Environment-dependent nanomorphology of TiN: the influence of surface vacancies Taehun Lee, a Bernard Delley, b Catherine Stampfl c and Aloysius Soon * a Received 23rd May 2012, Accepted 20th June 2012 DOI: 10.1039/c2nr31266b In this work, we present density-functional theory calculations to investigate the surface properties of TiN as a function of surface orientation and termination, as well as the influence of surface defects for various surface defect concentrations. We calculate both the surface energies (including vacancy formation) as a function of the nitrogen chemical potential, and plot the first-principles derived equilibrium crystal shape (ECS) under different growth conditions. We find that surface defects can considerably change the derived ECS of TiN (especially under nitrogen-lean conditions), highlighting the importance of surface defect consideration in modeling nanoparticle morphology. Introduction Titanium nitride (TiN) is one of the classical refractory transi- tion-metal nitrides and it crystallizes in the rocksalt (B1) struc- ture. The close-packed B1 structure hinders the migration of species, with the result of excellent thermal and chemical stability against, for example, oxidation or self-diffusion. 1–3 Because of its exceptional thermal and mechanical properties such as high melting point, high hardness, good thermal conductivity and high resistance to corrosion, TiN films have been used in various industrial applications such as hard wear-resistant coatings on cutting tools and corrosion-resistant coatings on mechanical components. 4 More recently, titanium nitride nanoparticles have been demonstrated to be a robust support material for the Pt catalyst used in the proton exchange membrane fuel cells (PEM FCs). 5–7 Electrodeposited Pt on TiN (or platinized TiN) often shows much higher catalytic performance than conventional Pt/C electrocatalysts as well as mitigating the CO poisoning effect (which is attributed to the presence of TiN). 8,9 With regards to its performance in these technological appli- cations, it has been found to be strongly dependent on both its surface morphology and orientation of nanocrystallite grains. 10–13 Although the exceptional thermal and mechanical properties of TiN are highly dependent on the film’s micro- structure, the underlying mechanisms and pathways leading to the development of preferred orientation in polycrystalline TiN layers are still poorly understood. The importance of having a good understanding of its preferred orientation is easily moti- vated by the fact that the morphology of isolated nanoparticles is dependent on the characteristics of exposed surface facets. 14 Moreover, several studies have also probed the electronic and mechanical properties of bulk TiN as a function of its point defects. 15–18 Nitrogen vacancies, V N , in particular, are regarded as the primary defects that control the composition ratio of sub- stoichiometric TiN. 19–21 However, the effect of sub-stoichiometry on the morphology of TiN still remains an open question. Furthermore, others have attempted to study the surface ener- getics and predict the shape of TiN nanoparticles via thermo- dynamics considerations, 22–25 however no explicit account of bulk or surface defects were included in these studies. Due to the high surface-to-volume ratios in TiN nano- structures, even small variations in the surface adsorption and defect behaviours, as well as the electronic properties of partic- ular surface facets present on the nanoparticles, can have a huge effect on the overall performance of this nanomaterial. In this present work, we perform first-principles density- functional theory (DFT) calculations to study the surface energetics of TiN as a function of surface orientation and termination. Surface defects (i.e. mono- and di-vacancies of Ti and N) for various surface defect concentrations are also inves- tigated and the surface energies of these defect surfaces are compared and contrasted with those of the clean, defect-free surfaces. Extending our DFT calculations by using ab initio atomistic thermodynamics and the Gibbs–Wulff theorem, 26,27 we then study the dependence of these surface energies as a function of the nitrogen chemical potential, and plot the first-principles derived nanomorphology under different growth conditions. Methodology All DFT calculations are performed using the all-electron DMol 3 code, 28,29 where we employ the generalized gradient approxima- tion (GGA) of Perdew et al. 30 for the exchange-correlation functional. This code employs fast converging three-dimensional numerical integrations to calculate the matrix elements occurring a Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Korea. E-mail: aloysius.soon@yonsei.ac.kr b Paul-Scherrer-Institut, CH-5232 Villigen PSI, Switzerland c School of Physics, The University of Sydney, Sydney, New South Wales 2006, Australia This journal is ª The Royal Society of Chemistry 2012 Nanoscale, 2012, 4, 5183–5188 | 5183 Dynamic Article Links C < Nanoscale Cite this: Nanoscale, 2012, 4, 5183 www.rsc.org/nanoscale PAPER