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Swift Heavy Ion Induced Optical and Electronic Modifications of Graphene-TiO Nanocomposites 2

Mukesh Mishra, Florian Meinerzhagen, Marika Y. Schleberger, Dinakar Kanjilal, and Tanuja Mohanty J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b07297 • Publication Date (Web): 17 Aug 2015 Downloaded from http://pubs.acs.org on August 24, 2015

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The Journal of Physical Chemistry

Swift Heavy Ion Induced Optical and Electronic Modifications of Graphene-TiO2 Nanocomposites M. Mishra,1 F. Meinerzhagen,2 M. Schleberger,2 D. Kanjilal3 and T. Mohanty1* 1

School of Physical Sciences, Jawaharlal Nehru University, New Delhi, 110067, India

2

Universität Duisburg-Essen, Fakultät für Physik and CENIDE, 47057 Duisburg, Germany

3

Inter University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi, 110067, India

ABSTRACT The effect of swift heavy ions irradiation on optical and electronic properties of chemically synthesized graphene-TiO2 nanocomposites is presented. Modification of surface properties of these nanocomposites by irradiation with three different ions and with varying fluence was analyzed by Raman spectroscopy, transmission scanning electron and scanning Kelvin probe microscopy techniques. Raman spectra of irradiated samples exhibit systematic changes in the characteristic peaks of both graphene and TiO2. The nanocrystallite dimension calculated from Raman peak intensity decreases with fluence indicating the occurrence of peripheral fragmentation. Furthermore, measurement of the surface contact potential difference using scanning Kelvin probe reveals that the work function of graphene-titanium dioxide nanocomposites can be effectively increased by more than 1 eV.

KEYWORDS: Graphene sheets, Titanium dioxide nanoparticles, Swift heavy ion irradiation, Raman spectra, Surface contact potential difference.

Address for correspondence: Dr. Tanuja Mohanty School of Physical Sciences Jawaharlal Nehru University New Delhi-110 067, India Tel: 91-11-26738802 Fax: 91-11-26741837 E-mail: [email protected], [email protected]

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INTRODUCTION Graphene is a two-dimensional single layer crystalline sheet of hexagonally bonded sp2 carbon atoms.1-3 With its remarkable properties such as high mobility of charge carriers, high thermal conductivity, and superior mechanical strength attracts the attention of researchers worldwide.1-8 Lots of efforts have been made to explore graphene in the form of nanocomposites as an energy material for advanced energy-conversion devices and energystorage devices.9-17 Chemically exfoliated graphene possesses an extremely high specific surface area (~2600 m2/g) and a large number of reactive edges that help to easily functionalize it and form composites with other nanomaterials.18-20 Active edges of graphene exhibit excellent electron transfer properties, rendering graphene a very good candidate for many functional devices.13,21 For electronic applications of graphene, it is crucial to find a way to alter the type and concentration of the charge carriers and band gap. Several approaches have been attempted to achieve controlled doping in graphene to modify type and concentration of the charge carriers and introduce a band gap. One such method is of chemical doping using adsorbents, which can be realized by the deposition of molecules or coating on top of graphene sheets21-23 or as demonstrated in this work, by preparing nanocomposites consisting of graphene and nanoparticles. In scenarios currently discussed such graphene-TiO2 nanocomposites appear very promising because of their extensive use in the field of solar cells and Li-ion batteries, photocatalysis, enhanced H2 production activity and water splitting.10-16 In this type of nanocomposites, graphene networks provide efficient pathways for electron transfer, while TiO2 nanoparticles prevent the restacking of the graphene nanosheets, resulting in an improvement in both electric conductivity and specific capacity. Furthermore, the electronic interaction and charge equilibration between graphene and TiO2 is expected to shift the Fermi level and decrease the conduction band potential leading to an enhancement of photocatalytic activity similar to that observed in case of graphene-Bi2WO6 composites.13-17 The shifting of the Fermi level which corresponds to a changes of the work function or surface potential of the materials considered here can be tuned in a systematic manner using swift heavy ion (SHI) irradiation.23,24 In addition, defects generated due to SHI irradiation can also be used to manipulate the work function of a given material. Although many reports exist on properties and application of G-TiO2, SHI induced effect in this system have not yet been investigated. This work presents investigation of SHI induced modification on optical as well as in surface electronic properties of G-TiO2 composites.


