Hydrogenated Fluorographene: A 2D Counterpart of Graphane with

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Hydrogenated Fluorographene: A 2D Counterpart of Graphane with Enhanced Nonlinear Optical Properties Ioannis Papadakis, Zoi Bouza, Stelios Couris, Athanasios B. Bourlinos, Vasilios Mouselimis, Antonios Kouloumpis, Dimitrios Gournis, Aristides Bakandritsos, Juri Ugolotti, and Radek Zboril J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08470 • Publication Date (Web): 22 Sep 2017 Downloaded from http://pubs.acs.org on September 24, 2017

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Hydrogenated Fluorographene: A 2D Counterpart of Graphane with Enhanced Nonlinear Optical Properties

Ioannis Papadakis1,2, Zoi Bouza1,2, Stelios Couris1,2*, Athanasios B. Bourlinos3,5, Vasilios Mouselimis3, Antonios Kouloumpis4, Dimitrios Gournis4, Aristides Bakandritsos5, Juri Ugolotti5, Radek Zboril5

1

Physics Department, University of Patras, 26504 Patras, Greece

2

Institue of Chemical Engineering Sciences (ICE-HT), Foundation for Research and

Technology-Hellas (FORTH), P.O. Box 1414, Patras 26504, Greece 3

Physics Department, University of Ioannina, 45110 Ioannina Greece

4

Department of Materials Science & Engineering, University of Ioannina, GR-45110

Ioannina, Greece 5

Regional Centre of Advanced Technologies and Materials, Department of Physical

Chemistry, Faculty of Science, Palacky University in Olomouc, Šlechtitelů 27, 779 00 Olomouc, Czech Republic *email: [email protected]

1

ACS Paragon Plus Environment


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Abstract We describe the benign wet chemical synthesis, characterization and third-order nonlinear optical properties of hydrogenated fluorographene, namely of a new 2D counterpart of hydrogenated

graphene

(graphane).

The

presence

of

hydrogen

in

hydrogenated

fluorographene was confirmed using infrared spectroscopy, x-ray photoelectron spectroscopy and thermal gravimetric analysis coupled with evolved gas analysis. The nonlinear optical properties of the derivative were investigated in the visible and infrared using ps laser excitation and compared to those of graphene and fluorographene. All samples were found to exhibit important nonlinear optical response, with hydrogenated fluorographene exhibiting the largest response under visible excitation (ca. one order of magnitude higher compared to graphene and fluorographene). This is among the record effects ever observed for any graphene-based materials, including graphene oxide presented elsewhere. The results reveal the importance of the nature of the functional group and degree of functionalization (i.e., fluorination and hydrogenation) on the nonlinear optical properties of graphenes. It is likely that highly polarized donor-π electron-acceptor regions within a layer result in such large optical nonlinearities.

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Introduction During the last years, graphene and its derivatives have been in the spotlight of significant scientific research interest. This interest has been boosted up by the several potential applications envisaged for graphene and graphene derivatives in photonic and optoelectronic technologies, ranging from optical limiters,1 to solar cells2 and several others, including systems for DNA sequencing,3, 4 spintronics,5 electrochemical energy storage6 and sensing,7 coating technologies8, 9 and composites.10 Graphene (G) is a two dimensional carbon allotrope with zero band gap.11-14 An interesting member of this family is graphene fluoride or fluorographene (CF),15, 16 which, however, is a two dimensional wide band gap semiconductor (i.e. ~3 eV) that is optically transparent in the visible part of the optical spectrum. The stronger the fluorination, the more the carbon bonds of graphene transform from sp2 to sp3 hybridization. Fluorographene holds great promises for applications in high performance materials, such as batteries,17 dielectrics,18 metal organic framework composites for oil adsorption/separation19,

