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Graphene Production and Future Use: An Investigation

Into the Materials Application as a Hydrogen Gas Sensor

Justin Nicolosi

Spring 2011

Magna Cum Laude

Bachelor of Science in Chemical Engineering

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Abstract

The history, potential applications, and current production routes of graphene are discussed.

Epitaxially grown graphene layers on a 4H-SiC (0001) sample were used to investigate the materials

application as a hydrogen gas sensor. Real time measurement of the devices current-time curves when

contacted by hydrogen gas at 1% concentration and at various isotherms using a HP 4145 semiconductor

parameter analyzer allowed the activation energy for the hydrogen adsorption reaction to be estimated

via the slope of an Arrhenius plot. Activation energies of .832, .396, and .057 eV were obtained for the

temperature ranges of 30 – 60, 60 – 100, and 100 – 170°C, respectively. The effects that varying

thicknesses of catalytic platinum metal deposits on the devices face had on the electrical resistance

change of the device when contacted by 1 % hydrogen gas were also investigated. An exponential decay

curve was fit to experimental data for the platinum coated devices’ response to hydrogen. The effects of

the silicon carbide substrate’s polarity on the electrical resistance change of the device when contacted

with a variety of hydrogen gas concentrations are shown. The device with silicon polarity shows a

decrease in resistance with adsorbed hydrogen. The device with carbon polarity shows an increase in

resistance with adsorbed hydrogen.

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Introduction and Background

Hydrogen gas is widely used in industry. It is used in the petroleum industry as a reactant to

hydrogenate hydrocarbons, sulfur containing compounds, and nitrogen containing compounds. It is also

used in the production of methane, saturation of unsaturated fats, production of ammonia, and

metallurgical applications (Ramachandran & Menon, 1998). Due to hydrogen’s relatively low lower

flammability limit in air (4% by volume), safety is a primary concern in its utilization. It is desirable to

design devices with responsive hydrogen gas detection capabilities to mitigate potential sources of

ignition at concentrations higher than the lower flammability limit.

Graphene, a monolayer derivative of graphite, is a two dimensional lattice of sp-2 hybridized

carbon atoms arranged in a six member ring honeycomb pattern. Based on its superior electrical

properties and pristine crystallinity, graphene has been suggested for use in tetra-hertz range medical

imaging, conductive composites, liquid crystal displays, field effect transistors, batteries, solar cells,

energy storage devices, microprocessors, and most pertinent to this paper, solid-state gas sensors (Geim

& Novoselov, 1997; “The Graphene Challenge,” 2008). In fact, results from the author’s recent

participation in research conducted at the University of Florida show that platinum coated graphene

devices can be used as a sensitive and repeatable detection mechanism for certain concentrations of

hydrogen gas.

Unknowingly, electron microscopist H. Ferenandez-Moran had almost isolated single layer

graphite (graphene) in 1960. Whilst looking for an electron-beam transparent support membrane,

Fernandez-Moran isolated a stack of graphene sheets ~ fifteen layers thick. However, significant

fascination with graphene did not take hold until the early 1990s, when the discovery of carbon

nanotubes (cylindrically rolled graphene sheets) and fullerenes (hollow spheres, ellipsoids, or tubes of

graphene) were discovered. Many attempts at segregating a graphene monolayer subsequently failed

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(Soldano, Mahmood, & Dujardan, 2010). It was thought impossible to segregate a monolayer sample of

graphene until 2004, when Novosolev et. al (2004) announced that they had studied the electric field

effect in several multilayer and monolayer samples of graphene isolated through the use of the simple

mechanism of repeatedly cleaving highly oriented pyrolitic graphite with standard sticky tape.

