Wall- and Hybridisation-Selective Synthesis of Nitrogen-Doped Double-Walled Carbon Nanotubes

: Controlled nitrogen-doping is a powerful methodology to modify the properties of carbon nanostructures and produce functional materials for electrocatalysis, energy conversion and storage, and sensing, among others. Herein, we report a wall- and hybridisation-selective synthetic methodology to produce double-walled carbon nanotubes with an inner tube doped exclusively with graphitic sp 2 -nitrogen atoms. Our measurements shed light on the fundamental properties of nitrogen-doped nanocarbons opening the door for developing their potential applications. carbon nanotubes with an inner tube doped with both pyridinic and graphitic nitrogen. Electrochemical studies and theoretical calculations confirm that the internal nitrogen-doped nanotube is able to transduce some of its properties across the external nanotube. Overall this method enables the synthesis of nitrogen-doped nanocarbons with an unprecedented level of control enabling the inves-tigation and subsequent harnessing of their functional properties.

graphitic lattice;( ii)t he bonding of the nitrogen atoms,a s principally three types of nitrogen (pyridinic, graphitic,a nd pyrrolic) can exist;(iii)wall-selectivity,inthe case of layered nanocarbons (e.g.d ouble-walled and multi-walled carbon nanotubes,orstacked layers of graphene). Among the abovementioned challenges,w all-selectivity has received little attention [15][16][17] and provides numerous opportunities for establishing and understanding the fundamental properties of nitrogen-doped nanocarbons and for developing their potential applications. [3] Herein, we report aw all-and hybridisation-selective synthetic methodology to produce double-walled carbon nanotubes (DWNT) with an inner tube doped exclusively with graphitic nitrogen atoms (Scheme 1). Such coaxial carbon nanotubes have been prepared in two steps by using an itrogen-rich polycyclica romatic hydrocarbon, namely dicyanopyrazophenanthroline 1,a sf eedstock and empty single-walled carbon nanotubes (SWNT) as reaction vessels. Firstly,d icyanopyrazophenanthroline 1 is sublimed into the Scheme 1. Synthetic approach for the preparation of coaxial N-SWNT@SWNT.
internal cavity of SWNT.Then, the filled SWNTs (1@SWNT) are exposed to an electron beam or thermally treated so the encapsulated dicyanopyrazophenanthroline 1 converts into ananotube to produce DWNT with an inner nanotube doped with nitrogen (N-SWNT@SWNT). Theg raphitic nitrogen content in the inner tube can be controlled with the annealing temperature to reach values up to 100 %, as demonstrated by acombination of Raman spectroscopy,high-resolution transmission electron microscopy (HR-TEM), energy-dispersive X-ray spectroscopy (EDX), X-ray photoemission spectroscopy (XPS) and cyclic voltammetry.
Dicyanopyrazophenanthroline 1 was selected as feedstock because of the high C/N ratio (16/6), the fact that all the nitrogen atoms are already incorporated in the p framework as sp 2 or sp nitrogen;a nd its ability to sublime.D icyanopyrazophenanthroline 1 was synthesised following ar eported procedure [18] and then to ensure ah igh level of purity it was further purified by chromatography on alumina and reprecipitation (see experimental procedures in supporting information).
Opened SWNT were filled with 1 by heating amixture of the compound with SWNT in 10 À6 mbar vacuum at 120 8 8C. HR-TEM imaging confirmed that the molecules become encapsulated inside the SWNT,yielding 1@SWNT ( Figure 1). Them olecules of 1 are disordered within the cavity of the SWNT,although in some areas,the molecules appear to stack within the nanotube cavity (Figure 1t op zoomed image). Extended exposure to the electron beam resulted in the transformation of the molecules within the nanotube into anew nanotube,thus forming aD WNT (N-SWNT@SWNT). Thef ormation of N-SWNT@SWNT is evidenced by the presence of long continuous structures coaxial to the external tube,terminated with internal caps (Figure 1bottom zoomed image).
