Evidence of Ion-Beam Induced Annealing in Graphene Oxide Film

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C: Physical Processes in Nanomaterials and Nanostructures

Evidence of Ion-Beam Induced Annealing in Graphene Oxide Film Using In-Situ X-ray Diffraction and Spectroscopy Techniques Chetna Tyagi, Saif Ahmad Khan, Indra Sulania, Ramcharan Meena, Devesh Kumar Avasthi, and Ambuj Tripathi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10699 • Publication Date (Web): 04 Apr 2018 Downloaded from http://pubs.acs.org on April 4, 2018

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The Journal of Physical Chemistry

Evidence of Ion-Beam Induced Annealing in Graphene Oxide Film using Insitu X-ray Diffraction and Spectroscopy Techniques Chetna Tyagi1, S.A. Khan1, Indra Sulania1, R. Meena1, D.K. Avasthi2, Ambuj Tripathi1* 1

Materials Science Group, Inter University Accelerator Centre, Aruna Asaf Ali Marg, New

Delhi, India-110067 2

Amity Institute of Nanotechnology, Amity University, Noida, Sector-125, Uttar Pradesh, India-

201313. *Email address: [email protected], Telephone: 011-26893955.

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Abstract Ion beam irradiation is one of the methods to tune the properties of Graphene oxide (GO) by modifying the ratio of sp2 and sp3 hybridization. However, the inherent defects present in GO during its synthesis deteriorate its properties (e.g. reduction efficiency) and are difficult to remove. We have earlier demonstrated the annealing of defects in carbon nanostructures (fullerene, CNT and graphene) at lower fluence of swift heavy ion (SHI) irradiation. In the present work, we have studied irradiation of GO film with 120 MeV Au ions at fluences ranging from 1010 to 1013 ions/cm2. In-situ XRD measurements showed the increase in crystallinity of GO film at low fluence. The irradiated samples showed an increase in the intensity of aromatic carbon bonds by Fourier Transform Infrared (FTIR) which indicates the maximization of graphitic regions for lower fluences up to 3×1011 ions/cm2. Higher fluences of ion beam irradiation indicated the presence of carbyne in Raman measurements. Thermal spike simulations were performed to understand the physical processes involved during ion beam irradiation by estimating the radii of core and halo of the tracks formed by SHI.


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Introduction The discovery of graphene has opened a wide door for the carbon materials in the field of electronics, energy storage materials, polymer nanocomposites and transparent electrodes due to its high strength, high thermal and electrical conductivity 1. Monolayer graphene is a zero band gap semi-metal having only sp2 hybridized carbon atoms. However, graphene oxide (GO) is a functionalized form of graphene due to the presence of oxygen-containing functional groups (COOH, -OH, -O-) on edges and basal planes 2. Due to the presence of disrupted sp2 network bonding, GO sheets contain both sp2 and sp3 hybridization 3. As, the stoichiometric ratio is not fixed, its structure is still under debate 4. GO, due to its functional groups, has good dispersibility in most of the organic solvents

5

and also provides a good matrix for nanocomposites 6. Apart

from its own applications, it is an intermediate step for the synthesis of large scale production of graphene which has immense industrial applications nowadays. GO can be tuned to obtain graphene-like properties and its synthesis is also quite easy as compared to pristine graphene. Reduction of GO leads to reduced graphene oxide which is similar in properties to pristine graphene. There are several reports on the reduction methods of GO, using chemical 7, thermal 8 and electrochemical

9

methods. These methods have their own demerits like introduction of

impurities in chemical method, creation of large defects in thermal process and condition of using a conducting substrate in electrochemical reduction. The swift heavy ion (SHI) irradiation is significant where a controlled and impurity free way of obtaining desired modifications is required