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In this work, graphene sheets of superior quality have been synthesized using the chemical exfoliation route, which involves ultrasonication of graphite flakes in the organic solvent N-Methyl-2-pyrrolidone (NMP) followed by a centrifugation process.19 The synthesized graphene sheets, were functionalized with titanium dioxide nanoparticles (TiO2 NPs). These thin films of nanocomposites of graphene-titanium dioxide (G-TiO2) were characterized initially by different spectroscopic techniques such as transmission electron microscope (TEM),25,26 field emission scanning electron microscope (FE-SEM) for surface morphology analysis,19-21 scanning Kelvin probe microscope (SKPM) for work function measurement22-24 and Raman spectroscopy to analyze its optical properties27,28, see Methods section for details. The surface and optical properties of G-TiO2 nanocomposites are modified by introduction of defects in a controllable manner using swift heavy ion irradiation (SHI) with different kinetic energy (100 MeV Ag, 70 MeV Si, and 40 MeV Ni). Swift heavy ions, while traversing through matter, lose their energy mainly by creating electronic excitation as well as ionization of the atoms in the material through inelastic collisions.23,27-29 Dense electronic excitation induced by electronic stopping (Se given in keV/nm) of SHIs during its passage through the material may create localized defects and disordered regions in the nanocomposites. The energy stored in electronic excitations is transferred rapidly to the phonons through electron-phonon relaxation in a period less than a picosecond. According to thermal spike model, these dense electronic excitations may lead to rise in local temperature of the film to more than a thousand degree, within a narrow cylindrical zone of a few nanometers around the ion path for time durations of picosecond.29,30 Swift heavy ion beams with Se > 2 keV/nm) are expected to change the surface potential of the system, which will significantly affect the conducting properties of the material.24,28 The SRIM-2013 software package was used to calculate the projectile range, electronic „Se‟ and nuclear „Sn‟ energy loss of ions by using the input data for graphite for graphene, Ti, and O for TiO2 NPs with a target density of 2.41 g/cm3.31 The values of electronic stopping power „Se‟ in G-TiO2 nanocomposites calculated from SRIM-2013 are ~ 2.8, 7.2, and 11.7 keV/nm for 70 MeV Si, 40 MeV Ni, and 100 MeV Ag ions respectively.31 In addition to inelastic collision induced electronic energy loss (Se), SHI also loose energy by elastic collision through nuclear energy loss (Sn), but at these high kinetic energies the electronic stopping power clearly dominates over the nuclear stopping power. The values of nuclear stopping power „Sn‟ in G-TiO2 nanocomposites were ~ 0.26x10-2, 2.8x10-2, and 5.7x10-2 keV/nm for 70 MeV Si, 40 MeV Ni, and 100 MeV Ag ions, respectively. The values for Se, Sn and the Se/Sn ratio for all ions are given in Table 1. 3 ACS Paragon Plus Environment


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Table 1: The values for Se, Sn and Se/Sn ratio for 100 MeV Ag ions, 40 Me1V Ni ions and 70 MeV Si ions in G-TiO2 thin films. 100 MeV Ag ions

40 MeV Ni ions

70 MeV Si ions

Se (keV/nm)

11.7

7.2

2.8

Sn (keV/nm)

5.7x10-2

2.8x10-2

0.26x10-2

Se/ Sn ratio

2.05x102

2.57x102

10.77x102

EXPERIMENTAL SECTION Synthesis of Graphene by liquid phase exfoliation method Graphene is synthesized by using direct solution phase exfoliation of graphite layers in the presence of NMP solvent. The interaction between the graphite and organic NMP solvent provides the sufficient energy required to make possible the process of exfoliation of graphite layers in organic solvent.19 In this liquid phase exfoliation process of graphite layers, graphite flakes (1 g, Alfa Aesar, 99.8 %, 325 mesh) were dispersed in the NMP (200 ml, Sigma Aldrich 99.5 %) solvent and sonicated in sonic bath (PCi analytics, 6.5 L) for 72 hours. After the sonication process, the resultant dispersion of graphite in NMP was centrifuged at 4000 rpm for 1 hour. The upper half of the dispersion was separated with the help of pipette. The final pipetted part of dispersion contains the resulting graphene sheets dispersed in NMP solvent.

