Defects in as‐grown
vs
. annealed rutile titania nanowires and their effect on properties
Authors: Folger Alena , Wisnet Andreas , Scheu Christina 
Published: December 2016

Semiconducting metal oxides play a key role in electrochemical and photo physical applications like photo catalysis and as electrode material in solar cells and Li‐ion batteries. Among these metal oxides, hydrothermally grown rutile TiO
2
nanowire arrays are promising as the 1 D structure possesses a large surface area and a directed electron path towards the substrate. However, the efficiency of these devices is also influenced by the defects inside the nanowires (dislocations, stacking faults, titanium interstitials and oxygen vacancies). A previous study showed that as‐grown nanowires have a high density of lattice defects.[1] However, these defects can be removed by an additional annealing step. Latest findings demonstrate that hybrid solar cells where those annealed nanowires are incorporated have significantly higher power‐conversion efficiency.[2] However, the removal of the defects is not fully understood so far.

In our present work, transmission electron microscopy (TEM) was used to study the changes within the nanowire during annealing. TEM investigations were performed at 200 kV using a JEOL JEM‐2200FS and at 300 kV using a FEI Titan Themis 60‐300. First results were obtained by e
x‐situ
TEM analysis of as‐grown TiO
2
nanowires and nanowires, which were annealed at 500 °C for 4 h. These
ex‐situ
analysis confirm that both, as‐grown and annealed nanowires, have the rutile crystal structure. Defects, present in the as‐grown state, can be removed by the thermal treatment. Concurrently, a structural transformation inside the nanowires occurs resulting in faceted voids of several nanometer in diameter. Using tilt series from ‐70° to +70° in high‐angle annular dark‐field (HAADF) scanning (S)TEM mode and the discrete iterative reconstruction technique (DIRT)[3], a TEM tomograph was obtained, which proved that these voids are solely formed inside the nanowire and not at the nanowire surface (Fig. 1). Further analysis including electron energy loss spectroscopy revealed changes of the oxidation state at the surface region of the voids during annealing, whereas the rutile TiO
2
crystal structure was maintained.

HAADF‐STEM
in‐situ
heating experiments, performed in a JEOL JEM‐2200FS with a DENSsolutions heating holder, enabled the direct observation of the structural changes inside the rutile TiO
2
nanowires (Fig. 2). Using a slow heating ramp of 3.3°C/min, a sudden formation of these voids at around 500 °C could be observed. Heating at lower temperatures did not affect the structure of the nanowire and also an additional heating after the transformation (600°C, 1h) did not change the size and shape of the voids. These
ex‐situ
and
in‐situ
observations are a decisive step to explain the mechanisms involved in this process in more detail.

The results of our TEM investigation were correlated to the properties of as‐grown and annealed TiO
2
nanowires. Here, we could show that the healing of the lattice defects upon annealing not only increased the performance of hybrid solar cells but also affects other properties of the nanowires e.g. chemical stability.