10-12

. Reduction in GO can be explained by two mechanisms, either by the de-

attachment of oxygen-containing functional groups from GO sheets leading to an increase in sp2 hybridized carbon atoms, or by annealing of the defects in carbon rings. But, both the mechanisms result in the increase in sp2 domains on expense of sp3 domains in GO sheets. SHI can also be used for the reduction of GO as reported by K. Hareesha et al. 13. K. Hareesha et al. used high fluences (1012 to 1013 ions/cm2) to increase the conductivity of GO indicating its reduction. But, the defects were created during the reduction as evident from Raman microscopy13. Though oxygen-moieties can be removed by a proper reduction route (chemical, thermal or electrochemical), the defects formed during the synthesis (during oxidation of graphite) or after reduction are difficult to anneal by post-treatment methods. Also, the oxygenmoieties attached to the defects and edges are quite difficult to remove rather than those attached with graphitic regions. Generally, these defects in GO, which remain even after reduction, are 3 ACS Paragon Plus Environment


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formed during its synthesis. Therefore, defects play an important role in determining whether the GO sheets would be able to reduce well or not

14

. So, the inherent defects in GO need to be

healed to reduce GO more efficiently 14. To the best of our knowledge, such an effect leading to annealing of even intrinsic defects has not been reported earlier. Our group has studied earlier the ion irradiation effects on carbon nanostructures (C60, C70, fullerene, graphene, CNTs etc.)

15-23

and observed the annealing effect in C60, fullerene, CNTs

and graphene at lower fluences using SHI. Similar phenomenon was observed in the case of polymer (polyvinylidene fluoride)

24

. Therefore, the behavior of GO under swift heavy ions at

lower fluences need to be investigated for the annealing of defects present in GO sheets. The present work focuses on the investigation of swift heavy ions induced physical processes in GO and its stability at higher fluence.

Experimental The commercially procured solutions of Graphene oxide (from Graphene Supermarket, USA) of two different concentrations (5 g/l and 500 mg/l) with ethanol and water as solvents were used. Using low concentration of GO solution, film obtained was not continuous and with high concentration, thick film was obtained. So, these solutions were mixed in an optimized ratio to get continuous good quality GO films for further measurements. 20 µl of this optimized solution was drop casted on SiO2/Si (300 nm SiO2) substrates and allowed to dry at room temperature. Films of ~ 600 nm thickness were obtained as measured by Oxygen-resonance Rutherford backscattering spectrometry (RBS) with energy 3.05 MeV He2+ ions

25

. The GO films were

characterized using D8 Bruker X-ray diffraction (XRD) with Cu Kα as source (at 1.54 Å wavelength) at 40 kV in locked coupled mode in the range of 100 to 160 (2θ). Raman microscopy was performed on GO films using Renishaw Invia Raman microscope (Argon ion laser, 514 nm) with x20 magnification. Argon laser with 5mW power was used in the range of 100 cm-1 to 3200 cm-1 for our studies. The exposure time of laser was 10 seconds for each spectrum. The functional groups analysis was done using Verter 70V Bruker Optik High-resolution Fourier transform infra-red (HR-FTIR) spectroscopy in the scan range of 400 to 3996 cm-1. Topography of the films was studied using Digital Instruments Nanoscope III a atomic force microscope


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(AFM) in the tapping mode. The morphology of films was observed with electrons of energy 25 keV using MIRA II LMH (TESCAN) field emission scanning electron microscope (FE-SEM). In-situ X-ray Diffraction studies were performed on GO sample in materials science beam line using 15 UD Pelletron at Inter University Accelerator Centre, New Delhi. Gold (Au9+) ion beam of energy 120 MeV with different fluences of 3 × 1010, 1 × 1011, 3 × 1011, 1 × 1012, 3 × 1012 and 1 × 1013 ions/cm2 were used for irradiation. The pressure inside the irradiation chamber was about 10-6 mbar and the average current used was 0.5 pnA. Sample of size 1×1 cm2 was used. XRD patterns were recorded at the same position of the sample after each fluence in vacuum. The time calculated for fluence below ~1010 ions/cm2 is less than 1 second and hence it was not feasible to perform irradiation for much lower fluences. Since in-situ XRD experiment was performed on one sample, additional irradiations were performed on identical samples of the same size using identical beam and fluences for other characterizations (Raman microscopy, FTIR spectroscopy, Atomic Force Microscopy and Scanning Electron Microscopy).