20

and

optical limiting.21-23 Furthermore, fluorographene is a unique substrate enabling further derivatization towards a broad set of 2D functional materials suitable for several applications and devices such as thiofluorographene sensors,7 hydroxofluorographene sustainable magnets24 and conductive cyanographene/graphene acid derivatives25 for biomedical applications. In all instances, modification takes place easily via the controllable substitution and defluorination of fluorographene by HS-, OH- or CN- groups respectively, leading to reconstructive derivatives with properties that are not manifested by graphene or fluorographene itself. Apparently, the type and degree of functionalization of fluorographene play a crucial role in revealing new aspects of graphene derivatives. In a different but closely related approach, herein the use of hydride ion H- to substitute fluorine with hydrogen in fluorographene is exploited using simple wet chemistry and a mild

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reducing agent (NaBH4). The method is safer and easier than hydrogenation with gaseous H2 at elevated temperature or the Birch reduction process.26-28 In this way, the so called hydrogenated fluorographene (CFH) is obtained (or fluorographane),29 a new member of graphene family which can be regarded as the analogous case of hydrogenated graphene (graphane).30-33

Previous

theoretical

calculations

have

shown

the

effect

of

graphane/fluorographene heterostructures on the electron states of the embedded lattices.34 Since such an effect could have a pronounced impact on the third-order nonlinear optical (NLO) properties of graphene-based materials for photonic/optoelectronic applications, we report here the wet chemical synthesis, characterization and detailed investigation of the nonlinear optical properties of hydrogenated fluorographene using both visible (532 nm) and infrared (1064 nm) 35 ps laser excitation. The presence of H in the new derivative was confirmed using a collection of techniques including infrared spectroscopy, x-ray photoelectron spectroscopy and thermal gravimetric analysis coupled with evolved gas analysis. Importantly, the new derivative exhibits excellent third-order nonlinear optical response under visible excitation, exceeding by about one order of magnitude those of graphene, fluorographene or even graphene oxide presented elsewhere.35 To the best of our knowledge, this is among the best nonlinear optical effects observed so far for 2D nanocarbons. This could be ascribed to highly polarized regions of donor-π electron-acceptor type within a layer.

Experimental Section Materials synthesis and characterization techniques Both graphene (G) and fluorographene (CF) were obtained by the liquid-phase exfoliation technique in dimethylformamide (DMF).36-38 CFH was obtained by hydride

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substitution of fluorine using sodium borohydride NaBH4 as the hydride source. The detailed syntheses of all samples are given in the SI sections. Nonlinear optical measurements The third-order NLO properties of G, CF and CFH were investigated by means of the Zscan technique39 both in the visible (532 nm) and infrared (1064 nm) using 35 ps laser pulses from a mode-locked Nd:YAG laser system operated at 10 Hz. The laser was focused into the sample by means of a 20 cm focal length quartz lens. The spot radii at the focus were determined to be 17.5 (at 532 nm) and 30 μm (at 1064 nm) respectively using a CCD camera. G and CF were dispersed in DMF, while CFH was dispersed in acetone. The samples were put in 1 mm thick glass cells for the optical measurements. Z-scan technique is a very powerful technique since it combines the experimental simplicity (being a single beam technique) with the capability for the simultaneous determination of the sign and magnitude of the nonlinear absorption and refraction of a sample, all from a single measurement. The nonlinear absorption and refraction of a sample are described by the nonlinear absorption coefficient (β) and the nonlinear refractive index parameter (γ΄), which are related to the imaginary (Imχ(3)) and real (Reχ(3)) parts of the thirdorder susceptibility χ(3), respectively. So, finally, both the sign and the magnitude of χ(3) can be obtained. In more detail, according to the standard Z-scan experimental procedures,39 the nonlinear absorption coefficient, β, and the nonlinear refractive index parameter, γ΄, are determined from the measurement of the sample transmission, as it moves along the propagation direction of a focused Gaussian laser beam, thus experiencing variable intensity. The sample transmission is measured in two different ways: (i) by collecting the transmitted laser beam just after the sample, and (ii) by measuring the transmitted laser beam after it has passed through a small aperture placed in the far field. The former transmission measurement