HR-TEM imaging clearly showed the propensity of 1 to convert into an anotube.T oi nvestigate in more detail the structural features of N-SWNT@SWNT,b ulk filling and annealing experiments were carried out with SWNT with diameters 1.4-1.7 nm in order to obtain sufficient material for structural characterisation. Thef illing of SWNT with 1 and the transformation process of 1@SWNT into N-SWNT@SWNT by annealing at different temperatures was monitored by resonance Raman spectroscopy and the samples were further analysed by HR-TEM, energy dispersive Xray spectroscopy (EDX) and by X-ray photoemission spectroscopy (XPS). Figure 1s hows the Raman spectra of the pristine SWNT, 1@SWNT and N-SWNT@SWNT formed by annealing at different temperatures.I na ll the spectra, the G-band, Dband, 2D-band and radial breathing mode (RBM) bands typical of SWNT can be clearly observed. In the spectrum of 1@SWNT,additional Raman bands corresponding to those of the spectra of 1 were detected ( Figure S1). TheR aman spectra show the appearance of new bands in the RBM region (Figure 2inset, 100-500 cm À1 ), of anew D-band and of anew contribution (shoulder) in the 2D-band with the increasing annealing temperatures.I nt he case of the samples obtained at 1300 and 1400 8 8C, the new bands in the RBM region are consistent with the formation of an internal tube within the SWNT cavity (N-SWNT@SWNT). Furthermore,t he Raman spectrum of 1@SWNT exhibits an itrile band consistent with the structure of precursor 1 that disappears upon annealing ( Figure S1).
To confirm that the precursor molecules are inside SWNT and that the new inner nanotube grows coaxially to the host nanotube, 1@SWNT and N-SWNT@SWNT were investigated by HR-TEM. Thei mages of 1@SWNT evidenced excellent filling rates of molecules inside the SWNT cavities (Figures 3a and S2). Also,the images of N-SWNT@SWNT samples obtained at 1300 8 8Ca nd 1400 8 8Cs howed long continuous structures within the SWNT consistent with the formation of well-structure inner nanotubes (Figures 3b and S3). In addition, caps on internal nanotubes can be clearly observed similar to those formed under the electron beam. EDX  provide the first evidence of incorporation of nitrogen in the nanotube with an ew emerging peak of nitrogen that is not observed in the empty SWNT sample (Figure 3c).
Then itrogen inclusion percentage and bonding on 1@SWNT and N-SWNT@SWNT was investigated by XPS (Figures 3d and S4;T able 1). TheN 1s region of the XPS spectrum of 1@SWNT indicates 10.6 %o fn itrogen present, distributed between pyridinic,n itrilic and graphitic nitrogen. Thepercentage of pyridinic and nitrilic nitrogen is higher than that of graphitic nitrogen, which agrees with the structure of molecule 1.T he presence of graphitic nitrogen on 1@SWNT was ascribed to the partial decomposition of 1 within the SWNT because of the high temperature used (300 8 8C) during the filling and cleaning processes.I mportantly,t he XPS spectrum of N-SWNT@SWNT obtained after annealing at 1300 8 8Cc onfirms the incorporation of 0.9 %o fn itrogen and shows the disappearance of nitrilic nitrogen-in agreement with Raman-and ah igher proportion of graphitic nitrogen versus pyridinic nitrogen. Them easured nitrogen incorporation indicates 2.4 %ofnitrogen in the inner tube (Table 1and   Table S1), which equates to two or more nitrogen atoms per nm in length. Remarkably,the N-SWNT@SWNT obtained at 1400 8 8Cshows only acontribution of graphitic nitrogen, while the total nitrogen inclusion percentage is virtually unchanged, which imply the formation of an internal nanotube doped exclusively with graphitic nitrogen.
