Caffeine as nitrogen source for Reduce Graphene 2 doping , and its functionalization with silver 3 nanowires 4

In this work, we propose an easy and a low cost method for the synthesis of 16 Nitrogen-Doped Graphene NDG and its silver nanowires NW functionalization NWGN. The 17 synthesis was performed using the improved graphene oxide method, chemical reduction of 18 graphene oxide in the presence of caffeine as green nitrogen source and the subsequently the silver 19 nanowires growth in the surface, by the chemical reductions salts in the presence of NG. Achieving 20 a homogeneous growing (coating) of graphene sheets. The samples were analyzed using 21 conventional characterization techniques: SEM-EDX, XRD, FT-IR, RAMAN, TEM, HRTEM, STEM 22 and XPS. 23

Besides, the properties of Graphene can be modified by the inclusion in their carbon bidimensional structure, foreign atoms as nitrogen [20][21][22], phosphorous and boron [23,24], or hetero-doping [25]; in this way, there are known three doping types for this: i) pyridinic doping [26], ii) graphitic or substitutional doping [27] and iii) pyrrolic doping [27].Each one has a certain quantity of atoms that can be included in the carbon hexagonal network.The final properties obtained in the doped Graphene, depends of the kind of atom [28,29]; in the present work, we have selected the nitrogen atom, in order to promote areas with reactivity [26,30,31]; the main objective is the N insertion in the hexagonal network by removing a carbon atom during the thermally reduction process in presence of the nitrogen source; this atom insertion induces structural defects in the honeycomb lattice, this doping, in a certain percentage concentration, it decreases the crystallinity of the material, and subsequently, changes in the graphene intrinsic properties [32,33].
The NDG has different physical-chemical properties in comparison with intrinsic graphene; due to the charge distribution is influenced by the nitrogen dopant and the doping types.
Furthermore insertion of foreign atom in the 2D network, favors the generation of activated sites for to promote cluster nucleation, growth nanostructures and anchoring different elements or molecules, in order to produce hybrid or functional materials [22,32,34].For the synthesis, there are two main routes to fabricate doped Graphene: Direct Doping Synthesis or bottom-up as CVD (chemical vapor deposition) that involves high temperatures, hydrocarbon source, inert atmosphere, high quality copper substrates [20]; MBE (molecular beam epitaxial), this method involves ultra high vacuum, hydrocarbon source, the yield of product is very poor; Plasma, that involves strong magnetic fields for ion implantation; it is important to note that these methods need special and costly infrastructure.The other methods known are Doping Post Treatment or top-down, the most used are: Arc-discharge, that employ two electrodes, controlled atmosphere and high currents; ball milling, that employs high energy to grind up graphite and mix with the doping source in order to promote the atom bonding in the defects; and the doping by hydrothermal methods or wet chemical, with this, we can obtain high yield, it involves autoclave reactors and in most of cases, dangerous doping sources [20,35].In our case, we use a combination of wet chemical methods and low temperature for heat treatment, a green nitrogen source (caffeine) Figure 1, in order to GOx impregnation via hydrothermal method, then a relative low temperature (600°C); the heat treatment was conducted in a reductive atmosphere (Ar-H 50%).Another goal is the fact that we have grown silver nanowires on the surface NDG; most of Research Groups functionalize carbon nanostructures with spherical shape nanoparticles.The functionalization experiments were carried out in an ethylene glycol reflux and silver salts, using the seedless method [37] in presence of NDG sheets.

Materials and Methods
Graphene Oxide (GOx), partially reduced Graphene (RG) impregnated with caffeine, Nitrogen Doped Graphene (NG) and Silver nanowires functionalized Nitrogen doped Graphene (NWNDG) were sintetized as following.Materials: the materials used in this study, were purchased from Sigma Aldrich, reagent grade, and used as is: graphite flakes CAS 7782-42-5, H2SO4 CAS 7664-93-9, H3PO4 CAS 7664-38-2, KMnO4 CAS 7722-64-7, AgNO3 CAS 7761-88-8, C8H10N4O2 CAS 58-08-2, PVP wt 30k CAS 9003-39-8, 30% H2O2 and distilled water.GOx Synthesis: for the synthesis of Graphene Oxide, we carried out as described Marcano et al [36].Our first stage product was slurry like, dark brown at 3%-6% in solids, hydrophilic.Caffeine GOx impregnation and Chemically partial reduction: hydrothermal process, the experiment was carried out in order to dissolve 1,3,7 trimethylxanthine (caffeine) and impregnated the entire GOx surface, as follow: 150mg of GOx was added in flask ball with 150 ml distilled H2O, the solution was mixed at 600 rpm 30 min and ultrasonic dispersion (40kHz/150W) was applied at the same time, in order to keep an homogeneous amber colloidal solution; in a hot stirrer plate, the temperature was increased to 90°C using at 300-400 rpm; Then was added into solution in the followed order: caffeine 15mg, ethylene glycol 15ml, and agent reductor NH4OH 15ml; the temperature was keeping at 90 min in a reflux mode, finally the solution turn black.After the impregnation processes with caffeine, the partially reduced RG or STWIN 300 kV and EDS EDAX.Functional groups were determined using Thermo Scientific i550 Nicolet ATR/FTIR spectrometer for powders.X-ray photoelectron spectroscopy XPS, was used to analyze the type of the nitrogen doped graphene.

