Visualization of one-dimensional diffusion and spontaneous segregation of hydrogen in single crystals of VO₂

Abstract

Hydrogen intercalation in solids is common, complicated, and very difficult to monitor. In a new approach to the problem, we have studied the profile of hydrogen diffusion in singlecrystal nanobeams and plates of VO2, exploiting the fact that hydrogen doping in this material leads to visible darkening near room temperature connected with the metalinsulator transition at 65 °C. We observe hydrogen diffusion along the rutile c-axis but not perpendicular to it, making this a highly one-dimensional diffusion system. We obtain an activated diffusion coefficient, ~. − ./ cm 2 sec -1 , applicable in metallic phase. In addition, we observe dramatic supercooling of the hydrogen-induced metallic phase and spontaneous segregation of the hydrogen into stripes implying that the diffusion process is highly nonlinear, even in the absence of defects. Similar complications may occur in hydrogen motion in other materials but are not revealed by conventional measurement techniques.KEYWORDS: vanadium dioxide, hydrogen doping, metal-insulator transition, 1D diffusion, optical microscopy TEXT Hydrogen dissolves in many solids, with effects on their physical properties which are relevant to storage 1 , catalysis, sensing 2 , and material degradation 3 . Understanding how hydrogen moves within a solid is thus important, but is very challenging because of the combination of the difficulty of detecting hydrogen, the complexity of the transport processes, and the sensitivity of kinetics and energetics to microscopic nonuniformity and defects 4 . It has traditionally been studied, predominantly in metals and semiconductors, by neutron scattering 5 , nuclear 6 and muon magnetic resonance 7 , and other techniques which do not yield spatial profiles. It is thus hard to know when deviations from a simple Fick's law behavior are relevant, such as when diffusion is guided by dislocations, grain boundaries or strain fields associated with twinning or structural modifications, or if there is spontaneous segregation. Recently, information on the spatial profile has been obtained using the change in color on hydrogenation of thin yttrium 8 and vanadium

In Situ Visualization of Fast Surface Ion Diffusion in Vanadium Dioxide Nanowires

“…For example, IL gating induced MIT in VO 2 has been attributed to electrochemical reaction involving oxygen vacancies, which have been found to diffuse at least 0.5 μm along the rutile c -axis . H atoms also result in the stabilization of the metal phase down to 2 K, and the diffusion constant of H has been determined to be around 10 –10 cm 2 /s at 100 °C along the rutile c -axis and is much slower along the a -axis. , …”

A Facile Strategy to Realize Rapid and Heavily Hydrogen-Doped VO2 and Study of Hydrogen Ion Diffusion Behavior

“…It was regarded that hydrogen ions diffuse directly along the c R axis of the rutile phase on the basis of a hydrogenated VO 2 nanobeam. 14,19,36 It should be noted that hydrogen diffusion behavior between the VO 2 nanobeam and the VO 2 film was different. The real hydrogen diffusion profiles in the VO 2 nanobeam depend on the energy barriers of the solid/gas interface (surface to subsurface) and inner bulk VO 2 , while for hydrogen in the VO 2 film, the hydrogen atom first overcame the sold/gas interface and then propagated along various directions in-plane (detailed discussion, Figures S14−S16 7d) first decreased and then recovered to pristine VO 2 , which could be reasonably ascribed to the polycrystalline property as displayed in Figure 7b.…”

“…Nevertheless, it remained a noble metal catalyst and was only achieved on specific substrates due to the requirement of domain matching. In addition, according to the previous literature, it is generally regarded that hydrogen ions diffuse directly along the c R axis of the rutile phase (equal to the a M axis of the monoclinic phase). ,, However, the real hydrogen diffusion profiles in a VO 2 nanobeam depend on the energy barriers of the solid/gas interface (surface to subsurface) and inner bulk VO 2 , which determined the transversal or longitudinal diffusion. In addition, Warnick et al employed first-principles molecular dynamics simulation to conduct hydrogen diffusion and claimed that the hydrogen moved along oxygen channels ( c R axis), while Natelson et al found that hydrogen tended to form an O–H bond with the nearest neighbor oxygen and swung from one oxygen to the next oxygen with a “sin 2 ” path, not a straight path, implying the complex diffusion behaviors .…”

A Novel Method for Notable Reducing Phase Transition Temperature of VO2 Films for Smart Energy Efficient Windows

“…where D is carrier diffusion coefficient, J is diffusion flux, and dn/dx is the carrier concentration gradient [44]. With temperature increasing, the diffusion coefficient D and carrier concentration gradient dn/dx in the vertical direction of the film surface increased too.…”