Results and Discussion Figure 1 shows the in-situ XRD plots for pristine and irradiated sample at different fluences. A peak at 12.910 in all the spectra is characteristic of (002) plane of GO. Figure 2 shows the variation of intensity of XRD peak with fluences. An increase in the intensity of the characteristic peak at initial fluence of 3×1010 ions/cm2indicates the increase in the crystallinity of the film26. Increase in crystallinity may be attributed to the annealing of GO film for initial fluence. Beyond the fluence of 3×1010 ions/cm2, a decrease in XRD peak intensity is observed which shows that GO tends to amorphize with ion fluence and is completely amorphized at the fluence of 1×1013 ions/cm2.



Figure 1. In-situ XRD plots of pristine and irradiated sample showing the characteristics peak at 12.910 (002) for pristine GO.

Figure 2. Plot showing the intensity of in-situ XRD peaks of pristine and sample irradiated with 120 MeV Au ions at different fluences.

Figure 3 shows the Raman spectra for pristine and irradiated samples. D peak (defect-induced peak) and G peak (graphitic peak) are the characteristics peaks of graphene oxide27 . The G peak


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occurs for all sp2 sites of both chains and rings and hence its intensity is independent of the presence of chains or rings. But, the D peak intensity depends on the presence of hexagonal lattice or opened up lattice 28. D and G peaks are at 1358.66 cm-1 and 1587.84 cm-1 respectively in case of pristine GO sample. Also the magnified part of the figure 3 shows the origin of a peak at ~ 2150 cm-1 at the fluence of 1×1012 ions/cm2. This peak (known as C peak) is due to the presence of some sp hybridized carbon atoms (carbyne) present in the chain structure and is indicated by an additional band at 1900-2300 cm-1 (a broad peak or hump) in Raman spectrum 29. It is reported that these sp carbon bonds form because of the shifting and rearrangement of electrons from the double bonds to triple bonding of carbon chains/rings if the temperature is increased above 2600 K

30

. According to the thermal spike model (explained in next section),

there is a transient rise in temperature (~ thousands of Kelvin) locally in GO and up to the radius of 6.0 nm. This temperature rise up to 2600 K leads to the formation of sp carbon bonds 30.

Figure 3. Raman spectra of pristine and irradiated samples showing D and G peaks. The region between 2000 and 2500 cm-1 is magnified and further magnified showing C peak (due to carbyne) centered at ~2160 cm-1 arising at the fluence of 1×1012 ions/cm2 shown by fitted continuous curves.



The disorder parameter (ID/IG) was determined for pristine and irradiated samples as plotted in Figure 4 (a). For pristine film, disorder parameter is 1.00 which decreased continuously till the highest fluence. The decrease in disorder parameter (ID/IG) in Raman spectra for lower fluences (till 3 × 1011 ions/cm2) is shown in encircled part of Figure 4 (a) which is due to the annealing of defects in GO films. However, the expected increase in ID/IG at higher fluence is not observed. This has been attributed to the fact that a large number of defects might have opened up the ring structure reducing the D peak intensity which further decreases the disorder parameter even at higher fluences. The opening of ring structure of the GO is evident from the peak arising at ~2160 cm-1 31

in the spectra for the samples irradiated at fluence 1x1012 ions/cm2 and observed till the

highest fluence i.e. 1x1013 ions/cm2 as discussed above. The position and full width at half maximum (FWHM) of G peak is plotted in Figure 4 (b) and (c) for different fluences. Encircled region in Figure 4 (b) shows a blue shift in G peak position in the low fluence region (3×1010 ions/cm2 to 3×1011 ions/cm2) which is due to the ion-induced strain in C=C structure resulting in the hardening of phonon modes

32-34

. With the increase in ion fluence, GO film is losing its

aromatic character and hence phonon modes are softening as indicated by the red shift in G peak position. Also, FWHM of G peak attributes to the structural disorder in carbon-based materials. In this case,

FWHM of G peak is decreasing for initial fluence till 3×1011 ions/cm2and

increasing for higher fluence as shown in figure 4 (c). These observations showed that sp2 domains are maximizing for low fluence 32-34.