Since Novosolev’s group isolated monolayer sheets of graphene by mechanical exfoliation,

several different approaches have been studied for the production of the material. Claire et al. (2004)

have shown a route to three layer thick epitaxial graphene production through the thermal decomposition

of carbides by the heating of SiC for 1-20 minutes at temperatures of 1250 – 1450 °C while under

vacuum. Strupinsky et al. (2011) have deposited monolayer thick epitaxial graphene sheets onto SiC

sheets by CVD with propane as the carbon precursor. Other laboratories have segregated monolayer

functionalized graphene oxide sheets by the complete oxidation of naturally occurring graphite flakes in

a solution of nitric acid, sulfuric acid, and potassium chlorate followed by immediate heating to 1050 °C

(Schniepp et al., 2006). Since the sheets created from oxidation are functionalized with oxidation

products (they contain phenol, carbonyl, and epoxy moieties), the graphene oxide sheets must be

chemically reduced to retain the specific electrical, mechanical, and optical properties of graphene

(Soldano et al., 2010). Gilje, Han, M. Wang, K. Wang, & Kaner (2007) have suggested a pathway for

reduction of graphite oxide first by dispersion of graphite oxide in mixtures of water and nonaqueous

solvents, followed by the spray coating of a heated Si/SiO2 substrate and subsequent reduction with

hydrazine vapor. The chemical reduction pathway does not always yield high quality graphene, because

sometimes the reduction step is incomplete. The wide variety of approaches to produce graphene show

scientists’ dedication in finding a commercially acceptable production pathway and affirm that

widespread production of graphene for potential applications such as gas sensors is on the horizon.

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Several research groups have investigated the gas sensing capability of graphene and graphitic

materials. Because of graphene’s high surface area and planar orientation, the entire volume is exposed

to the ambient, and adsorption sites are highly accessible to adsorbates. Both high surface area and

sensitive electronic properties make graphene ideally suited for use as a gas sensor. Schedin et al. (2007)

studied the resistivity changes of micromechanically cleaved graphene when exposed to the adsorbates

NO2, I2, NH3, H2O, ethanol, and CO, and concluded that the devices had sensitivities even down to

individual molecules. The group showed that the resistivity change was dependent on the type of gas,

and that the sign of change was relient on whether the adsorbate was an electron donor (NH3, CO) or an

electron acceptor (NO2, H2O). Since conductivity is proportional to charge carrier mobility and density,

it is apparent that the adsorbed gas molecules influenced one or both of these properties. Leenaerts,

Partoens, & Peeters (2008) studied the adsorption of H2O, NH3, CO, NO2, and NO on graphene, and

reported that NH3, CO, and H2O can be sensed at the ppb level. In addition to the sensitivity analysis,

the group also showed that the adsorption energies of gas molecules were dependent on both the

orientation and position of the gas molecule relative to the graphene surface. Arsat et al. (2009) used the

hydrazine reduced graphene oxide method previously discussed to produce graphene and detect H2 and

CO.

The rest of this paper will report and discuss the response of multilayer epitaxial graphene on a

silicon carbide substrate to hydrogen gas. Various focused studies were conducted at the University of

Florida and will be reported in this paper. The first of these studies utilized wide range temperature

measurements of the device’s response to 1% hydrogen gas to calculate an activation energy for the

hydrogen adsorption reaction. The second of the studies took an in depth look at the effect that coatings

of various platinum thicknesses had on the response of the device to hydrogen gas. The final study

investigates the effect that changing the polarity of the silicon carbide substrate has on the detection of a

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large range of hydrogen gas concentrations. The materials and methods used in the collection of data for

each study will be discussed along with relevant data and calculations.

Hydrogen Adsorption Energy Estimation by Arrhenius Plot

Materials and Methods

An Aixtron VP508 hot-wall reactor was used to epitaxially grow graphene layers on a Si-polarity

4H-SiC (0001) semi-insulating sample by chemical vapor deposition, a process which used propane as

the carbon precursor. Prior to graphene growth, the SiC substrate was etched in a H2-C3H8 mixture at

1600 °C and a pressure of 100 mbar, in order to obtain mono-atomic terraces on the substrate surface.

