The photoelectrochemical (PEC) activity of untreated (not anodized) and anodized nanotubular TiO 2 films (synthesized by the electrochemical anodization of Ti foil) was correlated with the phase composition of the film as a function of O 2 -annealing temperature at 400, 500 and 600 C. TiO 2 nanotubes have been shown to be more efficient than polycrystalline TiO 2 for the photocatalytic splitting of water. Raman spectroscopy was used to identify the amorphous and crystalline TiO 2 phases as well as the carbon species. The amorphous TiO 2 nanotubular array (unheated) exhibits a Raman spectrum consistent with TiO 6 8À octahedra having the same average structure as those present in the anatase and rutile phases of TiO 2 . Ratios of integrated Raman peaks were used as a semi-quantitative measure of the degree of crystallinity for rutile and the rutile/anatase weight ratio in the films. Results show that the anatase-to-rutile transformation on Ti metal initiates at much lower temperatures compared to polycrystalline TiO 2 and this is attributed to oxygen vacancies located at the metal/oxide interface. For untreated films, the amorphous TiO 2 crystallizes directly to rutile, and the photocurrent density increases almost linearly with rutile crystallinity as the O 2 -annealing temperature is increased; anatase does not form on untreated O 2 -annealed Ti foil. By comparison, amorphous TiO 2 nanotubular arrays are converted to about three times as much anatase as rutile at 400 C, where the photocurrent density is only slightly greater than the corresponding untreated film. At 500 C, however, the photocurrent density increases to 2.3Â that of the untreated-oxidized film, where $83% of the TiO 2 nanotubular film is rutile and $17% is anatase; this enhancement is attributed to the increase in surface area and photoactive sites of the rutile provided by the TiO 2 nanotubular array architecture acting as a support. At 600 C the rutile transformation continues ($92% rutile), but this is countered by the significant loss of surface area and surface photoactive sites due to degradation and collapse of the nanotubular structure as seen by SEM.

Organic–inorganic hybrid perovskite (PVSK) compounds are at the forefront of photovoltaic research, consistently surpassing silicon solar cells in power conversion efficiency. Possessing high refractive index, broad absorption spectrum, and superior quantum yields, hybrid PVSK thin films are theoretically also ideal candidates for luminescent solar concentrators (LSCs). In practice, however, the possibility of high self‐absorption in a continuous film, coupled with the inherent instability of PVSK materials, have hindered their use in this context. In this work, the viability of hybrid PVSK thin films as the active medium in planar LSCs is investigated. Using spectroscopic and photovoltaic measurements, variation of optical stability and device performance with different lead sources in the PVSK film precursors are monitored. The results display high optical efficiency in the range 15%–29% despite high self‐absorption losses, and the devices remain operational even after seven weeks in ambient conditions. Confirmed by Monte Carlo simulations, the superior performance is attributed to the high quantum yield and refractive index of the PVSKs. These results are encouraging not only for the implementation of PVSK thin films in LSCs, but additionally, for preparation of tandem devices to capture energy escaping as radiative exciton recombination in PVSK solar cells.

Hybrid nanocomposite films of ITO-coated, self-assembled porous nanostructures of tungsten trioxide (WO(3)) were fabricated using electrochemical anodization and sputtering. The morphology and chemical nature of the porous nanostructures were studied by Scanning Electron Microscopy (SEM) and X-ray Photoelectron Spectroscopy (XPS), respectively. The photoelectrochemical (PEC) properties of WO(3) porous nanostructures were studied in various alkaline electrolytes and compared with those of titania nanotubes. A new type of alkaline electrolyte containing a mixture of NaOH and KOH was proposed for the first time to the best of our knowledge and shown to improve the photocurrent response of the photoanodes. Here, we show that both the WO(3) nanostructures and titania nanotubes (used for comparison) exhibit superior photocurrent response in the mixture of NaOH and KOH than in other alkaline electrolytes. The WO(3) porous nanostructures suffered from surface corrosion resulting in a huge reduction in the photocurrent density as a function of time in the alkaline electrolytes. However, with a protective coating of ITO (100 nm), the surface corrosion of WO(3) porous nanostructures reduced drastically. A tremendous increase in the photocurrent density of as much as 340% was observed after the ITO was applied to the WO(3) porous nanostructures. The results suggest that the hybrid ITO/WO(3) nanocomposites could be potentially coupled with titania nanotubes in a multi-junction PEC cell to expand the light absorption capability in the solar spectrum for water splitting to generate hydrogen.

The nature of water interaction with tungsten nanorods (WNRs) fabricated by the glancing-angle deposition technique (GLAD)-using RF magnetron sputtering under various Ar pressures and substrate tilting angles and then subsequent coating with Teflon-has been studied and reported. Such nanostructured surfaces have shown strong water repellency properties with apparent water contact angles (AWCA) of as high as 160°, which were found to depend strongly upon the fabrication conditions. Variations in Ar pressure and the substrate tilting angle resulted in the generation of WNRs with different surface roughness and porosity properties. A theoretical model has been proposed to predict the observed high AWCAs measured at the nanostructure interfaces. The unique pyramidal tip geometry of WNRs generated at low Ar pressure with a high oblique angle reduced the solid fraction at the water interface, explaining the high AWCA measured on such surfaces. It was also found that the top geometrical morphologies controlling the total solid fraction of the WNRs are dependent upon and controlled by both the Ar pressure and substrate tilting angle. The water repellency of the tungsten nanorods with contact angles as high as 160° suggests that these coatings have enormous potential for robust superhydrophobic and anti-icing applications in harsh environments.

We have developed a framework for using temperature dependent static and dynamic photoluminescence (PL) of hybrid organic-inorganic perovskites (PVSKs) to characterize lattice defects in thin films, based on the presence of nanodomains at low temperature. Our high-stability PVSK films are fabricated using a novel continuous liquid interface propagation technique, and in the tetragonal phase (T > 120 K), they exhibit bi-exponential recombination from free charge carriers with an average PL lifetime of ∼200 ns. Below 120 K, the emergence of the orthorhombic phase is accompanied by a reduction in lifetimes by an order of magnitude, which we establish to be the result of a crossover from free carrier to exciton-dominated radiative recombination. Analysis of the PL as a function of excitation power at different temperatures provides direct evidence that the exciton binding energy is different in the two phases, and using these results, we present a theoretical approach to estimate this variable binding energy. Our findings explain this anomalous low temperature behavior for the first time, attributing it to an inherent fundamental property of the hybrid PVSKs that can be used as an effective probe of thin film quality.