Towards understanding the hydrogen molecule in ZnO

Abstract

The hydrogen molecule H 2 (or "hidden" hydrogen) in ZnO is studied by Raman scattering spectroscopy. It is shown that H 2 is practically a free rotator stable up to 700 • C, which is formed by two mobile interstitial hydrogen atoms. The concentration profile of the hydrogen molecule anticorrelates with hydrogen substituting for oxygen (H O ) created at the sample surface during the high temperature hydrogenation. The H 2 dissociation at elevated temperatures in the bulk regenerates interstitial (H BC ) hydrogen, whereas the formation of H O and hydrogen out-diffusion are dominant decay channels of the molecule near the surface. The latter mechanisms are responsible for the lower stability of H 2 in the subsurface region. An ortho-para conversion between the nuclear spin states 1 and 0 of the molecule at 79 K was found to occur within 7.5 h, whereas the back conversion at room temperature occurs faster than 0.5 h. A shift in frequency of the H 2 local vibrational mode with annealing time and temperature is associated with the thermal anneal of lattice imperfections. The coupling of transitions between rotational states of the molecule and lattice phonons influences the ro-vibrational properties. Interstitial lattice sites and/or larger vacancy clusters are preferred trapping centers for H 2 in ZnO.

Lateral interaction and spectroscopic constants of CO adsorbed on ZnO

“…This makes the calculation of spectroscopic constants important to future theoretical and experimental works that consider CO adsorption on ZnO, providing spectroscopic information regarding the Zn-C interaction [29][30][31]. Therefore, the calculation of the spectroscopic constants is of major importance and was performed as described by Paura et al [30].…”

Room temperature ferromagnetism promoted by defects at zinc sites in Mn-doped ZnO

“…10). At room temperature H C is 16 around 40 Oe and 100 Oe for the as prepared and for the annealed samples, respectively (upper-left 17 inset of Fig. 10).…”

“…The resulting powder was cold 15 compacted under pressure of 600 MPa in the form of pellets (green pellets). The green pellets were 16 finally sintered in oxygen atmosphere at 1400 °C for 4 h. Fraction of these samples were then post-17 annealed in a Ar (95%) and H 2 (5%) atmosphere at 600 °C for 3 h. Throughout the article the as 18 prepared sample will be labelled ZM3-O 2 and the post-annealed sample ZM3-ArH 2 . The thermal 19 treatment is not in order to introduce hydrogen specimens in the sample.…”

“…The Raman scattering results together with the XRD data and . 15 The short-range structural data provided by extended X-ray absorption fine structure (EXAFS) 16 spectroscopy offer element-specific insight, furnishing quantitative information about the numbers, The extraction and fitting of EXAFS spectra were performed using Multi-Platform Applications for 9 X-ray absorption (MAX). Ab initio FEFF8.2 [20] code was used to calculate the theoretical spectra.…”

“…Figure 5(a) shows a good agreement between experimental (lines) 15 and simulated (symbols) results. 16 The interatomic distance (R), the coordination number (N), and the reliability of the fit, determined 17 by a quality factor (QF) [32] and Debye−Waller factor (σ 2 ), relative to the fits are reported in Table 3. 18 The first shell (Mn-O) coordination number is around 4, consistent with a Mn 2+ substitutional for 19 tetrahedral Zn 2+ ions in the ZnO wurtzite structure.…”