Kamalakannan Ranganathan

Time-dependent evolution of the nitrogen configurations in N-doped graphene films

Large-area time-controlled N-doped graphene films were grown on a Cu foil using an ammonia-assisted atmospheric pressure chemical vapour deposition (APCVD) technique. The films were characterized using optical microscopy, Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). Raman spectroscopy was used to verify the doping level and lattice distortion in the graphene films, while the degree of N-doping (N/C at%) and nitrogen configuration were studied by XPS. The results showed that both total nitrogen content and configurations were strongly dependent on the growth time. Notably, at short growth time (2 min) pyridinic-rich films with high oxygen content (∼47.02%) were produced, and the total N-content reached a maximum of 4.68%. Interestingly longer growth times (20 min) also resulted in pyridinic-rich films, however both the nitrogen and oxygen content were at lower values of 2.84% and ∼26.07%, respectively. With increasing growth time, Raman spectra showed a decreasing doping level as seen by the decreasing ID/IG ratio values (1.2 to 0.9). Additionally, Raman peaks exhibited a systematic blue shift due to the compressive strain on the C–C bonds during the incorporation of N atoms into the graphene lattice. The study presents an in-depth understanding of how exposure time of N-dopants influences the bonding states of nitrogen atoms to carbon atoms, thereby dictating the resulting type of N-configurations as well as the overall nitrogen content.

Hollow carbon spheres and a hollow carbon sphere/polyvinylpyrrolidone composite as ammonia sensors

This study reports on the ammonia vapour sensing behaviour of hollow carbon spheres (HCSs), a hollow carbon sphere–polyvinylpyrrolidone (HCS/PVP) composite and annealed hollow carbon spheres (in a humid environment). For device fabrication, a surfactant assisted method was used to homogeneously disperse the hollow carbon spheres onto an interdigitated electrode. Conductance measurements (sensor response and recovery time) were performed at 20 °C and 40 °C. The sensor response was investigated by varying both ammonia concentration and relative humidity. The presence of amorphous domains and oxygenated groups on the pristine hollow carbon spheres resulted in a high relative humidity response. However, the detection of ammonia at high relative humidity using the pristine hollow carbon spheres was found to be negligible due to the inhibition of ammonia adsorption sites by the high concentration of water molecules. In contrast, a decline in conductivity at high relative humidity was recorded in the HCS/PVP sensors due to polymer swelling and plasticization. Annealing of the hollow carbon spheres resulted in a decrease in the amorphous domains in the carbon structure and a subsequent increase in the surface area. The topology of the response was determined as a function of these two variables (NH3 and H2O concentration) and analysed by applying a generalized tristimulus analysis to allow the ammonia concentration to be determined independently of the relative humidity. The pristine HCS, HCS/PVP and annealed HCS sensor responses to 74 ppm NH3 at ambient humidity were 6%, 86% and 196%, respectively. The ammonia sensitivity values (% per ppm) of the pristine HCSs, HCS/PVP and annealed HCSs were 0.08, 4 and 1.6, respectively. The annealed HCSs exhibited a good ammonia sensitivity to NH3 concentration (74–295 ppm) over a broad range of relative humidity (10–97%); indeed the values measured were higher than those reported for other nanomaterial based sensors. This study demonstrates the critical role played by humidity and surface chemistry in the ammonia sensing properties of hollow carbon spheres.

Synthesis and characterization of boron carbon oxynitride films with tunable composition using methane, boric acid and ammonia

Atomically thin films of boron carbon oxynitride (BCNO) are an intriguing new form of thin quaternary semiconducting hybrid films that have recently generated tremendous interest in the scientific community. These films are composition-tunable 2D materials which exhibit rich physical, chemical and electronic properties. Herein, a facile atmospheric pressure chemical vapour deposition (APCVD) synthesis and in-depth characterization of BCNO films with tunable composition, in which methane, ammonia and boric acid were used as precursor materials is reported. The different atomic compositions were achieved by adjusting the vaporization temperature of boric acid by varying the distance (i.e. 2 cm to 12 cm) between the boric acid and the growth substrate. From the XPS survey spectra, the atomic compositions of the BCNO films formed varied as follows: C (48–71 at%), B (2.34–12.8 at%), N (1.98–7.9 at%) and O (33–34 at%). Raman spectral analyses showed that the films exhibited vibrational modes from h-BN, B–C, and graphene domains. XPS data corroborated the Raman analyses by indicating that the films consisted of h-BN, B–O–B, and B2O2 domains embedded within a graphene-rich BCN matrix. Three band gap energies assigned to h-BN (5.87 ± 0.05–5.66 ± 0.06 eV), h-BCN (5.76 ± 0.01–5.22 ± 0.01 eV) and doped graphene (2.35 ± 0.06–1.74 ± 0.04 eV) domains were observed from the UV-vis spectra of BCNO films as a function of boric acid distance from the growth substrate. The narrowing optical energies were attributed to an increasing C-content with increasing growth distance, thus leading to C-doping in h-BN and h-BCN as well as in the formation of larger graphene domains. Finally, the in-depth analysis of the samples allowed for a mechanism of the BCNO formation to be proposed and hence provided further understanding of the growth of the BCNO films.

