Open AccessReduction in Work Functions of Transition-Metal Carbides and Oxycarbides upon Oxidation
Author(s) -
Wataru Hayami,
Shuai Tang,
Ta-Wei Chiu,
Jie Tang
Publication year - 2021
Publication title -
acs omega
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.779
H-Index - 40
ISSN - 2470-1343
DOI - 10.1021/acsomega.1c01671
Subject(s) - work function , transition metal , materials science , carbide , valence electron , fermi level , molybdenum , tungsten , work (physics) , metal , chemistry , crystallography , electron , atomic physics , thermodynamics , metallurgy , organic chemistry , physics , quantum mechanics , catalysis
Herein, the work functions of group 4 and group 5 transition-metal (Ti, Zr, Hf, V, Nb, and Ta) carbides and transition-metal oxycarbides (TMCOs) were investigated by first-principles calculations for their potential application as electron emitters. The work functions of both groups decreased as the substitution of carbon atoms with oxygen proceeded, and the reduction in group 4 was more than that of group 5. In particular, ZrC 1- x O x and HfC 1- x O x ( x ≥ 0.25) exhibited work functions of less than 3 eV, which were comparable with those of LaB 6 - and ZrO-coated tungsten. The reduction in the work functions could be explained by the rigid-band model of the electronic density of states. The increase in valence electrons increased the Fermi energy, while it demonstrated a less significant influence on the vacuum potential, resulting in a reduction in the work functions. The phonon dispersion curves indicated that the NaCl-type group 5 TMCOs were less stable than the group 4 TMCOs. This agrees with the experimental findings that TaC 1- x O x was not synthesized and NbC 1- x O x was synthesized only for smaller values of x (i.e., x < 0.28). From the viewpoints of the work functions and structural stabilities, group 4 (Ti, Zr, and Hf) TMCOs exhibit better potential for application as electron emitters than group 5 (V, Nb, and Ta) TMCOs.