Synthesis of G-TiO2 nanocomposites by sol gel method & irradiation conditions G-TiO2 nanocomposites are synthesized in-house by slowly adding TiO2 NPs into chemically exfoliated graphene with continuous stirring and then ultrasonicated for 30 min to prepare a homogeneous G-TiO2 nanocomposite sol.26 G-TiO2 thin films were grown on cleaned Si-substrate by spin coating from synthesized sol of G-TiO2. Initially thin films were dried at 100 ˚C for 1 hour; subsequently the samples were annealed at 300 ˚C for 1 hour. To observe the effect of SHI irradiation, these G-TiO2 nanocomposites thin films were irradiated with 100 MeV Ag ions and 70 MeV Si at five fluences (5x1011, 1x1012, 5x1012, 1x1013 and 5x1013 ions/cm2) using the 16 MV Pelletron accelerator at IUAC, New Delhi. GTiO2 nanocomposites thin films were irradiated with 40 MeV Ni ions at fluence 5x1011, 5x1012 and 1x1013 ions/cm2 using the accelerator facility at GANIL, Caen, France.


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Micro-Raman spectroscopy Pristine and SHI irradiated G-TiO2 nanocomposite thin films were characterized by micro-Raman spectroscopy (Renishaw inVia Raman microscope setup) using a green laser (wavelength 514 nm) with a 1 μm laser spot. It is well known that micro-Raman spectroscopy is a suitable tool to characterize graphene and its nanocomposites. Raman spectra of graphene consist of the characteristic peaks D (1350 cm-1), G (1585 cm-1) and 2D (2700 cm-1). The G band at 1585 cm-1 corresponds to an in-plane phonon vibration in sp2-bonded carbon, where opposite atoms vibrate with respect to each other. The D-Peak and the 2D-Peak are both double-resonance Raman process.32 In the case of the D peak, the two scattering processes consist of one elastic scattering event by defects of the crystal and one inelastic scattering event by emitting or absorbing a phonon. As a result of this the intensity of the D peak is affected by the defect density, disorder and edge chirality.33,34 In the case of the 2D-band, both processes are inelastic scattering events and two in-plane phonons are involved.33 Further, Raman spectra of G-TiO2 nanocomposite thin films show additional Raman active peaks of TiO2 at ~153 cm-1, attributed to the main Eg anatase vibration mode. Other vibration peaks at 400 cm–1 (B1g), 517 cm–1 (A1g), and 640 cm–1 (Eg), are characteristic peaks of anatase TiO2.27

Imaging techniques Characterization of G-TiO2 using transmission electron microscopy (TEM, JEOL 2100F) with an operating voltage of 200 kV was used to study the surface morphology and field emission scanning electron microscopy (FE-SEM, MIRA II LMH from TESCAN) was used for surface analysis. To study the effect of swift heavy ion irradiation on the surface electronic properties of these films, surface contact potential difference (CPD) values of GTiO2 nanocomposites are measured by scanning Kelvin probe microscopy (SKPM). The measured CPD value of irradiated and pristine films can be used to estimate the Fermi level shift.24,35 SKPM measures the surface contact surface potential difference (CPD) based on a noncontact atomic force microscope, which reveals the topography and the work function (φ) mapping of sample surface simultaneously.35 In general, scanning Kelvin probe microscopic setup consists of a vibrating gold tip as reference probe, integrated amplifier, Motorized Scanning X, Y, Z stages, Digital Control Unit. The working principle of SKPM setup is based on the vibrating parallel plate capacitor principle and it is used for investigation of surface morphological and electronic properties of thin films. According to principle of vibrating parallel plate capacitor, when two metallic plates are in electrically contact to each 5 ACS Paragon Plus Environment


