METHOD FOR NITROGEN DOPING OF SOLID MATERIALS

Abstract

A process for the nitrogen doping of a material includes a set of carbon atoms in the sp.sup.2 hybridization state. The process further includes the material not being oxidized beforehand, then placing the material in contact with dinitrogen. Irradiating the material and the dinitrogen placed in contact with a beam of electrons or of light ions whose energy is greater than or equal to 0.1 MeV, to obtain a material wherein some of the carbon atoms in the sp.sup.2 hybridization state is nitrogen-doped.

Claims

1. A process for the nitrogen doping of a material comprising a set of carbon atoms in the sp.sup.2 hybridization state, the process comprising: placing the material in contact with dinitrogen, wherein the material is oxidized beforehand; irradiating the material and the dinitrogen placed in contact with a beam of electrons or of light ions whose energy is greater than or equal to 0.1 MeV, to obtain a material wherein some of the carbon atoms in the sp.sup.2 hybridization state is nitrogen-doped.

2. The process as according to claim 1, wherein the placing in contact of the material takes place with a gaseous mixture including dinitrogen, in particular a gaseous mixture including air.

3. The process as according to claim 2, wherein the gaseous mixture also includes a gas selected from the group consisting of dioxygen, carbon dioxide, dihydrogen, helium, neon, argon, krypton, xenon and water vapor, and mixtures thereof.

4. The process as according to claim 1, wherein the placing in contact of the material takes place with: a gaseous mixture including helium and air; or a gaseous mixture including air and water vapor, in particular a saturated water vapor.

5. The process as according to claim 1, wherein the set of carbon atoms in the sp.sup.2 hybridization state is chosen from graphene, graphite, carbon nanotubes, fullerenes and fullerites, the set in particular being graphene or graphite.

6. The process as according to claim 1, wherein the set of carbon atoms in the sp2 hybridization state is supported on a substrate, chosen especially from metals, in particular Ni, semiconductors and insulators, in particular SiO.sub.2, glasses and polymers.

7. The process as according to claim 1, wherein the set of carbon atoms in the sp.sup.2 hybridization state is in powder form.

8. The process as according to claim 1, wherein the beam is an electron beam.

9. The process as according to claim 8, wherein the electrons have an energy ranging from 0.1 to 10 MeV, in particular from 0.4 to 2.5 MeV, more particularly from 1.0 to 2.5 MeV, the set of carbon atoms in the sp.sup.2 hybridization state especially being supported on a substrate.

10. The process as according to claim 8, wherein the electrons have an energy ranging from 2.5 to 10 MeV, the set of carbon atoms in the sp.sup.2 hybridization state especially being supported on a substrate or in powder form.

11. The process as according to claim 1, wherein the beam is a beam of light ions, chosen especially from H.sup.+ and He.sup.+.

12. The process as according to claim 11, wherein the light ions have an energy ranging from 0.1 MeV to 45 MeV.

13. The process as according to claim 1, wherein the irradiation time ranges from 1 minute to 72 hours, in particular from 5 minutes to 15 hours.

14. The process as according to claim 1, wherein: the flux of the electron beam ranges from 310 to 410.sup.13e.Math.cm.sup.2.Math.s.sup.1, in particular from 1.410.sup.12 to 1.910.sup.13 e.Math.cm.sup.2.Math.s.sup.1; or the flux of the light ion beam ranges from 310.sup.7 to 610.sup.11e.Math.cm.sup.2.Math.s.sup.1; and/or the fluence of the electron or light-ion beam ranges from 1.810.sup.9 to 10.sup.19 e.Math.cm.sup.2, in particular from 6.710.sup.15 to 7.210.sup.16 e.Math.cm.sup.2.

15. The process as according to claim 1, wherein, during the irradiation: the beam irradiates the dinitrogen, and then the material including a set of carbon atoms in the sp.sup.2 hybridization state; or the beam irradiates the material including a set of carbon atoms in the sp.sup.2 hybridization state, and then the dinitrogen.

16. The process as according to claim 1, wherein the set of nitrogen-doped carbon atoms in the sp.sup.2 hybridization state is also functionalized with carbon-oxygen bonds, in particular carboxyl and/or carbonyl groups.