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Figure 4. Plot of (a) Disorder parameter (b) G peak position (c) FWHM of G peak of GO film irradiated with different fluences of 120 MeV Au ions. Encircled regions are showing the maximization of sp2 domains for lower fluences.

The oxygen containing functional groups in pristine and irradiated samples were identified by Fourier transform infrared spectroscopy as shown in Figure 5. The FTIR showed that the absorption bands at 826.95 cm-1, 1088 cm-1, 1297.88 cm-1 and 1472.30 cm-1 correspond to C-H vibrations (out of plane), C-O alkoxyl stretching, C-O epoxy stretching and C-O carboxyl stretching respectively 35. The band at 1655.96 cm-1 corresponds to C=C aromatic stretching and 9 ACS Paragon Plus Environment


its intensity increased for initial fluences i.e. till 3x 1011 ions/cm2 which indicate the increase in the number of bonds contributing to that absorption band. It explains higher C=C aromatic content for lower fluence. A broad band around 3550 cm-1 indicates the presence of hydroxyl groups due to moisture as GO is hydrophilic in nature. The hydroxyl groups decreased with fluences as intercalated water molecules desorbed from the GO sheets. The carbyne peak (found in Raman spectra) is usually found in 2130 – 2150 cm-1 region and its intensity is usually very weak, if observed at all. Due to very few C-C triple bonds, this peak could not be observed in FTIR spectra 36.

Figure 5. The FTIR spectra showing the presence of oxygen-containing functional groups in pristine GO and their variation with 120 MeV Au-irradiated samples for two regions separately. The encircled part is zoomed in to clearly observe the variation of the C=C aromatic band (at 1655.96 cm-1) with fluence. The topography of the pristine and irradiated samples was studied using Atomic force microscopy as shown is figure 6. In 10 µm ×10 µm scan size, the surface root mean square (r.m.s.) roughness (Rq) of the pristine film is calculated as 41.2 nm. At initial fluence of 3×1010 ions/cm2, the roughness of the film increased to 67.0 nm due to the strain induced in the film 37 during ion-beam irradiation which saturates the dangling bonds present and hence resulted in the 10 ACS Paragon Plus Environment

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folding of the GO sheets as can be seen in figure 6 (a)

38

. With further irradiation (figure 6(b)-

(d)), water molecules intercalated between layers and attached functional groups are getting lost due to high temperature (as evident in FTIR spectra) reducing the roughness of the films to 53.0 nm and 40.4 nm for 3×1011 ions/cm2 and 3×1012 ions/cm2 respectively (shown in 6 figure (b-d)). This is further shown in a plot for an over view (Figure 6 (e)).

Figure 6. AFM images recorded in the scan size of 10 µm ×10 µm for (a) Pristine sample, irradiated with 120 MeV Au ions at fluences of (b) 3×1010 ions/cm2 (c) 3×1011 ions/cm2 and (d) 3×1012 ions/cm2. An overview of the surface roughness varying with fluence is plotted in (e). 11 ACS Paragon Plus Environment


The wide scan SEM micrographs for pristine and irradiated sample with highest fluence of 1×1013 ions/cm2 (given in Figure 7) showed no appreciable change in the morphology of the film after irradiation.

Figure 7. SEM micrographs of (a) Pristine sample (b) Irradiated with 120 MeV Au ions at fluence of 1x1013 ions/cm2 shows no appreciable change in the morphology of films.