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is the so-called “open-aperture” (OA) Z-scan while the latter one is known as “closedaperture” (CA) Z-scan. From the former Z-scan, the sign and the magnitude of the nonlinear absorption coefficient β of the sample can be determined by fitting the OA Z–scan with the following equation: T

   I 0 Leff 2  t   ln 1 exp   1  z 2 / z02    dt   I 0 Leff       2 2  z z 1 /   0   

1

(1)

where T is the normalized transmittance, I0 is the peak on-axis irradiance of the laser beam at the focus, z0 is the Rayleigh length and Leff = [1-exp(-α0L)]/α0 with α0 being the linear absorption coefficient at the laser wavelength and L denoting the thickness of the cell, which in the present case is 1 mm. The appearance of a minimum/maximum at the OA Z-scan indicates the sign of the nonlinear absorption coefficient β, corresponding to reverse saturable (RSA, β>0) or saturable absorption (SA, β 1, CFH inherits a significant sp3 fraction of fluorographene. Note that CF gives no Raman peaks since its strong luminescence suppresses the D and G bands.42 Atomic force microscopy (AFM) shows that the layer thickness of hydrogenated fluorographene (0.8-1 nm) is thinner than that of fluorographene (2-2.5 nm) (Figure 3). This can be ascribed to substitution of the bulkier F atoms by smaller hydrogen atoms, as well as, to reductive de-fluorination of the substrate towards polyaromatic regions.

Figure 3. AFM images and section analysis height profiles of (a) CF and (b) CFH. 9



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Based on XPS elemental analysis, the nominal atomic compositions of fluorographene and hydrogenated fluorographene were found to be CF~1.2 and C18F2.8O1.3 respectively (see e.g. inset survey spectra in Figure 4). According to the deconvolution of the C1s spectrum of fluorographene (Figure 4, bottom), there are no sp2 carbons, since practically every carbon atom is bonded to one F atom (or with 2 or 3 F atoms at the edges). Therefore all carbons appear at high binding energies centered at 287.7 eV (for a minor fraction of semi-ionic C-F), at 290.1 eV and at 291.8 eV for covalent C-F, CF2 and CF3 respectively (CF2 and CF3 are under the same component extending above 293 eV).43 After treatment with NaBH4, the XPS fingerprint is dramatically changed as shown at the top of Figure 4. Reductive defluorination leads to an extended sp2 carbon network, as indicated by the very intense carbon component at 284.8 eV. The component at 288.3 eV is ascribed to carbonyl groups (C=O) in accordance with the small band at 1700 cm-1 in the ATR spectrum. This component amounts to 4.4 at. % with respect to all atoms, in accordance with the ca. 5 at. % of O found from the XPS elemental analysis. The components centered from 286.9 to 292.7 eV are related to carbons close or bonded to F atoms.44 The blue component at 285.9 eV can be attributed to sp3 carbon atoms bonded to an element other than F or O. Taking into consideration the presence of C-H bonds from the ATR spectra, this component is ascribed to sp3 carbons bonded to hydrogen as suggested elsewhere for hydrogenated graphene.45 This component amounts to about 12.1% of carbon atoms and thus dictates an approximately 12.1% functionalization degree with hydrogen (e.g. C18H2.2F2.8O1.3). It should be pointed out that CFH’s synthesis takes place at room temperature for 3 days; these reaction conditions allow for an optimal control over the H/F composition and degree of graphitization. For instance, elevated temperatures might result in complete graphitization by hydride reduction.40,41 On the other hand, prolonged reaction times (e.g., for 2 weeks) in combination with adding more NaBH4 (×1.5) at room

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temperature in order to increase the H content, resulted in 1 nm thick nanosheets with a nominal XPS composition far from being considered ideal (C40F3.5NO5). In this case, extended defluorination, oxidation and N incorporation are simultaneously observed without affecting considerably the H content.