17. A material comprising a set of carbon atoms in the sp.sup.2 hybridization state, which may be obtained via a process as according to claim 1, of which part of the set of carbon atoms in the sp.sup.2 hybridization state is doped with nitrogen, and wherein: the atomic percentage of nitrogen of the set of nitrogen-doped carbon atoms in the sp.sup.2 hybridization state is greater than or equal to 0.5%; and the atomic percentage of nitrogen of the set of nitrogen-doped carbon atoms in the sp.sup.2 hybridization state in the form of nitro groups NO.sub.2, and/or nitrate groups ONO.sub.2 is greater than or equal to 0.1%; the set of nitrogen-doped carbon atoms in the sp.sup.2 hybridization state also being functionalized with groups including one or more carbon-oxygen bonds, more particularly carboxyl and/or carbonyl groups.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0108] FIG. 1 illustrates the resolution of the various components of the nitrogen XPS peaks, N1s-1, N1s-2, N1s-3, determined from the XPS data obtained for graphene on an Ni substrate (Graphene Supermarket) irradiated at the same electron energy, 2.5 MeV, at the same flux (1) and at the same fluence (14), namely G8 (example 2, FIG. 1A) and G9 (example 1, FIG. 1B). The thickness of the graphene on the Ni substrate ranges from 1 to 7 layers on domains of about 2 to 5 m (Graphene Supermarket).

[0109] FIG. 2 shows the analysis of the distribution of the bond energies of the photoelectrons obtained from the photoelectron spectroscopy (XPS) data for samples E3(2), A3(2) and G1(2) of example 7.

[0110] FIG. 3 shows the analysis of the distribution of the bond energies of the photoelectrons obtained from the photoelectron spectroscopy (XPS) data for samples E3(2) and E5(2) and G1(2) of example 7.

[0111] FIG. 4 shows the analysis of the distribution of the bond energies of the photoelectrons obtained from the photoelectron spectroscopy (XPS) data for samples E1 (2) and A5(2) and E9(2) of example 7.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0112] Materials

[0113] The graphene on Ni substrate comes from Graphene Supermarket (Graphene Laboratories Inc.); they are 10 mm10 mm samples. The graphene on SiO.sub.2 substrate comes from Graphenea (Graphena Monolayer Graphene film).

[0114] The electron beam irradiations were performed using a Pelletron accelerator, supplying electrons of high energy (150 keV-2.5 MeV) and at variable current (150 nA-200 A), and manufactured by the company NEC. The irradiations with an electron beam in the 2.5-10 MeV range are performed with a 10 MeV LINAC.

[0115] The irradiations with a light-ion beam in the 0.4-3 MeV range are performed with a Pelletron manufactured by the company NEC and in the 3-45 MeV range with a cyclotron.

[0116] The measurement of the percentages of elements at the surface of the irradiated samples is performed by X ray photoelectron spectrometry (XPS) using a THERMO-VG ESCALAB 250 spectrometer equipped with an RX K.fwdarw.Al source (1486.6 eV).

[0117] Methods

[0118] In examples 1 to 4 that follow, the irradiation geometry used is as follows: the beam passes through the gas layer in contact with the graphene on a substrate and then enters the graphene and then the substrate. The irradiation creates radicals in the gas layer at the surface and also defects in the crystal structure of the surface with partial destruction of the bonds between carbon.

[0119] The percentages of the elements at the surface of the irradiated samples are measured by XPS according to a procedure in which long pumping under vacuum is performed to condition the surfaces. This method differs from the standard XPS analysis procedures which perform thermal annealing at high temperature, about 450-550 C., over a short period (for example 10 min), before the measurements.

[0120] Determination of the electron extraction work Photoelectron emission spectroscopy (XPEEM) was used to perform an imaging spectroscopic analysis. In particular, maps of the electron extraction work were determined for a reference graphene/Ni layer and a treated layer (G9) using the procedure described by Wang et al. (Carbon 2015, 82, 360-367).

Example 1: Nitrogen Doping of Graphene on an Ni Substrate, Under a Helium/Air Atmosphere

[0121] Samples of graphene on an Ni substrate were irradiated under a helium/air atmosphere under the conditions described in table 1 below. The pressure of the mixture is 1 atm with a partial pressure of 0.9 atm for He and 0.1 atm for air.

[0122] The flux/flux0 ratios in table 1 correspond to the ratios of the ICE currents which make it possible to adjust the pelletron electron beam. The value flux0 is the nominal value of the number of electrons emitted per unit time and area for the minimal ICE current used in the present tests, and is indicated for each test in table 1. The nominal value fluence0 corresponds to irradiation for a time of one hour for an electron flux of value flux0.

