29664
Two - step thermochemical water - splitting cycle based on a Zn / ZnO redox pair is considered as a potential route for carbon - free production of hydrogen because the first hydrolysis step of the cycle highly depends on the method of preparation and the resultant particle characteristics , such as size , morphology , surface state , and initial oxide content .
 Zn, ZnO, carbon, hydrogen, oxide
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: Chemical 
Text: {0} 
Answer:
 Zn, ZnO, carbon, hydrogen, oxide