Shape and Morphology Effects on the Electronic Structure of TiO₂ Nanostructures: From Nanocrystals to Nanorods

We carry out an accurate computational analysis on the nature and distribution of electronic trap states in shape-tailored anatase TiO₂ structures, investigating the effect of the morphology on the electronic structure. Linear nanocrystal models up to 6 nm in length with various morphologies, reproducing both flattened and elongated rod-shaped TiO₂ nanocrystals, have been investigated by DFT calculations, to clarify the effect of the crystal facet percentage on the nanocrystal electronic structure, with particular reference to the energetics and distribution of trap states. The calculated densities of states below the conduction band edge have been very well fitted assuming an exponential distribution of energies and have been correlated with experimental capacitance data. In good agreement with the experimental phenomenology our calculations show that elongated rod-shaped nanocrystals with higher values of the ratio between (100) and (101) facets exhibit a relatively deeper distribution of trap states. Our results point at the crucial role of the nanocrystal morphology on the trap state density, highlighting the importance of a balance between the low-energy (101) and high-energy (100)/(001) surface facets in individual TiO₂ nanocrystals

Self-Cleaning Organic/Inorganic Photo-Sensors

We present the fabrication of a multifunctional, hybrid organic–inorganic micropatterned device, which is capable to act as a stable photosensor and, at the same time, displaying inherent superhydrophobic self-cleaning wetting characteristics. In this framework several arrays of epoxy photoresist square micropillars have been fabricated on n-doped crystalline silicon substrates and subsequently coated with a poly­(3-hexylthiophene-2,5-diyl) (P3HT) layer, giving rise to an array of organic/inorganic p–n junctions. Their photoconductivity has been measured under a solar light simulator at different illumination intensities. The current–voltage (<i>I</i>–<i>V</i>) curves show high rectifying characteristics, which are found to be directly correlated with the illumination intensity. The photoresponse occurs in extremely short times (within few tens of milliseconds range). The influence of the interpillar distance on the <i>I</i>–<i>V</i> characteristics of the sensors is also discussed. Moreover, the static and dynamic wetting properties of these organic/inorganic photosensors can be easily tuned by changing the pattern geometry. Measured static water contact angles range from 125° to 164°, as the distance between the pillars is increased from 14 to 120 μm while the contact angle hysteresis decreases from 36° down to 2°

Electrochemical Assessment of the Band-Edge Positioning in Shape-Tailored TiO₂‑Nanorod-Based Photoelectrodes for Dye Solar Cells

Three families of linear shaped TiO₂ anatase nanocrystals with variable aspect ratio (4, 8, 16) and two sets of branched TiO₂ anatase nanocrystals (in the form of open-framework sheaf-like nanorods and compact braid-like nanorod bundles, respectively) were employed to fabricate high-quality mesoporous photoelectrodes and then implemented into dye-sensitized solar cells to elucidate the intrinsic correlation holding between the photovoltaic performances and the structure of the nanocrystal building blocks. To this aim, the chemical capacitance and the charge-transfer resistance of the photoelectrodes were extrapolated from electrochemical impedance spectroscopy measurements and used to draw a quantitative energy diagram of the dye-sensitized solar cells realized, on the basis of which their photovoltaic performances have been discussed. It has thus been revealed that photoanodes made from braid-like branched-nanorod bundles exhibited the most favorable conditions to minimize recombination at the interface with the electrolyte due to their deep distribution of trap states, whereas linear-shaped nanorods with higher aspect-ratios result in more remarkable downshift of the conduction band edge

NiO/MAPbI_3‑xCl_<i>x</i>/PCBM: A Model Case for an Improved Understanding of Inverted Mesoscopic Solar Cells

A spectroscopic investigation focusing on the charge generation and transport in inverted p-type perovskite-based mesoscopic (Ms) solar cells is provided in this report. Nanocrystalline nickel oxide and PCBM are employed respectively as hole transporting scaffold and hole blocking layer to sandwich a perovskite light harvester. An efficient hole transfer process from perovskite to nickel oxide is assessed, through time-resolved photoluminescence and photoinduced absorption analyses, for both the employed absorbing species, namely MAPbI_3‑<i>x</i>Cl_<i>x</i> and MAPbI₃. A striking relevant difference between p-type and n-type perovskite-based solar cells emerges from the study

Influence of Porphyrinic Structure on Electron Transfer Processes at the Electrolyte/Dye/TiO₂ Interface in PSSCs: a Comparison between meso Push–Pull and β‑Pyrrolic Architectures

Time-resolved photophysical and photoelectrochemical investigations have been carried out to compare the electron transfer dynamics of a 2-β-substituted tetraarylporphyrinic dye (ZnB) and a 5,15-meso-disubstituted diarylporphyrinic one (ZnM) at the electrolyte/dye/TiO₂ interface in PSSCs. Although the meso push–pull structural arrangement has shown, up to now, to have the best performing architecture for solar cell applications, we have obtained superior energy conversion efficiencies for ZnB (6.1%) rather than for ZnM (3.9%), by using the I^–/I₃^–-based electrolyte. To gain deeper insights about these unexpected results, we have investigated whether the intrinsic structural features of the two different porphyrinic dyes can play a key role on electron transfer processes occurring at the dye-sensitized TiO₂ interface. We have found that charge injection yields into TiO₂ are quite similar for both dyes and that the regeneration efficiencies by I^–, are also comparable and in the range of 75–85%. Moreover, besides injection quantum yields above 80%, identical dye loading, for both ZnB and ZnM, has been evidenced by spectrophotometric measurements on transparent thin TiO₂ layers after the same adsorption period. Conversely, major differences have emerged by DC and AC (electrochemical impedance spectroscopy) photoelectrochemical investigations, pointing out a slower charge recombination rate when ZnB is adsorbed on TiO₂. This may result from its more sterically hindered macrocyclic core which, besides guaranteeing a decrease of π-staking aggregation of the dye, promotes a superior shielding of the TiO₂ surface against charge recombination involving oxidized species of the electrolyte

Ultrathin TiO₂(B) Nanorods with Superior Lithium-Ion Storage Performance

The peculiar architecture of a novel class of anisotropic TiO₂(B) nanocrystals, which were synthesized by an surfactant-assisted nonaqueous sol–gel route, was profitably exploited to fabricate highly efficient mesoporous electrodes for Li storage. These electrodes are composed of a continuous spongy network of interconnected nanoscale units with a rod-shaped profile that terminates into one or two bulgelike or branch-shaped apexes spanning areas of about 5 × 10 nm². This architecture transcribes into a superior cycling performance (a charge capacitance of 222 mAh g^–1 was achieved by a carbon-free TiO₂(B)-nanorods-based electrode vs 110 mAh g^–1 exhibited by a comparable TiO₂-anatase electrode) and good chemical stability (more than 90% of the initial capacity remains after 100 charging/discharging cycles). Their outstanding lithiation/delithiation capabilities were also exploited to fabricate electrochromic devices that revealed an excellent coloration efficiency (130 cm² C^–1 at 800 nm) upon the application of 1.5 V as well as an extremely fast electrochromic switching (coloration time ∼5 s)