[0123] The flux values on the graphene layers depend on the adjustment of the pelletron electron beam. The calibration curve, Itarget=f(ICE), makes it possible to determine them from the nominal flux values. For the tests performed in which the nominal flux values vary in ratios from 1 to 10 (table 1), the flux values vary in ratios from 1 to 11.9. The fluences vary in ratios from 1 to 10.8 instead of ratios from 1 to 14 for the nominal values (table 1).

[0124] The fluxes used vary within the range (1.410.sup.12 to 1.910.sup.13) e.Math.cm.sup.2.Math.s.sup.1 and the fluences in the range (6.710.sup.15 to 7.210.sup.16)e.Math.cm.sup.2 (table 1). It is possible to broaden the range of fluxes used, for example from (310.sup.11 to 410.sup.13) e.Math.cm.sup.2.Math.s.sup.1.

TABLE-US-00001 TABLE 1 List of the graphene samples irradiated with an electron beam emitted by the pelletron, and of the corresponding irradiation conditions. CE/ Flux/ Fluence/ Flux0 Flux Flux/ Fluence Fluence/ Exp. Irrad. time MeV A Flux0 Fluence0 e .Math. cm.sup.2 .Math. s.sup.1 e .Math. cm.sup.2 .Math. s.sup.1 Fxmin e .Math. cm.sup.2 Femin G9 14 h 2.5 1.2 1 14 1.44 10.sup.12 1.44 10.sup.12 1 7.27 10.sup.16 10.8 E9 1 + 1 + 1 = 3 h 1.5 1.2 1 3 1.87 10.sup.12 1.87 10.sup.12 1.29 2.0 .10.sup.16 3 E7 6 min 1 12 10 1 1.87 10.sup.13 1.87 10.sup.13 12.9 6.72 10.sup.15 1

[0125] The columns entitled MeV, CE/A, flux/flux0 and fluence/fluence0 indicate, respectively, the electron energy, the beam current, if the irradiation has taken place over the entire layer or not, the ratio of the irradiation fluxes and fluences at the lowest nominal values used. The flux and fluence columns indicate the flux and fluence values which irradiate the graphene monolayers on the substrate. The flux/fxmin and fluence/femin columns indicate the respective ratios thereof at the minimum values used in the tests.

[0126] Results

[0127] The percentages of the elements at the surface of the samples irradiated as indicated previously are collated in table 2 below.

TABLE-US-00002 TABLE 2 Percentages of the elements at the surface of the irradiated samples determined by XPS. G7-ref corresponds to a non-irradiated sample of graphene on an Ni substrate. XPS: atom (%) Exp. C 1s N 1s Ni 2p O 1s Si 2p G7-ref 79.54 0 4.74 9.47 6.3 G9 59.85 1.3 4.38 28.5 6 E9 69.69 0.9 3.13 20.1 6.2 E7 71.25 0.6 3.91 19 5.3

[0128] Thus, nitrogen doping is observed for both 1 MeV low-energy electrons and 2.5 MeV high-energy electrons in a flux range which varies from 1 to 10 and fluences which vary in a ratio from 1 to 14.

[0129] These data show that there is also functionalization of the carbon with groups including oxygen, in particular carboxyl and/or carbonyl groups.

Example 2: Nitrogen Doping of Graphene on an Ni Substrate, Under a Humid Atmosphere

[0130] Samples of graphene on an Ni substrate were irradiated under an atmosphere of air humidified by the presence of saturated water vapor, under the conditions described in table 3 below.

TABLE-US-00003 TABLE 3 List of graphene samples irradiated with an electron beam emitted by the pelletron, and the corresponding irradiation conditions. CE/ Flux/ Fluence/ Flux0 Flux Flux/ Fluence Fluence/ Exp. Irrad. time MeV A Flux0 Fluence0 e .Math. cm.sup.2 .Math. s.sup.1 e .Math. cm.sup.2 .Math. s.sup.1 Fxmin e .Math. cm.sup.2 Femin G8 14 h 2.5 1.2 1 14 1.44 10.sup.12 1.44 10.sup.12 1 7.27 10.sup.16 10.8 E10 1 + 1 + 1 = 3 h 1.5 1.2 1 3 1.87 10.sup.12 1.87 10.sup.12 1.29 2.01 10.sup.16 3

[0131] Results

[0132] The percentages of the elements at the surface of the samples irradiated as indicated previously are collated in table 4 below.

