Surface properties of CaNi5 hydrogen storage alloy

The Ca 2p, Ni 2p and O 1s emissions of the air-exposed sample of CaNi₅ indicate that all the calcium and most of the nickel are present in the oxidized state. Interestingly, it is observed that all the peaks are shifted by 3 to 4 eV towards higher binding energy values. This observation has been attributed to the presence of insulating oxide phases leading to partial charging of the sample during the experiment. Depth profiling by argon-ion sputtering removes the insulating oxide species and the sample etched for 7300 sec shows the presence of metallic nickel and mostly metallic calcium. XPS analysis indicates that preferential segregation of calcium in the CaNi₅ alloy is mainly due to induced surface reaction with oxygen.