DataSheet1_Monitoring the reduction of UO3 thin film by hydrogen atoms using valence-level spectroscopy: correlating the U5f1 signal to surface hydroxyls.docx

The reaction of a UO3 thin film with atomic hydrogen was studied by He(II) ultraviolet photoelectron spectroscopy (UPS) in the temperature range 190–300 K. UO3 reduction was instantaneously observed once it contacted H atoms at 10–7 torr. The reduction was manifested by the presence of U5f1 electrons in He(II) UPS at approximately 1.5 eV below the Fermi level. Based on the peak characteristics, the valence band shape (composed largely of O2p orbitals in addition to some contribution from U6d and U5f orbitals), and X-ray photoelectron spectroscopy (XPS) U4f lines, the reduction of U6+ in UO3 only results in the formation of U5+ cations and was largely limited to those on the surface. Associated with the reduction was the formation of surface hydroxyls (-OH species) due to the transfer of a proton of the H atom (H.) to surface oxygen ions, while the electron of H. is transferred to a U5f orbital. The pseudo-first-order rate constant of the initial rate of reduction at 10–7 torr and 190 K was found to be approximately 0.01 s–1. Qualitative analysis of the valence band before and after reduction indicates that O2p hybridization with U6d and U5f orbitals leads to well-distinguished features that are characteristic of UO3, U2O5, and UO2. These features, which were quantitatively reversed during the redox process, furthers the assessment of the stoichiometry of a given binary uranium oxide.

本研究采用He(II)紫外光电子能谱（Ultraviolet Photoelectron Spectroscopy，UPS），在190至300开尔文的温度区间内，探究了三氧化铀（UO₃）薄膜与原子氢的反应过程。当三氧化铀薄膜在10⁻⁷托的环境中与氢原子接触时，可即时观测到其还原反应。该还原反应的特征为：在He(II)紫外光电子能谱中，费米能级下方约1.5电子伏特处出现了U5f₁电子信号。结合峰形特征、价带结构（主要由O2p轨道构成，同时存在少量U6d与U5f轨道的贡献）以及X射线光电子能谱（X-ray Photoelectron Spectroscopy，XPS）的U4f谱线分析结果，可知三氧化铀中U⁶⁺的还原反应仅生成U⁵⁺阳离子，且还原过程主要局限于薄膜表面。伴随还原反应发生的还有表面羟基（-OH基团）的生成：氢原子（H·）的质子转移至表面氧离子，而其电子则转移至U5f轨道。在10⁻⁷托、190开尔文的条件下，还原初始速率的准一级速率常数约为0.01秒⁻¹。对还原反应前后的价带进行定性分析后发现：O2p轨道与U6d、U5f轨道的杂化作用，会形成分别对应三氧化铀（UO₃）、五氧化二铀（U₂O₅）与二氧化铀（UO₂）的特征鲜明的谱学特征。这些谱学特征在氧化还原过程中会发生定量可逆的变化，可进一步用于评估特定二元铀氧化物的化学计量比。