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other, the electrons flow from higher work function (WF) value to lower work function value of metallic plate, until their work function value equalize. This flow of electrons from higher to lower WF value creates a potential difference between two metallic plates due to the creation of equal and opposite charge; this potential difference is known as contact potential difference (CPD).35 Contact potential difference between the tip and sample surface forming parallel plate capacitor is defined as,

VCPD 

( -  ) tip sample e

(1)

Where φ denotes the work function and e is the elementary charge. The value of work function „φ‟ for each sample of SHI irradiated G-TiO2 thin films is measured by SKPM using a gold tip as reference surface (φAu~5.1 eV) under ambient environment. CPD mapping of thin film was performed by SKPM set up. During CPD measurement process, gold tip is allowed to approach closer to the sample surface and form parallel plate capacitor configuration between gold tip and sample surface. When these two parallel plates gold probe and sample surface are electrically connected, flow of charge carriers started from higher WF to lower WF until an equilibrium stated is not achieved between them by creating a potential gradient to stop such type of motion of carriers. Further to compensate such potential gradient equal and opposite external potential is applied. This external potential is measured by SKPM setup to find out the surface potential difference between gold probe and sample surface. The obtained CPD mapping thus shows the distribution of local work function. The values are estimated to be correct within 10 %. This value is given as an error bar in the corresponding figures.

RESULTS AND DISCUSSION Transmission electron microscope (TEM) images of G-TiO2 provide the distribution of TiO2 NPs and graphene sheets.25,26 From Figure 1a, one can see that in fact the TiO2 nanoparticles are non-homogeneously distributed among graphene sheets (graphene edges are marked). High resolution transmission electron microscope (HRTEM) images show that the TiO2 nanoparticles (~15 nm) are crystalline in nature with an interlayer spacing of ~ 0.33 nm and it‟s also seen that there is no sign of wrapping of graphene sheets over TiO2 NPs (see Figure 1b). In addition, FE-SEM images at the resolution limit of 200 nm revealed that the graphene sheets are non-homogenously heavily covered with TiO2 nanoparticles (Supporting Information Figure S1).


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Figure 1: (a) TEM (scale bar 20nm) and (b) HRTEM (scale bar 1nm) images of the G-TiO2 nanocomposite.

Raman spectra from our graphene samples were taken which show the characteristic features of multilayer graphene (see Figure 2a).36,37 The width of the 2D peak can be fitted with four Lorentzian peaks corresponding to few layer (less than 5 layers) graphene (see inset of Figure 2a). Spectra taken from the G-TiO2 nanocomposite show a distinct broadening of the 2D peak of graphene from a full width of half maximum (FWHM) of ~ 78 cm-1 to FWHM ~ 85 cm-1. This may be due to strain stemming from the interaction between graphene and the TiO2 nanoparticles.38 To explore the effect of SHI irradiation on surfaceelectronic and optical properties of the G-TiO2 nanocomposites, measurements like Raman, FE-SEM and surface contact potential difference (CPD) mappings were carried out on both pristine and SHI irradiated samples.

Figure 2: Raman spectrum of (a) graphene (inset shows a Lorentzian fitting of the 2D peak of graphene) and (b) G-TiO2 nanocomposite thin films.



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We begin by presenting the Raman spectra obtained from the pristine and ionirradiated G-TiO2 thin films see Figure 3 (a-c). For all ions, there occurs a systematic variation in the intensity of the D peak (at ~1350 cm-1) and G peak (at ~1585 cm-1) with ion fluence (Figure 3).

