Black TiO2: A review of its properties and conflicting trends

[Display omitted] •Black TiO2 photocatalyst has exhibited superior properties in environmental applications.•Defect species like Ti3+ and oxygen vacancies are believed to be responsible for enhanced photocatalytic activity.•Various factors which dictate the properties of black TiO2 make it difficult to understand the exact role of defect species.•Deeper insights on the areas of conflicting results surrounding black TiO2 are vital. Black titanium dioxide (TiO2) has been one of the most researched photocatalyst in recent years. Its ability to overcome the limitations of pristine TiO2 through enhanced visible light absorption and reduced recombination of photogenerated charge carriers have gathered the attention of the scientific community since its discovery in 2011. Numerous synthesis routes have been developed for black TiO2 owing to its superior activity as compared to pristine white TiO2 in various applications. Extensive studies have enabled understanding its superior activities and most studies reveal that defect species like Ti3+ and oxygen vacancies are responsible for the enhanced photoactivity. However, closer look reveals that mere presence of defects does not necessarily lead to improved performance of black samples. Studies show that the same defect species can have negative impact on its performance. In fact, different factors like defect concentration, defect location, valence band and conduction band levels, synthesis route, synthesis conditions, anatase/rutile ratio etc. play a key role in the working mechanisms of black TiO2 samples. In some cases, black TiO2 samples have actually shown reduced photoactivity as compared to the pristine ones. Such results have led to some conflicting ideas related to role of defects in overall photoactivity, photoactivity under visible spectrum, existence/co-existence of Ti3+ species and oxygen vacancies and their location, connection between band gap values and photocatalytic activity etc. The intricacies of black TiO2 are yet to be thoroughly understood as evidently seen in the differing properties reported in many studies. This review summarizes the significant properties of black TiO2 and identifies some of the conflicting points observed in literature where further light needs to be shed for better understanding.