TABLE-US-00004 TABLE 4 Percentages of the elements at the surface of the irradiated samples determined by XPS XPS: atom (%) Exp. C 1s N 1s Ni 2p O 1s Si 2p G7-ref 79.54 0 4.74 9.47 6.3 G8 57.56 4.2 4.59 29.1 4.6 E10 64.02 2.6 4.63 25.6 3.1

[0133] Thus, the nitrogen doping is observed for both 1.5 MeV energy electrons and 2.5 MeV high-energy electrons in a range of fluences which vary within a ratio from 1 to 4.67.

[0134] These data show that there is also functionalization of carbon with groups including oxygen, in particular carboxyl and/or carbonyl groups.

Example 3: Nitrogen Doping of Graphene on an SiO.SUB.2 .Substrate, Under a Helium/Air Atmosphere

[0135] Samples of graphene on an SiO.sub.2 substrate were irradiated under a helium/air atmosphere under the conditions described in table 5 below. The pressure of the mixture is 1 atm with a partial pressure of 0.9 atm for He and of 0.1 atm for air.

TABLE-US-00005 TABLE 5 Samples of graphene irradiated with an electron beam emitted by the pelletron, and corresponding irradiation conditions. CE/ Flux/ Fluence/ Flux0 Flux Flux/ Fluence Fluence/ Exp. Irrad. time MeV A Flux0 Fluence0 e .Math. cm.sup.2 .Math. s.sup.1 e .Math. cm.sup.2 .Math. s.sup.1 Fxmin e .Math. cm.sup.2 Femin Gs1 1 + 1 + 1 = 3 h 1.5 1.2 1 3 1.87 10.sup.12 1.87 10.sup.12 1.29 2.01 10.sup.16 1

[0136] Results

[0137] The percentages of the elements at the surface of the sample irradiated as indicated previously are collated in table 6 below.

TABLE-US-00006 TABLE 6 Percentages of the elements at the surface of the irradiated sample Gs1 determined by XPS XPS: atom (%) Exp. C 1s N 1s Ni 2p O 1s Si 2p G7-ref 79.54 0 4.74 9.47 6.3 Gs1 26.26 1.1 0 44.7 28

[0138] Thus, the amount of nitrogen introduced into the surface of the graphene on an SiO.sub.2 substrate is close to that introduced under the same conditions onto the surface of the graphene on an Ni substrate (sample E9 of example 1), showing the great versatility of the process of the presently disclosed subject matter as regards substrates bearing the irradiated sample.

[0139] The data of table 6 show that there is also functionalization of the carbon with groups including oxygen, in particular carboxyl and/or carbonyl groups.

Example 4: Configuration of the Nitrogen-Doped Samples

[0140] Analysis of the distribution of the bond energies of the photoelectrons obtained from the photoelectron spectroscopy (XPS) data shows that the nitrogen dopant has several bonding states which correspond to the energies of the ranges (a) 400.2-400.7, (b) 403.01-403.63 and (c) 407.18-407.59 eV, for G9 (example 1) and G8 (example 2).

[0141] FIG. 1 shows that the population of the various types of bond of the nitrogen dopant is greatly dependent on the atmosphere (helium/air or humid air) placed in contact with the sample to be doped, before irradiation at 2.5 MeV at low flux (1) and high fluence (14). The atmosphere of humid air type almost exclusively promotes nitrogen N1s-3. The atmosphere of helium/air type makes it possible to populate the three types of bond with a slightly higher probability for N1s-1 than for N1s-2 and N1s-3.

[0142] Table 7 below gives the domain and the mean values of the bond energies of the electrons corresponding to the various components N1s-n determined in the XPS spectra, and also the identification of these components.

TABLE-US-00007 TABLE 7 Component Emin-Emax N1s (dE) in eV E(eV) nature N1s-1 399.9-400.69 (0.7) 400.23 N-pyrrole; N-pyridone. N1s-2a 402.3(0) 402.30 N-N. N1s-2b 403.01-403-63(0.62) 403.26 N-pyridine oxide; N-nitroso. N1s-3 406.83-407.59(0.76) 407.20 N-nitro; N-nitrate

Example 5: Nitrogen Doping of Graphene with Modified Irradiation Geometry

[0143] In this example, the beam passes first through the substrate, and then the graphene, and finally arrives in the gas in contact with the graphene.