Figure 3: Raman spectra of G-TiO2 nanocomposites irradiated with (a) 100 MeV Ag ions (b) 40 MeV Ni ions and (c) 70 MeV Si ions with different fluences. For further analysis, the variation of the intensity of the D peak at ~1350 cm-1 with respect to the intensity of the G peak at ~1585 cm-1 and the intensity of 2D peak (appearing at ~2700 cm-1) with respect to the intensity of the G peak has been extracted from the Raman data. The values of the ID/IG and I2D/IG ratios of SHI irradiated G-TiO2 composites are given in Table 2. These intensity ratios ID/IG can be used to calculate the crystallite size (La) of graphene flakes.39 The change in the crystallite size of the graphene flakes in SHI irradiated G-TiO2 nanocomposite can then be estimated from the variation of ID/IG ratio using the Tuinstra-Koenig relation39 (2)

ID C(λ) = IG La

Where C (λ) = (2.4 x 10-10 nm-3) ∙ λ4. Thus, for λ = 514 nm, La = 16.752 nm / (ID/IG) corresponds to the crystallite size of the graphene flakes. The values of La for the different SHI irradiated G-TiO2 composite are also given in Table 2.


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Table 2: ID/IG, La and I2D/IG ratio of SHI irradiated G-TiO2 nanocomposites for different ion fluences. Fluence

Ag

(ions/cm2) ID/IG

Ni La

I2D/IG

Si

ID/IG

La

(nm)

ID/IG

(nm)

La

I2D/IG

(nm)

23.36

0.75

0.717

23.36

0.75

0.717

23.36

0.75

0.8123 20.62

0.60

0.76

22.04

0.67

0.728

23.01

0.71

1 x1012

0.83

20.18

0.49

--

--

--

0.726

23.07

0.68

5 x1012

0.91

18.41

0.335

0.86

19.48

0.55

0.731

22.92

0.65

1 x1013

0.95

17.63

0.2

0.91

18.41

0.51

0.741

22.61

0.62

5x1013

0.955

17.54

0.1

--

--

--

0.76

22.04

0.6

Pristine 5 x10

11

0.717

I2D/IG

The data shows that ID/IG increases with fluence for all three ions. The maximum damage is obviously achieved by the heaviest ion at high fluences. A plot of ID/IG vs fluence (Figure 4a) reveals that ID/IG shows an exponential dependence on the fluence. At a fixed λ, an increase of the ID/IG ratio with fluence can be correlated to an increase in ion induced disorder.40 The observed saturation of ID/IG ≈ 0.9 starting at fluences of 1x1011 ions/cm2 up till fluences of 5x1013 ions/cm2 (Figure 4a) may be due to an effective annihilation of defects as the disordered ion-track regions begin to overlap,41 see discussion below. The increase of the ID/IG ratio with fluence indicates a decrease in the average size of the sp2 domains42 from 22 nm down to 17.5 nm with increasing fluence (see Table 2). This effect may be attributed to irradiation induced fragmentation as observed in other SHI irradiated systems.43 As the percentage of decrease with fluence is quite low compared to the original size of flakes, we propose that fragmentation is most efficient at the boundary or edges of flakes. The rate of the increase is different for the respective ions and is found to be much more pronounced for the heavier ions. A linear fit of ID/IG vs Se (Figure 4b) shows that the rate of increase depends clearly on the electronic stopping power and is much more pronounced for the heavier ions with the higher stopping power. The maximum rate of increase in (ID/IG) ratio is observed in case of Ag ion irradiated G-TiO2, while the minimum change is observed for Si ion irradiation. This indicates that the electronic excitation is at the origin of the observed increase of ID/IG.



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Figure 4: (a) Exponential fitting of ID/IG ratio vs fluence and (b) Linear fitting of ID/IG ratio vs electronic energy loss Se of 100 MeV Ag, 40 MeV Ni and 70 MeV Si ion irradiated GTiO2 nanocomposites. The effect of SHI irradiation can also be clearly seen in the evolution of the 2D peak of G-TiO2 nanocomposites. From Figure 3, it can be seen that the FWHM of the 2D peak increases with fluence indicating increasing, SHI induced disorder in the G-TiO2 composite. The intensity of the 2D peak decreases and even disappears as the fluence of Ag ions increases beyond 1x1013 ions/cm2, where as in case of Si ions no significant change is found even at a fluence of 5x1013 ions/cm2 and Ni ion irradiation yields an effect in between the features observed for Ag and Si ion irradiation. Silver ions with a high Se value also affect the intensity of the Eg peak of TiO2 and again the variation of Eg peak with fluence shows an exponential decay of the peak intensity (for data on Eg peak see Supporting Information Figure S2). The observed large variation of the I2D/IG ratio with fluence (Supporting Information Figure S3) is in agreement with the data presented in Table 2. The high defect density effectively blocks the double resonance inelastic scattering events required for the occurrence of the 2D peak.

