Ivan Turkevych

Photocatalytic generation of hydrogen by core-shell WO 3 /BiVO 4 nanorods with ultimate water splitting efficiency

Efficient photocatalytic water splitting requires effective generation, separation and transfer of photo-induced charge carriers that can hardly be achieved simultaneously in a single material. Here we show that the effectiveness of each process can be separately maximized in a nanostructured heterojunction with extremely thin absorber layer. We demonstrate this concept on WO3/BiVO4+CoPi core-shell nanostructured photoanode that achieves near theoretical water splitting efficiency. BiVO4 is characterized by a high recombination rate of photogenerated carriers that have much shorter diffusion length than the thickness required for sufficient light absorption. This issue can be resolved by the combination of BiVO4 with more conductive WO3 nanorods in a form of core-shell heterojunction, where the BiVO4 absorber layer is thinner than the carrier diffusion length while it’s optical thickness is reestablished by light trapping in high aspect ratio nanostructures. Our photoanode demonstrates ultimate water splitting photocurrent of 6.72u2009mAu2009cm−2 under 1 sun illumination at 1.23u2009VRHE that corresponds to ~90% of the theoretically possible value for BiVO4. We also demonstrate a self-biased operation of the photoanode in tandem with a double-junction GaAs/InGaAsP photovoltaic cell with stable water splitting photocurrent of 6.56u2009mAu2009cm−2 that corresponds to the solar to hydrogen generation efficiency of 8.1%.

Nanostructured WO3/BiVO4 Photoanodes for Efficient Photoelectrochemical Water Splitting

Nanostructured photoanodes based on well-separated and vertically oriented WO3 nanorods capped with extremely thin BiVO4 absorber layers are fabricated by the combination of Glancing Angle Deposition and normal physical sputtering techniques. The optimized WO3 -NRs/BiVO4 photoanode modified with Co-Pi oxygen evolution co-catalyst shows remarkably stable photocurrents of 3.2 and 5.1 mA/cm(2) at 1.23 V versus a reversible hydrogen electrode in a stable Na2 SO4 electrolyte under simulated solar light at the standard 1 Sun and concentrated 2 Suns illumination, respectively. The photocurrent enhancement is attributed to the faster charge separation in the electronically thin BiVO4 layer and significantly reduced charge recombination. The enhanced light trapping in the nanostructured WO3 -NRs/BiVO4 photoanode effectively increases the optical thickness of the BiVO4 layer and results in efficient absorption of the incident light.

Photocatalytic Properties of TiO2 Nanostructures Fabricated by Means of Glancing Angle Deposition and Anodization

Structural, optical, and photocatalytic properties of various TiO 2 nanostructures prepared by glancing angle deposition (GLAD) and by electrochemical anodic oxidation of Ti have been studied. The TiO 2 nanorods were prepared on unheated glass substrates by using reactive sputtering of Ti in the GLAD regime. TiO 2 nanotubes and brush-type nanostructures were fabricated by anodic oxidation of flat Ti films and Ti nanorods prepared by GLAD, respectively. The optical studies revealed that the nanotubes and brush-type nanostructures possess antireflection properties. The photocatalytic activity of TiO 2 nanostructures was characterized by following decomposition of isopropanol under visible and UV light irradiation and found to be significantly higher in nanostructured samples than in their flat counterparts. Also, TiO 2 nanotubes and brush-type nanostructures showed superior photocatalytic activity in comparison with nanorods due to a significantly higher specific surface area.

Photoelectric Properties of Silicon Nanocrystals/P3HT Bulk-Heterojunction Ordered in Titanium Dioxide Nanotube Arrays

A silicon nanocrystals (Si-ncs) conjugated-polymer-based bulk-heterojunction represents a promising approach for low-cost hybrid solar cells. In this contribution, the bulk-heterojunction is based on Si-ncs prepared by electrochemical etching and poly(3-hexylthiophene) (P3HT) polymer. Photoelectric properties in parallel and vertical device-like configuration were investigated. Electronic interaction between the polymer and surfactant-free Si-ncs is achieved. Temperature-dependent photoluminescence and transport properties were studied and the ratio between the photo- and dark-conductivity of 1.7 was achieved at ambient conditions. Furthermore the porous titanium dioxide (TiO2) nanotubes’ template was used for vertical order of photosensitive Si-ncs/P3HT-based blend. The anodization of titanium foil in ethylene glycol-based electrolyte containing fluoride ions and subsequent thermal annealing were used to prepare anatase TiO2nanotube arrays. The arrays with nanotube inner diameter of 90 and 50 nm were used for vertical ordering of the Si-ncs/P3HT bulk-heterojunction.