Generation of open-ended, worm-like and graphene-like structures from layered spherical carbon materials

A study of the effects of size dispersion of Au@SiO2 spheres and silica sphere templates for the synthesis of hollow carbon structures was evaluated using a chemical vapor deposition (CVD) nanocasting method. The diameter of the template, the presence of the gold nanoparticles and the polyvinylpyrrolidone (to cap the Au particles) were found to determine the size, thickness and shape of the synthesized carbon nanostructures. The Au@monodispersed small-sized silica sphere (80–110 nm) template covered with carbon followed by removal of silica produced broken hollow carbon spheres, whereas an equivalent Au@monodispersed large-sized silica sphere (110–150 nm) template produced hollow carbon spheres with a complete carbon shell. Monodispersed and polydispersed pristine silica spheres without Au produced hollow carbon spheres with complete and deformed carbon shells, respectively. Polyvinylpyrrolidone addition to polydispersed SiO2 spheres, followed by carbonization with toluene (1 h) and SiO2 removal, produced wormlike carbon structures. Carbonization (and SiO2 removal) of Au@polydispersed silica spheres for a short carbonization time (1 h) gave a layered carbon nanosheet while at intermediate and longer carbonization times (2–4 h) gave nanotube-like (or worm-like) carbon structures. Raman spectra confirmed the formation of the graphitic nature of the carbon materials. These results highlight the potential use of Au@carbon coreshell structures for the generation of few layered graphene-like unusual nanostructures. As a proof of concept, the wormlike carbon structures were incorporated in organic solar cells and found to give a measurable photovoltaic response.

Generation of radical species in CVD grown pristine and N-doped solid carbon spheres using H2 and Ar as carrier gases

Solid carbon spheres (CSs, d ≈ 200 nm) were synthesized (yield, <40%) in a vertically oriented chemical vapor deposition (CVD) reactor using acetylene as a carbon source and Ar or H2 as the carrier gas. The CSs synthesized in the presence of H2 exhibited a broader thermal gravimetric derivative curve and a narrower paramagnetic signal than the CSs synthesized in Ar. Post synthesis doping of both types of CSs with nitrogen was achieved by passing acetonitrile at 800 °C for 1 h over the CSs in a CVD reactor. The N-doped CSs (NCSs) synthesized under both H2 and Ar displayed an increase in ID/IG ratios as obtained from Raman spectroscopy and showed an increase in the paramagnetic signal due to the presence of nitrogen induced defects compared to the undoped CSs. The NCSs synthesized in H2 had less graphitic-N (22%) than those produced in Ar (50%). The presence of a higher percentage of pyridinic-N and pyrrolic-N for the NCSs prepared with H2 as carrier gas suggested H2 etching effects on the CSs. Further, the N-doped carbon spheres obtained in the presence of H2 gave a higher N/C ratio (5.0) than in the presence of Ar (3.7). The introduction of edge defects and paramagnetic centers in CSs in the presence of H2 gas without the aid of a metal catalyst opens up a platform for regulating surface and catalytic reactions of CSs.

Single-step synthesis of crystalline h-BN quantum- and nanodots embedded in boron carbon nitride films

Herein we report on the synthesis and characterization of novel crystalline hexagonal boron nitride (h-BN) quantum- and nanodots embedded in large-area boron carbon nitride (BCN) films. The films were grown on a Cu substrate by an atmospheric pressure chemical vapour deposition technique. Methane, ammonia, and boric acid were used as precursors for C, N and B to grow these few atomic layer thick uniform films. We observed that both the size of the h-BN quantum/nanodots and thickness of the BCN films were influenced by the vaporization temperature of boric acid as well as the H3BO3 (g) flux over the Cu substrate. These growth conditions were easily achieved by changing the position of the solid boric acid in the reactor with respect to the Cu substrate. Atomic force microscope (AFM) and TEM analyses show a variation in the h-BN dot size distribution, ranging from nanodots (∼224 nm) to quantum dots (∼11 nm) as the B-source is placed further away from the Cu foil. The distance between the B-source and the Cu foil gave an increase in the C atomic composition (42 at% C-65 at% C) and a decrease in both B and N contents (18 at% B and 14 at% N to 8 at% B and 7 at% N). UV-vis absorption spectra showed a higher band gap energy for the quantum dots (5.90 eV) in comparison with the nanodots (5.68 eV) due to a quantum confinement effect. The results indicated that the position of the B-source and its reaction with ammonia plays a significant role in controlling the nucleation of the h-BN quantum- and nanodots. The films are proposed to be used in solar cells. A mechanism to explain the growth of h-BN quantum/nanodots in BCN films is reported.