Theelectrochemical properties of N-SWNT@SWNT were investigated by cyclic voltammetry in arotating disk electrode (Figures 4a and S5;T able S2). Thevoltammetric curves in an argon saturated aqueous 0.1m KOHs olution show no redox processes,but in an oxygen saturated solution, the voltammograms reveal ar eduction wave characteristic of the oxygen reduction reaction, [2-4, 6, 7, 9, 11, 13, 14] in which the presence and bonding of nitrogen atoms are directly linked to the electrode potentials. [19][20][21][22][23][24][25] Theglassy carbon (GC) electrodes containing N-SWNT@SWNT show more anodic potentials at 1mAcm À2 than those containing pristine SWNT (0.564 Vv s. NHE). Furthermore,t he potential at 1mAcm À2 of the N-SWNT@SWNT (1300 8 8C) with 83.8 %o fg raphitic nitrogen (0.580 Vv s. NHE) is more anodic than that of N-SWNT@SWNT (1400 8 8C) with 100.0 %o fg raphitic nitrogen (0.685 Vv s. NHE). Thev oltammograms confirm that the presence of graphitic nitrogen shifts the potential to more anodic values and also show that the nitrogen atoms of the internal nanotube are able to modulate the electrochemical properties even if they are encapsulated by the external carbon nanotube.
To investigate if the internal nitrogen-doped nanotube is able to transduce some of its properties across the external one,computational modelling at the tight-binding DFTB-D3 level [26] with the OB3 parameters set [27] was performed with the software DFTB + [28] on semiconducting and metallic    (Table S1).
[c] Pyridinic, nitrilic and graphitic nitrogen percentage ratio over the total nitrogen inclusion percentage measured by XPS. DWNT models with different doping degrees in the internal tube (Table S3). First, the extra electron of nitrogen at the doping levels measured yields metallic DWNTs in all cases through ad irect electron transfer mechanism (Table S4). In addition, analysis of the electron distribution indicates that there is adirect charge transfer from the doped internal tube to the external tube and that this transfer increases with N content (Table S4). Secondly,t he density of states (DOS) shows very little changes in the states of the external undoped SWNT,i nc ontrast to the electronic structure of the internal nanotubes,which is extensively altered due to the presence of nitrogen ( Figures S6 and S7). Thea nalysis of the frontier orbitals shows that there are levels from the internal and external tubes in as imilar range of energies and that the nitrogen atoms area and the external tubes are coupled ( Figure S8). This can be seen in the HOMO molecular orbital, showing enhanced electronic densities near the nitrogen atoms in the internal (Figure 4b,l eft) and external tubes (Figure 4b,r ight). Thee lectronic doping of the external SWNT induced by the internal tube creates potential differences (Table S4), which are consistent with the ones observed by the cyclic voltammetry.
To conclude,wehave reported awall-and hybridisationselective synthetic methodology to prepare nitrogen-doped double-walled carbon nanotubes (N-SWNT@SWNT). A nitrogen-rich polycyclic aromatic hydrocarbon (1)h as been inserted into the cavity of SWNT and then by exposing the samples to an electron beam or to annealing temperatures over 1300 8 8C, the encapsulated molecules are transformed into an itrogen-doped carbon nanotube within the SWNT cavity.T he transformation can be monitored by Raman spectroscopy and HR-TEM. XPS confirms the inclusion of % 1% of nitrogen in the newly formed carbon nanotube and also illustrates that the hybridisation of the nitrogen atoms can be controlled with the annealing temperature.F or instance,t hermal annealing at 1400 8 8Cp roduces doublewalled carbon nanotubes with an inner tube doped exclusively with graphitic nitrogen, while annealing at 1300 8 8C produces double-walled carbon nanotubes with an inner tube doped with both pyridinic and graphitic nitrogen. Electrochemical studies and theoretical calculations confirm that the internal nitrogen-doped nanotube is able to transduce some of its properties across the external nanotube.O verall this method enables the synthesis of nitrogen-doped nanocarbons with an unprecedented level of control enabling the investigation and subsequent harnessing of their functional properties.