19487
Hierarchical TiO2 nanotube arrays grown on Ti foil are yielded by subjecting electrochemically anodized , vertically oriented TiO2 nanotube arrays to hydrothermal processing .
 TiO2, Ti, TiO2
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: Chemical 
Text: {0} 
Answer:
 TiO2, Ti, TiO2