GOx Morphology
The pristine or initial material were graphite flakes and according to the SEM analysis, each one has 1 mm lateral size or square and 20 m of thickness in Figure 2a and Figure 2b, that is equals to 60,000 stacked sheets of single layers; the SEM micrographs reveals that the morphology obtained for GOx is composed by large single layers, according to the scale bar, they are up 500 m Figure 2c; TEM micrographs confirm that GOx is composed by continuous single layers, Figure 2d.XRD analysis Figure 3a shows a comparison between graphite and GOx, for graphite, the main peak of intensity at 2=26.1° corresponds to the graphite planes [002] with an interlayer distance d=0.34nm, meanwhile for GOx, the main intensity peak is near 2=10°, that corresponds to interlayer distance d=1.2 nm, this is due the oxygen and functional groups intercalation (OH and COOH) in graphite planes as a result of the treatments with strong acid agents; the RAMAN spectra Figure 3b, compares graphite and GOx resulted from the experiment; for graphite (see the inset: free-defects model) the spectrum shows the typical G band at 1574 cm -1 , it is first order vibration of the layers meanwhile for GOx (see the inset: model with defects, oxygen and functional groups), the spectrum shows two characteristic peaks: the band G at 1597 cm -1 and the band D at cm -1 , that confirm the change in the surface of basal planes due the oxidation and functional groups attached; as is showing in the FTIR spectra in Figure 3c, there are the following functional groups in the GOx: C-O at 1037 cm-1, OH at 1308 cm -1 , C=C at 1621 cm -1 , OH at 1673 cm -1 , COOH at 2518 cm -1 and all of them promote changes in the interlayer distance that results in a facile route for exfoliation and to obtain GOx single layers, due the decreasing of attraction forces (energy) between neighbors planes.
HRTEM micrographs Figure 3d, shows a large single layer GOx.At this point, it is important to mention that this method involves an easy experimental set-up and a friendly-green nitrogen source, that according to CN Rao et.al [38].6b; the goal of nitrogen doping and its type was found in the XPS spectra around 400 eV: the two main peaks of energy corresponds to pyridine doping type (GN-p) at 398.9 eV and quaternary or substitutional doping type (GN-q) at 400.6 eV Figure 6c, this results are consistently to the results of other groups [20,30,32,33,39].In Figure 6d are the models of the obtained system in each stage: graphite is the pristine system, GOx is the result of acid treatment for oxidation and functional groups intercalation, the reduce graphene RG and NDG with the use of caffeine as source of nitrogen.

Silver NW Functionalization
After nitrogen doping, the silver nanowires functionalization on Graphene was analyzed via SEM, HRTEM-STEM/EDX mappings, in order to see the shape and distribution of Ag nanoparticles on the entire NDG sheet.It is important to mention that we made a previous silver nanoparticles functionalization experiment with two types of Graphenes, both at the same conditions: the first one was RG (reduced Graphene) and the second one NDG (Nitrogen Doped Graphene), in order to analyze and compare the surface reactivity of the NDG; the results were characterized with TEM, we found that the NDG promotes more Ag nanoparticles nucleation, because its reactivity, it means that the nitrogen atoms act as active sites on the graphene surface; according to the TEM micrographs Figure 7a and Figure 7b, most of the NDG surface were covered with silver nanoparticles, and the average size in wide range 20-40 nm with spherical shape, meanwhile for RG the average size in wide range 10-20 nm in spherical shape too.The average length of silver nanowires obtained in the surface of NDG was a 1 to 2 m and 20-50 nm, it means that the aspect ratio wide/large is in the range of 20-50, SEM micrograph see Figure 8a, TEM micrographs see Figure 8b to Figure 8e.

Conclusions
It is demonstrated that this new proposal experimental method for the synthesis of nitrogen doped Graphene, carried out using caffeine, and its nanowires functionalization it is a useful, easy and green method; We found that the surface of NDG is more reactive due to the content of Nitrogen, and promotes the nucleation of silver nanowires and nanoparticles in the active sites in the surface, and according to these results, it is possibility to used the new method to produce NDG or NW to make Graphene electrodes or circuits interconnected by silver nanowires and used in electronics applications.
Author Contributions: Conceptualization, Methodology, DRG and AZO; SEM/TEM/RAMAN Characterization Analysis, DRG, JJCR and AGR; XPS measurements, HT; Discussion AZO, JJCR and IGE; Supervision, AZO.All authors were involved in the writing process of the manuscript.