[0144] Samples of graphene on an Ni, Cu, SiO.sub.2 or SiC substrate are irradiated under an atmosphere of N.sub.2, N.sub.2/H.sub.2(5%), N.sub.2/saturating water vapor, N.sub.2/H.sub.2(5%)/saturating water vapor, N.sub.2/alkene, N.sub.2/alkyne, N.sub.2/H.sub.2S, N.sub.2/O.sub.2, N.sub.2/SO.sub.2, He (0.9 atm)/air (0.1), or air (saturating water vapor) (see in particular examples 1 and 2) with electrons which have at the graphene/gas interface energies in the 0.4-2.5 MeV range for flux and fluence conditions, respectively, in the ranges (310 to 410.sup.13) e.Math.cm.sup.2.Math.s.sup.1 and (1.810.sup.13 to 110.sup.19) e.Math.cm.sup.2.

Example 6: Nitrogen Doping of Graphite Composites

[0145] Disks (8-10 mm diameter) of graphite composites (PVDF) BMA-5 used in industry as structural elements in fuel cells, were irradiated under oxidative conditions under an atmosphere of He (0.9 atm)/air(0.1 atm), air (saturating water vapor), at 1.5 MeV under flux and fluence conditions comparable to those of graphene, in particular under the following conditions:

TABLE-US-00008 TABLE 8 Irrrad. CE/ Flux/ Fluence/ Flux0 Flux Flux/ Fluence Exp. time MeV A Flux0 Fluence0 e .Math. cm.sup.2 .Math. s.sup.1 e .Math. cm.sup.2 .Math. s.sup.1 Fxmin e .Math. cm.sup.2 Atmosphere BMA- 3 h = 1 h + 1.5 1.2 1 3 1.44E+12 1.44E+12 1.00E+00 1.56E+16 He (0.9 atm)/ MP_1 1 h + 1 h air 5-5 (0.1) BMA- 3 h = 1 h + 1.5 1.2 1 3 1.44E+12 1.44E+12 1.00E+00 1.56E+16 air + water MP_1 1 h + 1 h vapor 5-6

[0146] These disks, once doped with nitrogen, were then tested as redox electrode with respect to vanadium redox couples used for rechargeable flow batteries of vanadium redox type. It turns out that the electron exchange rates are appreciably modified for both the oxidation and reduction reactions.

Example 7: Nitrogen Doping of Graphene on an Ni Substrate, Under Various Atmospheres

[0147] Samples of graphene on an Ni substrate were irradiated under an atmosphere as described in table 9 below, under the conditions described in table 10 below.

TABLE-US-00009 TABLE 10 List of graphene samples irradiated with an electron beam emitted by the pelletron, and the corresponding irradiation conditions. Experiment Atmosphere A3(2) H.sub.2 (3%)/N.sub.2 1.1 bar E3(2) N.sub.2 1.1 bar E5(2) N.sub.2 0.6 bar; Air 0.4 bar G1(2) N.sub.2 1.1 bar; saturating water vapor A5(2) H.sub.2 (3%)/N.sub.2 1.1 bar E1(2) N.sub.2 1.1 bar E9(2) N.sub.2 1.1 bar; saturating water vapor CE/ Flux/ Flux0 Flux Flux/ Fluence Fluence/ Exp. Irrad. time MeV A Flux0 e .Math. cm.sup.2 .Math. s.sup.1 e .Math. cm.sup.2 .Math. s.sup.1 Fxmin e .Math. cm.sup.2 Femin A3(2) 1 + 1 + 1 = 3 h 1.5 1.2 1 2.51 10.sup.12 2.51 10.sup.12 1 2.71 10.sup.16 3 E3(2) 1 + 1 + 1 = 3 h 1.5 1.2 1 2.51 10.sup.12 2.51 10.sup.12 1 2.71 10.sup.16 3 E5(2) 1 + 1 + 1 = 3 h 1.5 1.2 1 2.51 10.sup.12 2.51 10.sup.12 1 2.71 10.sup.16 3 G1(2) 1 + 1 + 1 = 3 h 1.5 1.2 1 2.51 10.sup.12 2.51 10.sup.12 1 2.71 10.sup.16 3 A5(2) 14 h 19 1.5 1.2 1 2.51 10.sup.12 2.51 10.sup.12 1 1.30 10.sup.17 14.32 E1(2) 13 h 21 1.5 1.2 1 2.51 10.sup.12 2.51 10.sup.12 1 1.21 10.sup.17 13.35 E9(2) 14 h 28 1.5 1.2 1 2.51 10.sup.12 2.51 10.sup.12 1 1.30 10.sup.17 14.47

[0148] Results

[0149] Analysis of the distribution of the bond energies of the photoelectrons obtained from the photoelectron spectroscopy (XPS) data of the samples mentioned above is given in FIGS. 2, 3 and 4.