 1578 cm-1 peak increase, [41, 42, ,44, 45] attributed to C=C stretch vibration of non-substituted aromatic bonds. At the same time, the stretch vibrations of the C=O bond (1738 cm-1) increases.
The spectral changes in the C=O region were used to probe the oxidation reaction, as presented in detail in Figure 2b. As the temperature increases, we observe three qualitative characteristics: a) a peak widening, b) a peak shift to higher wavenumbers (it moves from 1725 cm-1 to 1744 cm-1), and c) a new peak appearing at 1837 cm-1 (for temperatures higher than 150C). These observations indicate both intra- or/and inter- molecular anhydride and ester formation. In general, anhydrides are formed between two adjacent carboxyl groups and cause two well-defined absorbance peaks at
ACCEPTED MANUSCRIPT ~1775 cm-1 and at ~1835 cm-1 (the band at 1775 cm-1, overlapped by the initial C=O stretching, causes the peak shift to higher wavenumbers). In addition, newly formed esters between a hydroxyl and a neighboring carboxyl group, may also cause the carbonyl peak shift to higher wavenumbers
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(in general, ester carbonyl bonds stretch at higher wavenumbers than acids, aldehydes and ketones).
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Figure 2. a) FT-IR spectra of GO films heated at various temperatures. Inset: Schematic
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representation of GO. b) Zoom-in of the carbonyl band after each heating step. As the temperature increases, the band gets wider and shifts from 1725 cm-1 to 1744 cm-1. The shoulder at 1835 cm-1
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denotes anhydride formation.
Finally, if we calculate the respective peak areas, we may quantify all the aforementioned qualitative spectral observations. An illustration of such quantification showing the evolution of GO
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functionalities (OH, C=O, and C=C) during thermal reduction is presented in Figure 3. Figure 3 also shows the electrical conductivity enhancement of GO proceeding with temperature. Electrical conductivity enhances with temperature predominantly as a consequence of the elimination of hydroxyl and epoxy groups along with the simultaneous gradual restoration of the graphene sp2 network. The conductivity begins to increase above 90C indicating that a graphene-like structure starts to generate, and exhibits a percolation-type behavior up to 150C. Above this critical temperature and for the tested range of temperatures no further significant conductivity increase is observed.
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Figure 3. Removal of hydroxyl functionalities, formation of carbonyls groups, and simultaneous
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restoration of the sp2 graphene network (C=C), along with electrical conductivity enhancement as a function of heating temperature. At 150°C the rGO structure has been formed: the OH groups have
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decreased, the amount of C=C bonds have increased and the electrical conductivity has enhanced
3.2 Gas sensor response
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sharply.
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In order to probe the sensor performance, five sensors were fabricated and exposed to water vapors. The deposited GO sensitive layer was reduced at 120C, 150C, 180C, 200C and 300C for 1 h. In all cases, a wide range of vapour concentrations (500 – 20,000 ppm) were introduced in the
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chamber interchanged with dry N2 intervals to allow water molecules to desorb and the sensor to recover. In Fig. 4a the normalized response ΔR/Ro(%) of the rGO sensors upon exposure to water vapors is depicted. ΔR/Ro(%) is defined as : Δ
Δ
where Ro is the initial sensor resistance prior to exposure to analyte vapors and R the sensor resistance during testing in the presence of various analyte concentrations. We observe that the rGO sensors respond with varying sensitivity depending on reduction temperature: the sensor with GO reduced at 150C shows higher response, thus higher sensitivity, comparing to the other sensors. The above are illustrated in Figure 4b.
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(a)
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Figure 4. a) Normalized response of the rGO device upon exposure to water vapors, operating at
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room temperature. The GO sensing layer was heated at 120C, 150C, 180C, 200C and 300C for 1 h. b) comparison of all rGO sensor responses at 10,000 ppm H2O: the reduced at 150C sensor
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shows the highest response.
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The above observations may be explained if we take into account the evolution of GO functionalities during heating as evidenced by the FT-IR spectra. If we correlate the sensor response
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to the data shown in Figure 3, we notice that the highest response is achieved once the rGO structure is formed for the first time, which coincides to the exact temperature when the electrical conductivity increases sharply, i.e. 150C. At this temperature, rGO has become conductive enough
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but at the same time retains a small amount of OH groups. On the other hand, the response at 300C is the lowest. As hydroxyl functionalities are eliminated at higher reduction temperatures, the rGO droplet becomes denser. In other words, the distance between the GO layers decreases.[46] Hence, comparing to the initial three-dimensional (3D) GO structure, as reduction temperature increases water molecules penetrate into the resulting inner rGO layers with accelerated difficulty. This reasoning may also explain the increased but noisy sensor response observed at 120C reduction temperature, i.e. low-conductivity structures are likely to present noisy responses while hydrophilic 3D structures interact strongly with water vapors, hence the response is enhanced. Thus, the optimal reduction temperature lies where rGO structure is conductive enough, however still hydrophilic and not too compact. Additional experiments with methanol and ethanol vapors confirmed the higher response observed at 150C. Nevertheless, the sensor response at these vapors was significantly lower (ΔR/R < 2%) than water vapors, since they interact only with the top few layers of the rGO drop. [47]
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4. CONCLUSIONS In summary, the chemical structure of GO films versus mild to moderate heating temperatures (90-
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300C) in ambient air was studied by FT-IR spectroscopy. Thermal treatment gradually removed
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GO hydroxyl and off-the plane epoxy groups, and restored C=C sp2 bonds. At the same time, atmospheric oxygen promoted GO oxidation by forming C=O containing functional groups that
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remained in the final product. All structural changes proceeded drastically up to a critical temperature (150C). In addition, the electrical conductivity enhancement exhibited a percolationtype behavior up to that critical temperature. A humidity rGO sensor was fabricated and evaluated
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for different reduction temperatures. It was revealed that the sensor sensitivity can be optimized through controlling the reduction degree by adjusting the reduction temperature. This study points
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to ways for facilitating the design of graphene materials with functional sites across a range of applications.
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ACKNOWLEDGMENTS
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This work was partially funded by the State Scholarships Foundation (IKY) (Grant number: 22056/13) under the IKY fellowships of excellence for postgraduate studies in Greece-Siemens
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Graphical abstract
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ACCEPTED MANUSCRIPT Highlights
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Mild heating of graphene oxide in ambient air renders it conductive. Heating in ambient air triggers concomitant redox reactions. The product is reduced graphene oxide with oxygen containing functionalities. A gas sensor with GO reduced at different temperatures, is fabricated. A critical temperature exists for sensor performance optimization.
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