For an in-detail growth description as well as the characterization techniques used, the reader is referred

to the article by Strupinski et al. (2011). Krupka and Strupinski (2010) measured the conductivity of

such graphene layers using post-dielectric resonators as 5 x 105 – 5 x 106 S/m. The sample had a surface

root mean square roughness of 1.827Ǻ. Ti/Au metal contacts were deposited on the sample using an e-

beam evaporator with a hexagonally shaped TEM grid. After the metal contacts were formed, a 5 nm Pt

film was deposited to function as a catalyst for hydrogen cracking. A four point probe was used to find a

sheet conductivity of 1000 ohms/square for a similarly deposited platinum layer on a glass slide. The

device was then wire bonded for electrical connection. The experiments were conducted in a sealed

quartz tube surrounded by a tube furnace for temperature manipulation. The sample was also electrically

connected to a HP 4145 semiconductor parameter analyzer for data output. The electrical resistivity

change of the device vs. time was recorded while the sample was subjected to a constant external bias of

.025 V, and an environment containing 1% H2 and 99% N2. Molar flow of the mixture was manipulated

through use of mass flow controllers (Chu et al., 2011a).

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Results and Discussion

The current-voltage plot of the device with and without platinum deposits is shown in

figure 1.

Figure 1: Current vs. Voltage for the Device. Reproduced with permission from Chu et al. (2011a).

The linear plot is evidence that the device possesses Ohmic contact characteristics. This plot also shows

a large increase in current for the device with platinum deposits. Giovannetti, Khomyakov, Brocks,

Karpan, van den Brink, & Kelly (2008) showed that the interaction between graphene and platinum

shifts the Fermi level of free standing graphene downwards. This change means that holes are donated

by the platinum and graphene becomes p-type doped. This donation of holes to graphene can give

explanation for the increased conductivity observed when platinum deposits are added.

It is well known that platinum can act as a catalyst for reactions involving hydrogen. In the case

of graphene, the platinum serves to crack the hydrogen molecule, allowing for easier diffusion to the

graphene/Pt surface boundary. In order to accurately represent the response of the platinum-coated

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graphene device, the contribution to the resistance change that the platinum itself makes must be taken

into account. Figure 2 shows a comparison between a sample of platinum on graphene and a sample of

platinum on glass at a temperature of 175 °C and .025 V bias when contacted with 1% hydrogen gas.

The chart plots the change in resistance over the initial resistance in percent (y-axis) vs. time.

Figure 2: Comparison of platinum on graphene with platinum on glass when contacted with 1% H2.

Reproduced with permission from Chu et al. (2011a).

The sample was contacted with pure nitrogen gas for the first 250 seconds, and 1% hydrogen thereafter.

The platinum along with graphene showed a larger change in resistance as well as increased sensitivity

at the onset of hydrogen contact.

Figure 3 shows the intrinsic response of the graphene device without platinum at various

temperatures and figure 4 shows the temperature vs. current change plot. The graphs shows that the

resistance of the device is decreased (current is increased), and that the current change is temperature

dependent. The measurements taken at higher temperatures show larger resistance decreases than the

measurements taken at lower temperatures.

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Figure 3: Response of graphene without platinum at various temperatures when contacted with 1% H2.

Figure 4: Temperature vs. Current Change for the graphene device without platinum.

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Measurements for the platinum device when contacted with 1% hydrogen were taken with the

aim of obtaining an activation energy for hydrogen detection. The percent change in resistance of the

device vs. temperature displays sigmoidal behavior and is plotted in figure 5.

Figure 5: Resistance Change vs. Temperature for the device with Platinum.

Reproduced with permission from Chu et al. (2011a).

To obtain the activation energy, an Arrhenius plot of the form

ln k=ln A−Ea

R ( 1T ) (1)

must be obtained. To this aim, the natural log of the percent change in resistance was plotted against 1/T

at varying temperatures. The result is shown in figure 6, with three accompanying least squares lines fit

for the temperature ranges of 30 – 60, 60 – 100, and 100 – 170 °C.

