Prediction of standard enthalpies of formation of boron nitride nanocones

Fahimeh Shojaie

doi:10.3906/fiz-1908-7

Abstract

Prediction of the standard enthalpy of formation (ΔfH0298) of boron nitride nanocone (BNNCs) structures with disclination angles 60°, 120°, and 180°is recommended to be performed by the isodesmic reaction approach. NH3, BH3, and N2H4 were selected as key reference compounds. In order to calculate ΔfH0298 of nanocones, we must first calculate the enthalpies of nanocone rings at their apexes. For this purpose, ΔfH0298 values of 40 different structures, such as boron, nitrogen, and hydrogen compounds, have been calculated by combining Gaussian-4 (G4) theory calculations with the isodesmic and other balanced reactions approach. At each stage of the calculations, the previously estimated enthalpies of formation of nanocones were used as reference points for new molecules in the isodesmic and other balanced reactions. The results of enthalpies of formation of reference compounds were then used as reference values to estimate the enthalpy of formation of rings of BNNCs at their apexes. Finally, enthalpies of formation of BNNCs with disclination angles of 60°, 120°, and 180°and cone heights of 1-5 Å, 3-7 Å, and 4-8 Å were calculated. The results show that the reactions are highly exothermic. An increase in cone height causes the enthalpy of formation of boron nitride nanocone structures to become more negative.

Highlights

Nanocones are a novel form of nanostructure that were discovered in 1992 as caps at the end of nanotubes [1]
The disclination angle determines the properties of nanocones as this angle depends on the internal pore space and surface area of nanocones
A nanocone with a disclination angle of 60°has a pentagonal ring at its apex, i.e. this cone has a defective line at its wall, which is formed by a B–B or N–N bond

Summary

Introduction

Nanocones are a novel form of nanostructure that were discovered in 1992 as caps at the end of nanotubes [1]. Electronic structures [2,13,22], mechanical properties [23], and effects of the electric field [24,25] on boron nitride conical structures with different disclination angles have been studied. The Gaussian-4 (G4) theory is a composite method and the goal is to obtain accurate energy without requiring extensive computer resources [31] This approach achieves overall accuracy of 3.3 kJ/mol for a set of 270 accurate experimental enthalpies of formation of nanocones. Structures of BNNCs with disclination angles of 60°, 120°, and 180°were considered for determination of the enthalpy of formation of nanocones. A nanocone with a disclination angle of 60°has a pentagonal ring at its apex, i.e. this cone has a defective line at its wall, which is formed by a B–B or N–N bond. In BNNCs, the hydrogen atoms have been used to saturate the ending atoms (60°, 120°, and 180°) in order to avoid dangling effects

Computational modeling

Results and discussion

Conclusions