Work function In order to correlate the SHI induced defects with the changes of the electronic properties, CPD maps of the 100 MeV Ag, 40 MeV Ni and 70 MeV Si ion irradiated G-TiO2 nanocomposites have been measured (CPD maps and their 2D false color image are shown in Supporting Information Figure S4, Figure S5 and Figure S6, respectively). The mapped value of CPD over pristine and SHI irradiated G-TiO2 thin films are inhomogeneous; it may be due to non-homogenous distribution of TiO2 NPs on graphene sheets as shown in its TEM and FESEM images (Figure 1 and Figure S1). We take average value of measured CPD values for 10 ACS Paragon Plus Environment

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better representation. From these we have extracted the variation of the average work function φ (with 10 % error) of the SHI irradiated G-TiO2 nanocomposites as function of fluence (Figure 5). We find that φ increases with increasing ion fluence for all irradiated TiO2 nanocomposites. Again, saturation is observed for higher fluences, initiating at a fluence of 1x1011 ions/cm2. The largest change from initially 4.6 eV (pristine samples) up to 5.7 eV is achieved with Ag ions at fluences larger than 5x1012 ions/cm2. Note that the work function of pristine, suspended graphene is 4.5-4.9 eV

14, 44

and that of TiO2 is 4.5 to 6.2 eV.45,46 Thus,

depending on the value for TiO2, the work function of the G-TiO2 composite can have a value in a wide range. Variation of CPD with fluence (Supporting Information Figure S7) corresponds quite well with the variation of I2D/IG varies with fluence (see Figure S3). The I2D/IG ratio is however gets affected by either irradiation induced disorder or by doping effects. A better indication to what might be the origin of the change in work function is provided by the shift of the G mode as structural defects alone would not directly affect the position of the G peak. From Figure 5b one can see that except for very low fluences, where a notable minimum is observed, the shift of the Raman G peak shows the same trend as the CPD value for the heaviest ion, i.e. silver. This indicates that the observed shift in work function is at least for high stopping powers indeed related to an effective doping of graphene.

Figure 5: (a) Work function φ vs fluence of 100 MeV Ag, 40 MeV Ni and 70 MeV Si ion irradiated G-TiO2 nanocomposites. (b) Position of the Raman G peak as a function of fluence for the ions studied here.

Doping of graphene via SHI irradiation may be achieved by atoms ejected from the TiO2 NPs which are then embedded in the graphene sheets. For TiO2 it has been reported that SHI induced surface changes yield a nearly linear proportionality with fluence.24 For G-TiO2 11 ACS Paragon Plus Environment


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nanocomposites there is not much known yet about the exact nature of the defects caused by swift heavy ion irradiation. However, in case of Si beams with Se smaller than the threshold values of Se required for the latent track formation in TiO2 and graphite, (Seth = 6.2 keV/nm, Seth = 7 keV/nm)47 the resulting defects should be caused predominantly by direct collisions giving rise mainly to the formation of point defects. These point defects have a much weaker effect on the variation of ID/IG, I2D/IG and φ. On the other hand, the high value of Se (11.7 keV/nm) delivered by the 100 MeV Ag ion beam has a much more pronounced effect. Here the energy density is sufficient to create latent tracks in TiO2 and may also be able to cause extended defects in graphene either due to substrate induced doping23 or by direct structural damage,27 resulting in an efficient modification of the G-TiO2 nanocomposite. From the Se value of the Ag ion beam, the latent track radius in TiO2 can be estimated using Szenes model 48,49

given by

 Se    Seth 

R 2 = a 2 (0)ln e 



Se    2.7Seth 

R 2 = a 2 (0) e 

For 0 ≤ Re ˂ a(0)

