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However , recombination processes occurring in parallel at the TiO ( 2 ) - QDs - electrolyte triple junction constitute one of the major limitations for further improvement of QDSSCs .
 TiO ( 2 )
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: Chemical 
Text: {0} 
Answer:
 TiO ( 2 )