20864
Sulfur - nitrogen co - doped three - dimensional carbon foams with hierarchical pore structures as efficient metal - free electrocatalysts for oxygen reduction reactions .
 Chemical: Sulfur; Chemical: nitrogen; Chemical: carbon; Chemical: oxygen
Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: Chemical 
Text: {0} 
Answer:
 Chemical: Sulfur; Chemical: nitrogen; Chemical: carbon; Chemical: oxygen