(3)

For a(0) ≤ Re

(4)

And S = πρcT a 2 (0)/g eth 0

(5)

Where a(0), ρ, c, and T0, denote the Gaussian width of the thermal spike, density, average specific heat and the difference between the melting point „Tm‟ and the irradiation temperature „Tir‟ respectively. The parameter denotes g the efficiency with which the energy is transferred into the electronic system and thus depends on the velocity of the ion. For ions having energy 1 MeV per nucleon traversing an insulator it typically has a value of around 0.4.48 Assuming the band gap energy in the G-TiO2 nanocomposite to be approximately the same as for TiO2 (3.2 eV ), a(0) can be estimated to be 6.0 from the variation of Eg vs a(0).48 From equation (4), the latent track radius in G-TiO2 due to 100 MeV Ag ion irradiation can then be estimated to be ~ 4.7 nm. Thus, at a fluence of 1x1013 ions/cm2, the G-TiO2 nanocomposite surface is fully covered with latent tracks. This is the value where we observe the complete disappearance of the 2D peak intensity (Figure 1a) and where the maximum values of ID/IG (Figure 4a), φ and for the shift of the G mode (Figure 5) are achieved, followed by saturation. This indicates that at intermediate fluences the electronic excitation results in a significant defect creation causing doping effects (in addition to the point defects due to elastic collisions) whereas at even higher fluences, when tracks begin to overlap, a 12 ACS Paragon Plus Environment

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competing process must exist. As the temperatures in the track radius are sufficiently high to induce phase transitions such as melting, it is indeed possible to achieve annealing effects in the vicinity of the impact area. This effect has been reported for a variety of SHI irradiated materials such as SiC, VO2 and PbTiO3.50-52 and most recently also for graphene.40 We thus attribute the observed saturation to an effective defect annihilation mechanism which limits the maximum achievable shift of the work function. From the observed trend in Figure 2b one can however extrapolate that ions with an even higher stopping power might be able to increase the work function even further. This together with the rather low threshold of less than 5 x 1011 ions/cm2 provides us with a powerful tool for work function tuning of G-TiO2 nanocomposites.

CONCLUSION Chemically synthesized G-TiO2 nanocomposite films were irradiated by SHI ion beams with varying energy and fluence to study the effects of ion interaction on its electrical and optical properties. The ion induced defects are found to contribute to the variation in Raman intensity as well as surface potential difference. We could show that electronic excitation caused by swift heavy ions is an effective tool to modify the work function of the G-TiO2 nanocomposite. The large change of the work function of almost 1 eV enables a shift of the Fermi level and a decrease of the conduction band potential which could lead to an enhancement of photocatalytic activity as well as water splitting efficiency of this nanocomposite system. Our studies also reveal that even higher changes in work function may be achieved by tuning the SHI irradiation conditions. ACKNOWLEDGEMENTS The authors Tanuja Mohanty and Marika Schleberger convey their gratitude to DSTDAAD for funding the project "PAC-SPS-TM-DAAD-07110713-641". M. S. also acknowledges support from the DFG SPP 1459: “Graphene”. We are thankful to AIRF, JNU Delhi for TEM facility. One of the authors MM is thankful to CSIR for providing SRF fellowship. We thank H. Lebius for helping us with the irradiation at GANIL and O. Ochedowski for fruitful discussions.

Supporting Information Supporting Information is available free of charge via the Internet at http://pubs.acs.org. . 13 ACS Paragon Plus Environment


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Vacancies in PbTiO3 Thin Films Generated Heat-Treated in Hydrogen Atmosphere. Jpn. J. Appl. Phys. 2008, 47, 7510.