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Figure 6: Arrhenius Plot for the Device with Platinum. Reproduced with permission from Chu et al. (2011a).

From the slopes obtained for the least squares fit lines, we obtain activation energies for the hydrogen

adsorption reaction of .832, .396, and .057 eV for the temperature ranges of 30 – 60, 60 – 100, and 100 –

170 °C, respectively (Chu et al., 2011a). Ivanovskaya, Zobelli, Teillet-Billy, Rougeau, Sidis, & Briddon

(2010) report that literature lists binding energies in the range of .47 eV to 1.44 eV, with most values in

the .6 eV to .85 eV range. The first calculated value is within the published range of values, however the

values calculated at higher temperatures are below the reported range. The decrease in activation energy

observed at higher temperatures could possibly be explained by the greater translational motion

observed in gases at increased temperature. The increased translational motion may have possibly

increased the local hydrogen concentration at the surface of the device. Boukhvalov, Katsnelson, &

Lichtenstein (2008) showed that the activation energy of chemisorbed hydrogen on graphene was

lowered at high relative concentrations for certain spacial configurations.

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Platinum Thicknesses Effect on Hydrogen Gas Detection by Platinum Coated Graphene

Materials and Methods

The CVD growth technique, Ti/Au contact addition, and etching production methods used for

the hydrogen detection study previously discussed were preserved and the reader is referred to the

previous materials and methods section. The sample utilized for measurements possessed a root mean

square roughness of 3.34 Ǻ. After metal contact addition, Pt was deposited by e-beam evaporation to a

thickness of 1 nm or 4nm. The Ti/Au contacts and platinum deposits were also placed on glass slides, to

measure the response of the platinum film alone. The device was then wire bonded for electrical

connection. The experiments were conducted in a sealed quartz tube surrounded by a tube furnace for

temperature manipulation. The sample was also electrically connected to a HP 4145 semiconductor

property analyzer for data output (Chu et al., 2011b).

For the measurements, pure N2 contacted the samples for 200 s followed by the introduction of

1% hydrogen gas. The total flow rate of the gases was controlled by mass flow controllers and remained

at a constant 300 sccm (Chu et al., 2011b).

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Results and Discussion

The current-voltage plots for both devices are shown in figures 7 and 8.

Figure 7: Current-voltage plot for 1 nm platinum deposit devices. Reproduced with permission from Chu et al. (2011b).

Figure 8: Current-voltage plot for 4nm platinum deposit devices. Reproduced with permission from Chu et al. (2011b).

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Once again, because of the linear fashion of these curves, the devices exhibit Ohmic contact

characteristics. As the graphs display, the conductivities of both platinum coated graphene devices were

higher than each of the platinum and graphene samples alone.

Figure 9 shows the response of reference platinum sensors placed on glass when contacted with

both N2 and 1 % hydrogen gas.

Figure 9: Percent resistance change of reference devices. Reproduced with permission from Chu et al. (2011b).

The glass with 1 nm platinum thickness had no apparent response to the hydrogen. In addition, the 4 nm

device showed limited sensitivity with the resistance change being only about 1%.

The response of the devices to 1 % hydrogen gas for the 4 nm platinum thickness on glass, 4 nm

platinum on graphene, and 1 nm platinum on graphene are shown in figure 10. The 4 nm platinum on

glass, although previously shown to have the largest response, did not show a response on this scale.

This chart does show that the presence of graphene increases the sensitivity of platinum films to

hydrogen adsorption. This is readily observable by the ~ 15 % difference in resistivity reduction

between the 4 nm platinum on glass and 4 nm platinum on graphene.

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Figure 10: Response of various devices to hydrogen gas. Reproduced with permission from Chu et al. (2011b).

