Photoelectron Spectroscopic Studies of the Electronic Structure and Bonding in TiC and TiN

Abstract

Titanium carbide (TiC) and titanium nitride (TiN) possess remarkable physical properties, such as extremely high hardness and melting point, that promote their use as antiwear materials under harsh tribological conditions. These physical properties must arise from chemical bonding phenomena that result from the inclusion of the non-metal atom within the metallic matrix, and these bonding phenomena should be apparent in measurements of the valenceband electronic structures of TiC and TiN. This paper explores the surface electronic structure and bonding in TiC(lOO) and TiN(l 10) with core and valence level photoelectron spectroscopies (PES's) using X-rays (1486.6 eV) and synchrotron radiation in the range 28-180 eV. Intensity changes in the valence-band features are followed as a function of incident photon energy; these changes are then compared to theoretical atomic photoionization cross sections to determine the atomic origins of these features. Resonant PES at the Ti 3p absorption edge is used to determine titanium 3d contributions to the valence band and to show differences in the electronic structures in TiC and TiN. A new resonance phenomenon near the Ti 3s edge in TiC was observed, and its possible assignment is discussed. The electronic structure and bonding in these materials is well described by molecular orbital theory, where the Ti and non-metal ions in their formal oxidation states (e.g., Ti4+ and C4-in TiC) undergo covalent bonding interactions. Overall, the PES results indicate greater covalent mixing for TiC as compared to TiN, consistent with the differences in the electronegativities of the atoms. Specifically, stronger covalent interactions between the C 2s, 2p and the Ti 3d, 4s, 4p levels must occur to explain the spectroscopic differences between TiC and TiN. In addition, there is no evidence for an occupied TiC valence level having predominantly Ti character (unlike TiN), precluding the existence of direct Ti-Ti bonding in TiC. Any such orbital overlap is significantly affected by the carbon atoms in the lattice.

Interaction of oxygen with ZrC(001) and VC(001): Photoemission and first-principles studies

“…8,14,15,18,25 In these materials, the chemical bonding exhibits a mixed character. 25,[30][31][32][33] For example, the valence bands of VC show a strong hybridization of the V͑3d͒ and C͑2p͒ states as expected for a covalent compound, 25,31 but there is, nevertheless, a rather large degree of ionicity in the V -C bonds with a positive Mulliken charge of ϳ1e on the V atoms. 25 Among the carbides, the magnitude of the metal→ carbon charge transfer increases following the VC Ͻ TiC Ͻ ZrC Ͻ TaC sequence, and there can be large variations in the relative density of the C͑2p͒ and metal nd states near the Fermi level.…”

“…ZrC and VC are among the most extensively studied transition metal carbides. 7,8,21,25,31 They adopt a cubic NaCl lattice, 7,37 as TiC, and their ͑001͒ face contains the same number of metal and C atoms.…”

X-ray spectroscopic study of the electronic structure of visible-light responsive N-, C- and S-doped TiO2

“…[6][7][8][9][10][11][12][13][14] Titanium nitride (TiN) and carbide (TiC) are both metallic conductors with a partially filled band at the Fermi level (E F ) and a chemical bond of simultaneously metallic, covalent, and ionic character, [15][16][17][18][19][20][21] while titanium sulfide (TiS 2 ) is regarded as a semiconductor or semimetal, 22,23 with a bandgap of about 0.9 eV. 24 It is known that TiN, TiC, and TiS 2 can be oxidized to become TiO 2 via high-temperature sintering in air or oxygen.…”

“…27 X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES) are powerful element-specific probes of the electronic structure of materials, and have been applied extensively to study pure TiO 2 . [16][17][18][19][20][21]24,[27][28][29][30][31][32][33][34][35][36][37][38][39] In XAS, a core-level electron absorbs an x-ray photon and is excited to an unoccupied state above E F . 15,17,27,38 In XES, a valence band electron relaxes to a core hole created by the x-ray absorption process via the emission of an x-ray photon.…”

Interaction of Oxygen with the Polar HfC(111) Surface: Angle-Resolved Photoemission Study

“…The (100) surface is composed of equal numbers of metal and carbon atoms, and the electronic structure can be viewed as a cross section of the bulk electronic structure [2]. The highest occupied level of the group IV TMC has been proved to be composed of C 2p orbitals [27]. Thus, on the (100) surface, oxygen should attack preferentially the surface C atoms, which should result in the desorption of C atoms as CO or CO 2 .…”