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Figure caption Figure 1: (a) TEM (scale bar 20nm) and (b) HRTEM (scale bar 1nm) images of the G-TiO2 nanocomposites. Figure 2: Raman spectrum of (a) graphene (inset shows a Lorentzian fitting of the 2D peak of graphene) and (b) G-TiO2 nanocomposites thin films. Figure 3: Raman spectra of G-TiO2 nanocomposites irradiated with (a) 100 MeV Ag ions (b) 40 MeV Ni ions and (c) 70 MeV Si ions with different fluences. Figure 4: (a) Exponential fitting of ID/IG ratio vs fluence and (b) Linear fitting of ID/IG ratio vs electronic energy loss Se of 100 MeV Ag, 40 MeV Ni and 70 MeV Si ion irradiated GTiO2 nanocomposites. Figure 5: (a) Work function φ vs fluence of 100 MeV Ag, 40 MeV Ni and 70 MeV Si ion irradiated G-TiO2 nanocomposites. (b) Position of the Raman G peak as a function of fluence for the ions studied here. Supporting information Figure caption Figure S1: FE-SEM image of (a) pristine and (b-d) 1x1013 ions/cm2 swift heavy ion (Si, Ni and Ag ions, respectively) irradiated G-TiO2 nanocomposites thin film (scale bar 200 nm). Figure S2: (a) Raman spectra of TiO2 and (b) Exponential fitting of intensity of Eg peak of TiO2 vs fluence of 100 MeV Ag ion irradiated G-TiO2 nanocomposite thin films (with error bar of 10%). Figure S3: (a) Exponential fitting of I2D/IG ratio vs fluence and (b) linear fitting of I2D/IG ratio vs electronic energy loss Se of 100 MeV Ag, 40 MeV Ni and 70 MeV Si-ion irradiated GTiO2 nanocomposite thin films. Figure S4: CPD maps and their 2D false color image of (a) Pristine (b) 5x1011, (c) 1x1012, (d) 5x1012, (e) 1x1013 and (f) 5 x1013 ions/cm2 100 MeV Ag ion irradiated G-TiO2 nanocomposites. Figure S5: CPD maps and their 2D false color image of (a) Pristine, (b) 5x1011, (c) 5x1012 and (d) 1x1013 ions/cm2 40 MeV Ni ion irradiated G-TiO2 nanocomposites. Figure S6: CPD maps and their 2D false color image of (a) Pristine (b) 5x1011, (c) 1x1012, (d) 5x1012, (e) 1x1013 and (f) 5x1013 ions/cm2 70 MeV Si ion irradiated G-TiO2 nanocomposites. Figure S7: Exponential fitting of CPD vs fluence plot of 100 MeV Ag, 40 MeV Ni and 70 MeV Si ion irradiated G-TiO2 nanocomposite thin films with 10 % error bar.



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Figure 1: (a) TEM (scale bar 20nm) and (b) HRTEM (scale bar 1nm) images of the G-TiO2 nanocomposite. 24x8mm (600 x 600 DPI)

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Figure 2: Raman spectrum of (a) graphene (inset shows a Lorentzian fitting of the 2D peak of graphene) and (b) G-TiO2 nanocomposite thin films. 790x232mm (96 x 96 DPI)


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Figure 3: Raman spectra of G-TiO2 nanocomposites irradiated with (a) 100 MeV Ag ions (b) 40 MeV Ni ions and (c) 70 MeV Si ions with different fluences. 354x96mm (300 x 300 DPI)



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Figure 4: (a) Exponential fitting of ID/IG ratio vs fluence and (b) Linear fitting of ID/IG ratio vs electronic energy loss Se of 100 MeV Ag, 40 MeV Ni and 70 MeV Si ion irradiated G-TiO2 nanocomposites. 487x199mm (96 x 96 DPI)


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Figure 5: (a) Work function φ vs fluence of 100 MeV Ag, 40 MeV Ni and 70 MeV Si ion irradiated G-TiO2 nanocomposites. (b) Position of the Raman G peak as a function of fluence for the ions studied here. 621x237mm (96 x 96 DPI)



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Swift Heavy Ion Induced Optical and Electronic Modifications of

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