Novel Approach to the Processing of Meso-macroporous Thin Films of Graphite and in-situ Graphite-Noble Metal Nanocomposites †

1. Experimental procedure The porous PVDF films are processed from a solution containing 15% PVDF and 12% NH4NO3 (AN) in C3H7NO (DMF). For the in-situ nanocomposites, l 2% of AgNO3 (SN) or 2 % of chloroplatinic acid (H2PtCl6) (Pt) were added to the solution under stirring. A clean stainless steel substrate (AISI 316) was used as substrate and the PVDF films were deposited by dip-coating. The films were first allowed to dry for 10 minutes at room temperature and subsequently tempered at 150°C for 12h. Pyrolysis was performed in a tube furnace at 550°C under flowing nitrogen and a ramp of 2°C/min for both heating and cooling. The samples were investigated by X-ray diffraction (XRD) (X'Pert Pro, PANalytical, Holland) in grazing incident geometry with fixed angle of 1.5°, step size 0.05° using monochromatic Cu Kα radiation (λ = 1.5418 Å) and a scanning range (2θ) of 10–90°. A Bruker Raman microscope (532 nm laser diode) was used to acquire spectra over a range of 100−3700 cm−1, with a spectral resolution of 35 cm−1, using a backscattering configuration with a 20× objective. Data were collected on numerous spots on the sample and recorded with a fully focused laser power of 5 mW. Each spectrum was accumulated ten times with an integration time of 15 s. The Raman signal was recorded using a CCD camera. Silicon substrate Raman peak position (520 cm–1) was used to calibrate spectral frequency. The nano-structured surface was characterized using a high-resolution scanning electron microscope (Ultra Plus, ZEISS, Germany).The electrochemical experiments were performed at room temperature in a standard three electrode cell using fresh H2SO4 (0.5M) as electrolyte, which is degassed (>30 min.) prior to each experiment The cyclic voltammetry (CV) measurements, range between 0.2 V and 0.8 V at different scan rates, were recorded using an electrochemical workstation (Zahner, IM6e, Germany). A Pt mesh and Hydroflex probe (Reversible H2 reference electrode) were used as counter and reference electrodes, respectively. All potentials are referred to normal hydrogen electrode (NHE).

