Tuning the Fermi Level and the Kinetics of Surface States of TiO₂ Nanorods by Means of Ammonia Treatments

Ammonia-induced reduction treatment of titanium dioxide rutile nanorods has been performed, where the treatment triggered a synergistic surface modification of titania electrodes that enhanced its overall photoelectrochemical performance, besides introducing a new absorption band in the 420–480 nm range. A physical model has been proposed to reveal the role of each fundamental interfacial property on the observed behavior. On the one hand, by tuning the Fermi level position, charge separation was optimized by adjusting the depletion region width to maximize the potential drop inside titanium dioxide and also filling the surface states, which in turn decreased electron–hole recombination. On the other hand, by increasing the density of surface holes traps (identified as surface hydroxyl groups), the average hole lifetime was extended, depicting a more efficient hole transfer to electrolyte species. The proposed model could serve as a rationale for controlled interfacial adjustment of nanostructured photoelectrodes tailoring them for the required application

Visible Photoluminescence Components of Solution-Grown ZnO Nanowires: Influence of the Surface Depletion Layer

Arrays of electrodeposited ZnO nanowires (NWs) were used to illustrate the dependence of the ZnO visible photoluminescence (PL) emission on the extension of the surface depletion layer and obtain further insight into the localization of the related states. With this goal in mind, three sets of measurements were carried out: (i) analysis of the PL spectra of ZnO:Cl NWs as a function of their carrier concentration; (ii) analysis of the PL spectra of ZnO:Cl/ZnO core–shell NWs as a function of the thickness of their intrinsic ZnO shell; (iii) in situ analysis of the PL dependence on the polarization of ZnO:Cl photoelectrodes. The obtained experimental results evidenced that the yellow and orange emissions from electrodeposited ZnO NWs are correlated with the extension of the NWs surface depletion region. This result points out the surface localization of the states at the origin of these transitions. On the other hand, the green emission that dominates the visible part of the PL spectra in annealed ZnO NWs showed no dependence on the surface band bending, thus pointing toward its origin in the bulk

Polarity-Driven Polytypic Branching in Cu-Based Quaternary Chalcogenide Nanostructures

An appropriate way of realizing property nanoengineering in complex quaternary chalcogenide nanocrystals is presented for Cu₂Cd_<i>x</i>SnSe_<i>y</i>(CCTSe) polypods. The pivotal role of the polarity in determining morphology, growth, and the polytypic branching mechanism is demonstrated. Polarity is considered to be responsible for the formation of an initial seed that takes the form of a tetrahedron with four cation-polar facets. Size and shape confinement of the intermediate pentatetrahedral seed is also attributed to polarity, as their external facets are anion-polar. The final polypod extensions also branch out as a result of a cation-polarity-driven mechanism. Aberration-corrected scanning transmission electron microscopy is used to identify stannite cation ordering, while <i>ab initio</i> studies are used to show the influence of cation ordering/distortion, stoichiometry, and polytypic structural change on the electronic band structure