Direct Imaging of the Recombination/Reduction Sites in Porous TiO2 Electrodes

Modification of charge transport in nanostructured TiO2-xSchottky diodes via post fabrication annealing

An ordered, crack-free array of anatase TiO₂nanotubes were prepared via anodization and annealing at 550 °C. Oxygen vacancies are known to improve photocatalytic activity in TiO₂and were introduced using a facile low vacuum annealing. Stoichiometric and nonstoichiometric films were followed by a conformal Pt deposition at 250 °C using atomic layer deposition resulting in a current rectifying device, or Schottky diode. Here, we investigated the current-voltage effects of a post thermal treatment using 350 °C and 400 °C. This paper presents reproducible improvements in the charge transport behavior after a 350 °C post-fabrication annealing process. Oxygen vacancies resulted in the formation of a shunt resistance behavior of 10⁵Ω and no overall performance improvements compared to stoichiometric films. Additionally, thermal treatments offered trap-based shifts from multiple trap distributions to a single trap distribution. This work hereby provides valuable experimental observations into understanding the effects of oxygen vacancies and effectively modifying the electronic properties of a conformal Pt/TiO_2-xnanostructured junction using a facile post-thermal treatment.

Incorporation of Nanostructured Carbon Composite Materials into Counter Electrodes for Highly Efficient Dye-Sensitized Solar Cells

Dye-sensitized solar cells (DSSCs) composed of nanostructured carbon composite materials-stacked counter electrodes (CEs) were fabricated in the present study. As the potential replacement of expensive platinum (Pt) thin film, various carbon composite materials, including zero-dimensional carbon nanoparticles (CNPs), one-dimensional multiwalled carbon nanotubes (MWCNTs), and two-dimensional graphene flakes (GFs) as a suitable charge transfer medium were deposited on the surface of CEs using a screen printing process. As the results, CNPs were found to result in deteriorating the charge transfer from CE to liquid electrolyte due to the formation of highly aggregated structures with very low specific surface area. However, MWCNTs and MWCNTs-added carbon composites (e.g., CNP/MWCNT, MWCNT/GF, CNP/MWCNT/GF) were found to enhance the charge transfer from CE to liquid electrolyte due to the formation of highly networked structures with high specific surface area. The resulting PCE of DSSCs composed of pure MWCNTs- and MWCNTs-added carbon composites-based CEs were very similar with that of DSSCs composed of Pt-based CEs. This suggests that the nanostructured carbon materials especially composed of MWCNTs and their composites are one of the promising candidates to replace the expensive Pt in the CEs of DSSCs.

Facet-Dependent Diol-Induced Density of States of Anatase TiO2 Crystal Surface

Owing to their fundamental importance and practical applications, anatase TiO2 crystals with well-defined {001} and {101} facets attracted intensive research interests. In this study, we systematically investigated solvent dependence of the photoreaction of the different coexposed crystal facets during noble metal photodeposition. By examining the deposition position in each solvent, we revealed that solvents play a pivotal role on the facet selectivity. On the basis of density functional theory calculations, the solvent molecules were found to modify both the crystal facet electronic structure and the {001}-{001} heterojunction. These modifications are not only the origin of diverse charge-carrier pathways but are also responsible for carrier accumulation at specific facets that increase their reductive power. These findings are vital for a better understanding of photocatalytic materials and an improved design for the next-generation materials.

Citrate-Capped Hybrid Au-TiO2 Nanomaterial for Facile and Enhanced Electrochemical Hydrazine Oxidation

Effective and facile electrochemical oxidation of chemical fuels is pivotal for fuel cell applications. Herein, we report the electrocatalytic oxidation of hydrazine on a citrate-capped Au-TiO₂-modified glassy carbon electrode, which follows two different oxidation paths. These two pathways of hydrazine oxidation are ascribed to occur on Au and the activated TiO₂ surface of the Au-TiO₂ hybrid electrocatalyst. This activation was achieved through molecular capping of the Au-TiO₂ surface by citrate, which leads to favorable hydrazine oxidation with a lower Tafel slope compared to that of the clean surface of the respective materials, that is, Au and TiO₂.

Open circuit potential build-up in perovskite solar cells from dark conditions to 1 sun

The high open-circuit potential (Voc) achieved by perovskite solar cells (PSCs) is one of the keys to their success. The Voc analysis is essential to understand their working mechanisms. A large number of CH3NH3PbI3-xClx PSCs were fabricated on single large-area substrates and their Voc dependencies on illumination intensity, I0, were measured showing three distinctive regions. Similar results obtained in Al2O3 based PSCs relate the effect to the compact TiO2 rather than the mesoporous oxide. We propose that two working mechanisms control the Voc in PSCs. The rise of Voc at low I0 is determined by the employed semiconductor n-type contact (TiO2 or MgO coated TiO2). In contrast, at I0 close to AM1.5G, the employed oxide does not affect the achieved voltage. Thus, a change of regime from an oxide-dominated EFn (as in the dye sensitized solar cells) to an EFn, directly determined by the CH3NH3PbI3-xClx absorber is suggested.