Carbon based materials are of paramount importance in various applications, including fuel cells, 1,2 gas sensing, 3 hard coatings 4 and energy storage. 1,[5][6][7][8] Among the urry of structures and morphologies of these materials graphite in its various forms is probably most familiar and nds widespread applications (see references above and references therein). Additional functionalities may be generated by designing graphite-metal nanocomposites. For instance carbon and diamond-like-carbon (DLC)-Ag-nanoparticles (Ag-NPs) were applied as bactericidal coatings; 9 Ag-NPs incorporated into DLC were also shown to improve life time in aerospace applications involving oxygen ion bombardment. 4 Ag-Sn-graphite nanocomposites are suitable as electrodes for fuel cells, 10 and carbon-Pt-nanocomposites are well known as electrocatalyst. 1,2 Carbon nanomaterials are also particularly suitable for high-performance supercapacitors 1,6-8,11 because they can exhibit high surface area, adequate mesoporosity and high chemical stability. For instance carbon nanotubes (CNT) were reported with specic capacitances in the range from 100 to 180 F g À1 , depending on electrode morphology and on whether the CNT are multi or single walled. 11 Today state of the art double layer capacitors rely, however, on activated porous graphite that is unsurpassed both from the point of view of material and production cost. These advantages are however mitigated by the relatively low specic capacitance that can be achieved with this material. 6,11 The main limiting factor has been identied in the inadequate pore structure that rather consists of more micropores (pore diameter <2 nm) and less mesopores, thus limiting electrolyte penetration. 12 Designing graphite electrodes with high mesoporosity can therefore boost the performance of these materials while maintaining their cost-effectiveness.
In the present communication we describe a versatile method for preparing supported porous monolithic and in situ metal-graphite nanocomposites, starting from porous polyvinylidene uoride (PVDF). We demonstrate that these lms are promising candidates for supercapacitor applications, but other applications, as outlined above, may readily be explored.
For the preparation of porous PVDF lms we refer to our previous publication. 12 Briey, the PVDF powder is dissolved in N,N-dimethylfromamide (C 3 H 7 NO, DMF). To this solution we add ammonium nitrate (NH 4 NO 3 which is also well soluble in DMF). The nal solution obtained is clear and has a honey-like viscosity of approximately 280 mPa s. The solution can be applied to a substrate, in our case a stainless steel sheet or oxidized silicon, using solution deposition methods, and may also be screen printed. It is during deposition that the system self-organizes so that specic exothermic reactions occur, 12 giving rise to hierarchical open porosity, as illustrated in Fig. 1. For the in situ nanocomposites, either AgNO 3 or chloroplatinic acid (H 2 PtCl 6 ) is added to the polymeric solution. The lms are subsequently pyrolysed in a tube furnace at 550 C for 30 to 60 minutes under owing nitrogen.
Careful XRD experiments using grazing incidence and long acquisition time (Fig. 1S, ESI †) did not reveal any lines characteristic of graphite or graphite oxides 13 thus implying that the graphite phase is amorphous. The Raman scattering spectra conrm the ndings of XRD, and show the G and D peaks characteristic of amorphous nanographite, 3 Fig. 2. The intensity where E laser (eV) is the energy of the laser excitation, in our case 2.33 eV. The domain size obtained is 22 nm (see below for comparison with microscopic images). The Ag-and Pt-nanocomposites show very similar Raman spectra with negligible changes in the G peak width and the I D /I G peak ratio, denoting no remarkable changes in the microstructural dimensions. The topography of the graphite layer largely replicates that of the porous PVDF lm, but there are areas where only mesoporosity can be seen. This might be amenable to partial melting of the PVDF lm and collapse of the pores during pyrolysis. Cracks are not present despite the high thermal expansion mismatch between the stainless steel substrate and the graphite lm. Fig. 3 shows the topography of the graphite lm with its macro-mesoporous structure with a pore size range from approximately 10 to 250 nm. The cross-section of Fig. 3d shows that the lm is approximately 1 mm thick with an overall uniform appearance, and a porosity network throughout the   lm thickness. The grain morphology and the apparent grain size are better revealed by the AFM micrographs shown in Fig. 4. The lms consist of stacks of nano-platelets having an almost uniform size distribution with of 20 to 40 nm, rather consistent with the Raman scattering data. Micrographs of the in situ nanocomposite of graphite and silver are shown in Fig. 5. The Ag-NPs are scattered throughout the graphite lm surface with a broad size range from 20 to 80 nm, but isolated particles with larger size were also observed. It is interesting to note that the Ag-NPs are present not only on the surface but also over the full lm thickness. These lms are particularly suitable for applications involving abrasion but necessitating a continuous supply of, e.g. Ag-ions, for bactericidal coatings.
The graphite-Pt nanocomposites are characterized by much smaller Pt-NPs as illustrated in Fig. 6 (see also Fig. 1S and 2S, ESI † for XRD and EDS analysis, respectively). Although the SEM micrograph shows large isolated clusters, the overall size determined from the AFM phase image, Fig. 6b, ranges from 10 to 18 nm, in agreement with the size obtained from the XRD 111-Pt peak using the Scherrer formula. Also in this case, the Pt-NPs are present both on the surface and in the regions beneath.
We have chosen to explore the charge storage capacity of the macro-mesoporous graphite lms in an aqueous solution of 0.5 M H 2 SO 4 . Other applications involving the nanocomposite lms will be reported in a later stage. The cyclic voltammograms of the lms that have the microstructure shown in Fig. 3 are displayed in Fig. 7. The CV curves are fairly similar to those known for carbon materials, and the specic capacitances of 180 AE 40 F g À1 obtained lie in the range of those known for porous activated carbon and other carbon nanomaterial electrodes. In comparison to the plain lm (see Fig. 3S, on-line ESI †) the specic capacitance increases by approximately three-fold which underscores the role played by the porosity for this specic application. It should, however, be pointed out that our nanographite lms did not undergo any activation treatment. Further, because the charge collector (stainless steel) also supports the lm during pyrolysis the formation of an oxide scales during this treatment (see Fig. 2S † where the oxide layer is Fe 3 O 4 ) should lead to a high contact resistance, consequently impacting negatively lm performance. We surmise that lm performance could be boosted by employing supports with a thin Au-layer and using an activation treatment.

Conclusions
In conclusion we have shown a versatile processing route for porous carbon processing on a substrate, starting from a porous PVDF lm. Amorphous nanographite platelets are obtained at the fairly low pyrolysis temperature of 550 C. The graphite lms can be deposited on any thermally resistant substrates making a broad range of applications possible. Via the addition of metal salts to the precursor solution we have shown the possibility for in situ graphite-metal nanocomposite processing on graphite-Ag-NPs and graphite-Pt-NPs with rather homogenous distribution of the particles throughout the lm thickness. Finally we have shown one potential application of the porous nanographite lms for energy storage.