Pan et al. Nanoscale Research Letters 2011, 6:97 http://www.nanoscalereslett.com/content/6/1/97 Recently, Gracia and Kroll [27] reported a new nano- tube, g-C3N4 nanotube, based on density-functional-the- ory calculations. The unique porous structure of these materials (Figure 1) makes the TM doping controllable and the clustering avoidable because each pore can only host one atom. In this study, we present our first-princi- ples study on the metal-functionalized g-C3N4 nano- tubes. We show that the ferromagnetic g-C3N4 nanotube can be achieved by functionalizing the g-C3N4 nanotube with non-magnetic atoms. Methods The first-principles calculation was carried out based on the density function theory (DFT) and the Perdew- Burke-Eznerhof generalized gradient approximation (PBE-GGA) [28,29]. The projector augmented wave (PAW) scheme as incorporated in the Vienna ab initio simulation package (VASP) was used [30,31]. The Mon- khorst and Pack scheme of k-point sampling was used for integration over the first Brillouin zone [32]. The geometry of the g-C3N4 monolayer was first optimized to obtain the lattice constants with a vacuum space of 12 Å used to minimize the inter-layer interaction. A 5 × 1 × 1 grid for k-point sampling and an energy cutoff of 400 eV were used for the bulk and monolayer. The g- C3N4 nanotube is obtained by rolling up the g-C3N4 monolayer into a cylinder along the axial (x) direction (Figure 1), which is defined as a zigzag tube, adopting a similar terminology used in other nanotubes [33,34]. In our study, we chose a nanotube with an index of (4, 0), labeled as g-C3N4-zz4. A 1 × 1 × 3 grid for k-point sam- pling and an energy cutoff of 400 eV were consistently used in our calculations. Excellent convergence was obtained using these parameters, and the total energy was converged to 2.0 × 10-5 eV/atom. A large supercell Figure 1 (Color online) Atomic configuration of monolayer g-C3N4. The wrapping vector of the nanotube is along the x axis. The red circle indicates the functionalizing site. Page 2 of 7 dimension with a wall-wall distance of 10 Å in the plane perpendicular to the tube axis was used to avoid any interaction between the nanotube and its images in neighboring cells. The functionalization was achieved by positioning the metal elements at the center of triangular pores, one atom per pore, labeled as g-C3N4-zz4-TM (Figure 1). The TMs, including Ti, Cr, Mn, Fe, Co, Ni, and W (widely used doping elements in DMS), were used for the functionalization. The formation energy was esti- mated from Ef = (Etot(tube + doping) − Etot(tube) − nm doping) / n where Etot(tube + doping) and Etot(tube) are total ener- gies of the g-C3N4 with and without doping, respec- tively; μdoping is the chemical potential of the functionalizing metal element, calculated from the metal bulk, and n is the number of the metal atoms. Results and discussion After geometry optimization, the calculated in-plane repeating period of the g-C3N4 monolayer in the x direction is 7.129 Å, consistent with the experimental value [19]. For the nanotube, it is seen that the hepta- zine structures in the optimized geometry of g-C3N4-zz4 remain almost as flat as in the monolayer after the opti- mization (Figure 2a). A large curvature with the up C-N-C angle reduced to 106° is observed at the hepta- zine-heptazine N connection, consistent with the litera- ture [21]. However, the lattice distortion both in the heptazine and at the connection is less than 0.5%. The optimized g-C3N4-zz4-TM (TM is Ti, Cr, Mn, Fe, Co, Ni, or W) (see Figure 2b) looks more like a cylinder than the unfunctionalized g-C3N4-zz4 (Figure 2a). The curvature in the g-C3N4-zz4-TM is uniform and the heptazine structure is not a plane. Interestingly, TM atom forms four bonds with the four neighboring edge nitrogen atoms with a bonding length of approximately 2.1 Å and a distance of about 2.7 Å from the other two edge nitrogen atoms. The calculated functionalization energies are 0.14, 3.19, 0.57, 1.72, 2.69, 2.11 and 4.05 eV for Ti, Cr, Mn, Fe, Co, Ni, and W, respectively (see Table 1), indicating that the functionalization of Ti is relatively easy than that of other elements because of the lower formation energy. From the calculated exchange energies (the energy difference between anti-ferromagnetic and ferromagnetic states, Eexch = EAFM - EFM) of the metal-functionalized g-C3N4-zz4 (see Table 1), we can see that W and Ti favor the ferromagnetic coupling with the exchange energies of 180 and 95 meV/pair of TMs, respectively. The Fe-functionalized nanotube should be non-magnetic due to the weak exchange energy (4 meV/pair of TMs).