Photoinduced Reversible Structural Transformations in Free-Standing CH3NH3PbI3 Perovskite Films

In the pursuit to better understand the mechanisms of perovskite solar cells we performed Raman and photoluminescence measurements of free-standing CH3NH3PbI3 films, comparing dark with working conditions. The films, grown on a glass substrate and sealed by a thin glass coverslip, were measured subsequent to dark and white-light pretreatments. The extremely slow changes we observe in both the Raman and photoluminescence cannot be regarded as electronic processes, which are much faster. Thus, the most probable explanation is of slow photoinduced structural changes. The CH3NH3PbI3 transformation between the dark and the light structures is reversible, with faster rates for the changes under illumination. The results seem to clarify several common observations associated with solar cell mechanisms, like performance improvement under light soaking. More important is the call for solar-cell-related investigation of CH3NH3PbI3 to take the photoinduced structural changes into consideration when measuring and interpreting the results.

Modulating the interaction between gold and TiO2nanowires for enhanced solar driven photoelectrocatalytic hydrogen generation

The interaction strength of Au nanoparticles with pristine and nitrogen doped TiO₂nanowire surfaces was analysed using density functional theory and their significance in enhancing the solar driven photoelectrocatalytic properties was elucidated.

Green synthesis of anatase TiO(2) nanocrystals with diverse shapes and their exposed facets-dependent photoredox activity

The exposed facets of a crystal are known to be one of the key factors to its physical, chemical and electronic properties. Herein, we demonstrate the role of amines on the controlled synthesis of TiO2 nanocrystals (NCs) with diverse shapes and different exposed facets. The chemical, physical and electronic properties of the as-synthesized TiO2 NCs were evaluated and their photoredox activity was tested. It was found that the intrinsic photoredox activity of TiO2 NCs can be enhanced by controlling the chemical environment of the surface, i.e.; through morphology evolution. In particular, the rod shape TiO2 NCs with ∼25% of {101} and ∼75% of {100}/{010} exposed facets show 3.7 and 3.1 times higher photocatalytic activity than that of commercial Degussa P25 TiO2 toward the degradation of methyl orange and methylene blue, respectively. The higher activity of the rod shape TiO2 NCs is ascribed to the facetsphilic nature of the photogenerated carriers within the NCs. The photocatalytic activity of TiO2 NCs are found to be in the order of {101}+{100}/{010} (nanorods) > {101}+{001}+{100}/{010} (nanocuboids and nanocapsules) > {101} (nanoellipsoids) > {001} (nanosheets) providing the direct evidence of exposed facets-depended photocatalytic activity.

Extremely Slow Photoconductivity Response of CH3NH3PbI3 Perovskites Suggesting Structural Changes under Working Conditions

Photoconductivity measurements of CH3NH3PbI3 deposited between two dielectric-protected Au electrodes show extremely slow response. The CH3NH3PbI3, bridging a gap of ∼2000 nm, was subjected to a DC bias and cycles of 5 min illumination and varying dark duration. The approach to steady -state photocurrent lasted tens of seconds with a strong dependence on the dark duration preceding the illumination. On the basis of DFT calculations, we propose that under light + bias the methylammonium ions are freed to rotate and align along the electric field, thus modifying the structure of the inorganic scaffold. While ions alignment is expected to be fast, the adjustment of the inorganic scaffold seems to last seconds as reflected in the extremely slow photoconductivity response. We propose that under working conditions a modified, photostable, perovskite structure is formed, depending on the bias and illumination parameters. Our findings seem to clarify the origin of the well-known hysteresis in perovskite solar cells.

Relaxation of Electron Carriers in the Density of States of Nanocrystalline TiO2

Band gap localized states and surface states play a dominant role in the application of nanocrystalline metal oxides to photovoltaics and solar fuel production. Electrons injected in nanocrystalline TiO2 by voltage or photogeneration are mainly located in band gap states. Therefore, charging a nanoparticulate semiconductor network allows one to recover the density of states (DOS) in the energy axis. However, shallow traps remain in equilibrium with the conduction band electrons, while deep traps do not. We show that the characteristic peak of the apparent DOS mixes an exponential DOS and a monoenergetic surface state. A model that incorporates the trap's kinetics proves to be very efficient to assess the important parameters that determine both contributions via variation of charging rate. Contrary to the common theory, we demonstrate that the peculiar capacitance peak of nanocrystalline TiO2 can be mainly attributed, in some cases, to deep traps in the exponential distribution.