21203
The protective function of PAH layers and enhanced antietching capability of Pd atom are responsible for the formation of the Pd icosahedra .
 PAH, Pd, Pd icosahedra
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: Chemical 
Text: {0} 
Answer:
 PAH, Pd, Pd icosahedra