Current perspectives of anodized TiO<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="d1e1461" altimg="si3.svg"><mml:msub><mml:mrow/><mml:mrow><mml:mi mathvariant="bold">2</mml:mi></mml:mrow></mml:msub></mml:math> nanotubes towards photodegradation of formaldehyde: A short review