Theoretical approach In the present work, interaction of ion beam with the target material is explained by Thermal spike model according to which local energy density deposited by impinging ion along the ion path raises the temperature of electronic subsystem governed by equation (1). The electronic temperature is transferred to lattice via electron phonon coupling represented by equation (2). It causes transient temperature spike in time regime of the order of pico seconds. The space and temperature evolution in a cylindrical geometry is governed by the non-linear differential coupled heat transport equations given below:

1 , … … … … … 1

1 … … … … … … … … … . 2 12 ACS Paragon Plus Environment

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where C, T and K are the specific heat, temperature, and thermal conductivity values for the electronic (denoted by e) and atomic (denoted by a) subsystems. g is the electron−phonon (e−ph) coupling constant and A (r,t) is the initial energy density function. Electronic specific heat, Ce can be considered as constant

10, 39-42

. The initial energy density function depends on the ion

beam energy and hence electronic energy loss (Se). It is defined using two distribution functions, F(r) 43, and Gaussian distribution, G(t), of the delta ray energy deposition in time (t) as:

A(r , t ) = b × S e × G (t ) × F (r )  (t − t 0 )2  = b × S e × exp −  × F (r ) 2 t 0  Where, b is normalization constant and to denotes the time (in pico seconds) required by the electron for attaining thermal equilibrium 40. The normalization constant is chosen in such a way that ∞

∫∫

t =0

r = rm

r =0

A(r , t )2πrdrdt = S e

Here, rm denotes the maximum radius of the ion beam path formed by the ions. In the simulation code, equations 1 and 2 are solved numerically in a cylindrical geometry with space boundary of 300 nm radius. This radius is relatively large as compared to the region affected by an incident ion. The high temperature raised varies along the radial distance from the centre of the track which leads to the simultaneous events: defects annealing and the damage of lattice. The temperature at the centre of ion track (known as track core) is highest and transiently reaches a temperature more than the melting point of the target. The quenching of transiently melted lattice leads to the amorphization of the material in that region. The temperature far from the centre of the ion track (known as track halo) is lower than the melting point but it is higher than the annealing temperature of the lattice. The radii of track halo and core are the parameters that decide the 13 ACS Paragon Plus Environment


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dominance of the two competing phenomena. Larger halo region attributes to the defects annealing for lower fluences, whereas at higher fluences, core region starts overlapping causing an irreversible damage which leads to the amorphization of the target 18. In our case, Au ions of energy 120 MeV were used to bombard on GO film. The energy loss of the ions is primarily due to the in-elastic collisions, known as electronics energy loss (Se) with the target rather than the elastic collisions, known as the nuclear energy loss (Sn). Table 1 represents the different parameters using SRIM code designed for studying ion-beam interaction.

Table1. Parameters obtained using SRIM code for a known target and ion beam energy Ion with

Target (elements/

Electronic energy

Nuclear energy

Projected range of

energy

density)

loss (Se)

loss (Se)

ions in target

Gold (Au)

C, O, H

1.51 keV/ Å

18.73 eV/Å

15.33 µm

120 MeV

~ 2 g/cm3

The radial and temporal distribution of temperature of the target bombarding with SHI can be simulated using in-elastic thermal spike (i-TS) code

44

. Different parameters required for i-TS

simulation are tabulated in Table 2. The incident ion energy is calculated as the ratio of total energy of ion beam to the atomic mass of ion. It is a measured value from Pelletron accurately used as an input parameter in simulation code and remains same for the whole experiment.

Table 2. Parameters for thermal spike simulations estimating the track core and halo radii created in GO by SHI irradiation.

Physical constant / Parameter

Value

Melting point

3873 K

Boiling point

5100 K

18

Electron mean free path

3e-7 cm

45

Thermal conductivity

8.8 W/m-K

46

Latent heat of fusion

1200 J/g

Electronic Specific heat

3.6 J/cm3

Density

~ 2 g/cm2 14 ACS Paragon Plus Environment

Reference

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0.1291 mm2 s-1

Thermal Diffusivity

48

Electronic stopping power of 120 MeV Au in GO 1.539 keV/Å

SRIM code

Incident ion energy

-

0.609 MeV/u

The temporal and radial distribution of the temperature in GO can be simulated and plotted in 3D as shown in figure 8. Different color codes for different temperatures varying with different radial distance from the track centre indicate the sudden rise in temperature near the track centre for very few seconds (

Evidence of Ion-Beam Induced Annealing in Graphene Oxide Film

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