  A template-free process for the synthesis of nanocrystalline TiO2
hierarchical microstructures by reactive Pulsed Laser Deposition (PLD) is here
presented. By a proper choice of deposition parameters a fine control over the
morphology of TiO2 microstructures is demonstrated, going from classical
compact/columnar films to a dense forest of distinct hierarchical assemblies of
ultrafine nanoparticles (<10 nm), up to a more disordered, aerogel-type
structure. Correspondingly, film density varies with respect to bulk TiO2
anatase, with a degree of porosity going from 48% to over 90%. These structures
are stable with respect to heat treatment at 400 centigrade degrees, which
results in crystalline ordering but not in morphological changes down to the
nanoscale. Both as deposited and annealed films exhibit very promising
photocatalytic properties, even superior to standard Degussa P25 powder, as
demonstrated by the degradation of stearic acid as a model molecule. The
observed kinetics are correlated to the peculiar morphology of the PLD grown
material. We show that the 3D multi-scale hierarchical morphology enhances
reaction kinetics and creates an ideal environment for mass transport and
photon absorption, maximizing the surface area-to-volume ratio while at the
same time providing readily accessible porosity through the large inter-tree
spaces that act as distributing channels. The reported strategy provides a
versatile technique to fabricate high aspect ratio 3D titania microstuctures
through a hierarchical assembly of ultrafine nanoparticles. Beyond
photocatalytic and catalytic applications, this kind of material could be of
interest for those applications where high surface-to-volume and efficient mass
transport are required at the same time.