Also apparent in figure 10 is the ~65 % greater reduction in resistance of the 1 nm platinum film on

graphene when compared with the 4 nm platinum film on graphene. Another noteworthy point about the

graph is the time it took for the sensor to reach a value close to the value at the end of the exposure. For

the 4 nm devices, it took only about 25 seconds to reach about 50% of the total steady state resistance

change. For the 1 nm device it took about 60 seconds to reach 50% of the total steady state resistance

change. Although the response time for the 1 nm device was almost double the 4 nm device, it showed

much larger resistance change. This shows that the sensitivity and time required for a representative

value may be directly correlated to the thickness of the platinum film (Chu et al., 2011b).

The green lines in figure 10 show the values of the resistance change predicted by curves fit to

the experimental data. A second order exponential function (eqn. 2) was used to fit the data.

ΔRRo

=−a1 [1−exp(−t /τ1 )]−a2 [1−exp(−t /τ2 )] (2)

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where τ1, τ 2,a1, and a2 are constants that are dependent on which device’s response you are modeling

(Chu et al., 2011b). As can be seen, the model fits the experimental data almost “to a T.” The parameters

in the equation are shown below in table 1.

Sample a1 a2 τ1 τ2

4nm Pt on glass 0.004 0.008 321 14.34nm Pt on graphene 0.051 0.125 662 15.91nm Pt on graphene 0.143 0.77 2130 58.4

Table 1: Parameters for use in Eqn. 2. Reproduced with permission from Chu et al. (2011b).

Effect of Silicon Carbide Polarity on Graphene’s Response to Hydrogen Gas

Although not discussed in great detail, the research group did investigate the effect that the face

polarity of the silicon carbide substrate had on measurements. All prior measurements discussed

previously were conducted with a Si-polarity SiC substrate. The measurements discussed next were

isothermally (25 °C) collected for both a Si-polarity and C-polarity substrate, across a range of hydrogen

concentrations from 50 ppm – 20 % H2. Figure 11 shows the response of a C-polarity graphene device

when contacted with various concentrations of hydrogen gas. The interesting quality about the response

of this device is that it has an opposite response than the Silicon polarity graphene devices. That is, when

a Si-polarity device is contacted with hydrogen gas, the resistance is decreased and the current

increased. In contrast, the C-polarity device shows an increase in resistance and a corresponding

decrease in current. Also, a heightened response is obtained for higher concentrations of hydrogen gas.

However, after a certain point, the increase in resistance change for consecutive concentrations seems to

level off. This is probably due to a saturation of the device with adsorbed hydrogen.

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Figure 11: Resistance change of the C-polarity device when contacted with various hydrogen concentrations.

For comparison, the graphs of the Si-polarity devices response when contacted with the entire spectrum

of hydrogen concentrations shown in the chart above are shown in figures 12 and 13.

Figure 12: Resistance change of the Si-polarity device when contacted with various hydrogen concentrations.

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Figure 13: Resistance change of the Si-polarity device when contacted with various hydrogen concentrations.

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Conclusion

As hydrogen is continually used in industrial applications, graphene based devices have shown

promise for potential application as hydrogen gas sensors. The various aspects of graphene device

construction discussed in this paper provide a small piece to the puzzle that is the formulation of an ideal

graphene based hydrogen sensor. From the Arrhenius plot for the platinum coated graphene device,

activation energies for the hydrogen adsorption reaction of .832, .396, and .057 eV were obtained for the

temperature ranges of 30 – 60, 60 – 100, and 100 – 170°C respectively. With further investigation, the

determination of activation energies for the hydrogen adsorption reaction on various devices could

provide temperature ranges at which the use of the device is applicable. In the study of the platinum

thicknesses effect, the device with the 1 nm platinum deposit showed higher sensitivity and slower

response time when compared to the device with the 4 nm deposit. Future determination of the ideal

catalytic platinum metal deposition thickness on the face of the graphene device could provide peak

sensitivity and response time. A change in the polarity of the substrate causes an inversion in response.

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