Figure 4. Deconvoluted carbon components based on the high resolution-XPS C1s core level spectra and survey spectra (insets) of the starting material of fluorographene (bottom) and hydrogenated fluorographene (top). Circles represent experimental data; the red line represents the sum of the deconvoluted components.

Thermal gravimetric analysis (TGA) performed under Ar showed a total mass loss at 1000 °C of 30.93 % (Figure 5). The simultaneous evolved gas analysis (EGA-QMS) was 11



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focused on m/z 2, 19 and 20 (Figure 5). The evolutions of gases of m/z 2, attributed to H2, and of m/z 20, attributed to HF, have a very similar shape, with the one of m/z 2 resulting in an ion current ca. 1 order of magnitude lower. The emission of m/z 19, attributed to F, has a similar order of magnitude as that of m/z 20 but a significantly different shape. The emissions take place within a comparable range of temperatures (from 150 to 650 °C), with m/z 19 slightly shifted to higher temperatures. It can be inferred that the fluorine and hydrogen emitted during the thermal treatment partly interact forming HF. The formation of HF decreases when no more hydrogen seems to be available, thus mostly fluorine only is visible at higher temperatures. It must be noted that the trace of m/z 20 is normally negligible in analogous experiments conducted on pristine fluorographene.

Figure 5. TGA (in black) and EGA-QMS (in red, green and blue for m/z 2, 19 and 20, respectively) traces of sample CFH under thermal treatment under Ar in the range 40-1000 °C. In the ion current axis, a break was applied to allow insertion of all traces.

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In Figures 6a, b representative OA Z-scans of G and CF dispersions in DMF (0.1 mg/ml) obtained with visible and infrared excitation respectively are shown. In Figures 6c, d similar OA Z-scans of two CFH dispersions of different concentration (0.001 and 0.01 mg/ml) in acetone are also shown. As can be seen, all OA Z-scans exhibited a maximum, indicating saturable absorption (SA) behavior, as expected, because of the resonant excitation conditions occurring under Vis-NIR excitation of graphene.46 The G and CF dispersions studied exhibited SA for laser intensities 5 to 45 GW/cm2; the CFH dispersions, since they exhibited significantly larger nonlinear absorption, where diluted by 10 and 100 times compared to the G and CF dispersions, and were showing SA behavior for laser intensities in the 20-60 GW/cm2 range under visible and infrared excitation respectively. Interestingly, the OA Z-scans of CFH exhibited broadening above some incident laser intensity, suggesting the onset of saturation of the SA, while further increase of laser intensity resulted into the formation of a transmission minimum exactly where the transmission maximum was occurring under low intensity conditions. This situation reflects the manifestation of multiphoton excitation processes becoming effective at high laser intensity. It is worth noting that CFH exhibited saturation of its SA behavior at both excitation regimes. In order to determine the value of the laser intensity Is at which the onset of saturation of the SA behavior occurs, the following intensity-dependent absorption coefficient a(I) was considered: a( I )  applied.47,

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a0   I and the procedure described in details elsewhere was 1  (I / Is )

The corresponding laser intensities for the onset of the SA behavior were

determined to be about 33 and 49 GW/cm2 for 532 and 1064 nm laser excitation respectively.

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Figure 6. Graphene (G) and fluorographene (CF) OA Z-scans at: a) 532 and b) 1064 nm. Corresponding hydrogenated fluorographene (CFH) OA Z-scans at: (c) 532 and (d) 1064 nm.

From the analysis of the “divided” Z-scans, G, CF and CFH nonlinear refraction were determined. As shown in Figure 7, G and CF exhibited same sign nonlinear refraction while CF exhibited opposite sign. In particular, G and CFH exhibited a valley-peak transmission configuration, corresponding to self-focusing (Reχ(3)>0), while CF exhibited a peak-valley configuration, corresponding to self-defocusing (Reχ(3)

Hydrogenated Fluorographene: A 2D Counterpart of Graphane with

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