Figure 1 .
Figure 1.Graphical representation for the experimental process: Graphite to NWNDG.

Preprints
(www.preprints.org)| NOT PEER-REVIEWED | Posted: 25 July 2018 doi:10.20944/preprints201807.0469.v1chemically reduced Graphene was filtered and dried in oven 70-80°C during 4 hours.Nitrogen doping: NDG was obtained as following: the RG impregnate with caffeine was annealed at 650°C by 30 min, under reductive atmosphere composed by a mix of gases in equal proportions Ar/H 50-50, into a quartz tube reactor.NDG Silver NW functionalization NWNDG: In a three necked flask, 150mg NDG was added to 150ml ethylene glycol in a reflux setup, the solution at 150°C and at 800-1000 rpm; after, simultaneously 50 ml ethylene glycol solutions PVP with concentration 0.6M and AgNO3 with a concentration 0.1M were dropping in the main solution and keeping refluxing and mixing by 180 min.The final product was filtered and dried in a convection oven 90°C 3 hours.Characterization: the characterization was carried out with the following techniques: X-Ray Diffraction (XRD), measurements in Bruker D8 Advance diffractometer.The vibrational characteristics of the Graphenes were analyzed in a HORIBA Xplora Plus Raman spectrometer using 0.5mW -1mW 785 nm laser.Morphological and chemical composition characteristics were determined with the following electron microscopes: environmental scanning electron microscope E-SEM JEOL 6610LV equipped with EDS Oxford.Transmission electron microscopes: TEM JEOL 100CX 100kV, HRTEM/STEM JEOL 2100 200kv with EDS Oxford, FEG HRTEM FEI Tecnai G2 20

Figure 3 .Figure 4 .
Figure 3. a) XRD of Graphite and GOx showing the increase of interlayer distance in plane (002), from 0.34nm to 1.2nm; b) RAMAN spectra for Graphite and GOx showing D band and G after intercalation; c) FTIR shows OH and COOH functional groups presents in GOx; d) TEM of GOx.
There are several methods to produce nitrogen doped Graphene that involves expensive experimental setup as CVD, ball milling, plasma enhanced CVD, Arc-discharge, dangerous chemicals, reagents and gases like ammonium or large periods of synthesis and high temperature.In order to explore the average distribution of nitrogen in the obtained doped graphene sample, we use high resolution 200 kV STEM bright field in an isolated single layer Figure 5a, in an EDX line scan analysis Figure 5b, carbon (red line) and nitrogen (green line) are presented in a normalized graph, both signals are in the same path that indicates the homogeneous distribution of both elements, meanwhile an EDX surface elemental mappings are presented in an individual frames for each element: carbon distribution in color red Figure 5c is present in the graphene zone and in the grid formvar specimen support, and nitrogen distribution in color aqua-blue Figure5d, is just present in the graphene region, note that it is not present in the specimen support.

Figure 5 .
Figure 5. (a) STEM bright field of NDG; (b) normalized Elemental EDX line scan of NG: carbon is colored red, Nitrogen is colored green; (c) y (d) elemental EDX mapping of NG: carbon distribution is colored red, meanwhile nitrogen distribution is colored blue-aqua.Regarding the nature or type of nitrogen doping, the binding energy between neighbor atoms, is unique for each material, and the properties can change if we modify experimentally this binding, in this case, we analyze via high resolution X-ray photoelectron spectroscopy (XPS), the result binding in each stage was shown, we divide the spectra in region of energy to analyze the type of

Figure 6 .
Figure 6.(a) XPS binding energy for C-C sp 2 around 284 eV for NDG, graphite, RG, meanwhile for GOx, the bonding is drift to the right; (b) C-O bonding only is present in GOx; (c) XPS in the range 390 eV to 410 eV shows two peaks, that correspond to doping quaternary or substitutional in 400.6 eV and pyridinic at 398 eV;(d) models for Graphite, GOx, RG and NDG.

Figure 7 .
Figure 7. (a) TEM of RG with Ag NP functionalization; (b) TEM of NDG with Ag nanoparticles functionalization; both samples were conducted under the same experimental conditions.

Figure 8 .
Figure 8.(a) SEM shows silver nanowires in the surface of GN; (b), (c), (d) y (e) TEM of silver nanowires anchored to NDG surface.The STEM image with an elemental mapping (EDX), shows in the yellow frame: silver nanowires and nanoparticles distribution; in the blue frame: carbon distribution, and nitrogen distribution is in the red one.Figure9.

Figure 9 .Figure 9 .
Figure 9. EDX elemental mapping of NDG decorated with Ag nanowires and nanoparticles distribution.