Photovoltaic Rudorffites: Lead-Free Silver Bismuth Halides Alternative to Hybrid Lead Halide Perovskites

Hybrid CPbX3 (C: Cs, CH3 NH3 ; X: Br, I) perovskites possess excellent photovoltaic properties but are highly toxic, which hinders their practical application. Unfortunately, all Pb-free alternatives based on Sn and Ge are extremely unstable. Although stable and non-toxic C2 ABX6 double perovskites based on alternating corner-shared AX6 and BX6 octahedra (A=Ag, Cu; B=Bi, Sb) are possible, they have indirect and wide band gaps of over 2u2005eV. However, is it necessary to keep the corner-shared perovskite structure to retain good photovoltaic properties? Here, we demonstrate another family of photovoltaic halides based on edge-shared AX6 and BX6 octahedra with the general formula Aa Bb Xx (x=a+3u2009b) such as Ag3 BiI6 , Ag2 BiI5 , AgBiI4 , AgBi2 I7 . As perovskites were named after their prototype oxide CaTiO3 discovered by Lev Perovski, we propose to name these new ABX halides as rudorffites after Walter Rüdorff, who discovered their prototype oxide NaVO2 . We studied structural and optoelectronic properties of several highly stable and promising Ag-Bi-I photovoltaic rudorffites that feature direct band gaps in the range of 1.79-1.83u2005eV and demonstrated a proof-of-concept FTO/c-m-TiO2 /Ag3 BiI6 /PTAA/Au (FTO: fluorine-doped tin oxide, PTAA: poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine], c: compact, m: mesoporous) solar cell with photoconversion efficiency of 4.3u2009%.

Hierarchically Organized Micro/Nano-Structures of TiO2

We have developed a technique for the fabrication of hierarchically organized micro/nano-structures of TiO2 by means of anodic oxidation of Ti nanorods prepared by glancing angle deposition. The fabricated nanostructures consist of small TiO2 nanotubes that form brush-type shells around long central oxide cores. This multiscale organization of the nanostructures satisfies requirements for large and accessible surface area while providing direct path for electrons, which are desirable for photocatalytic applications.

Ordered titanium dioxide nanotubes filled with photoluminescent surfactant-free silicon nanocrystals

The electrophoretic filling of titanium dioxide (TiO(2)) nanotubes with surfactant-free silicon nanocrystals (Si-ncs) dispersed in ethanol and the water is shown. We illustrate a simple and scalable room temperature approach that allows natural selection and deposition of the smallest-supernatant Si-ncs on TiO(2) nanotubular template. The anatase TiO(2) nanotubes with diameters of 50 nm were used to template Si-ncs with a higher (>10 times) deposition rate of Si-ncs dispersed in ethanol. Lower agglomeration in ethanol and the deposition of Si-ncs with smaller diameter deeper inside the TiO(2) nanotube resulted in a redshift of the photoluminescence maximum of about 60 nm. A potential application of the room temperature photoluminescent Si-ncs/TiO(2) nanocomposite for hybrid solar cells is demonstrated. Enhanced Si-ncs electrophoretic deposition in ethanol improved both the open-circuit photovoltage and short-circuit photocurrent.

Tandem photovoltaic–photoelectrochemical GaAs/InGaAsP–WO3/BiVO4 device for solar hydrogen generation

We demonstrated highly efficient solar hydrogen generation via water splitting by photovoltaic–photoelectrochemical (PV–PEC) tandem device based on GaAs/InGaAsP (PV cell) and WO3/BiVO4 core/shell nanorods (PEC cell). We utilized extremely thin absorber (ETA) concept to design the WO3/BiVO4 core/shell heterojunction nanorods and obtained the highest efficiencies of generation, separation and transfer of the photo-induced charge carriers that are possible for the WO3/BiVO4 material combination. The PV–PEC tandem shows stable water splitting photocurrent of 6.56 mAcm−2 under standard AM1.5G solar light that corresponds to the record solar-to-hydrogen (STH) conversion efficiency of 8.1%.

Ubiquitous element approach to plasmonic enhanced photocatalytic water splitting: the case of Ti@TiO2 core-shell nanostructure

We demonstrate a new approach to plasmonic enhanced photocatalytic water splitting by developing a novel core-shell Ti@TiO2 brush nanostructure where an elongated Ti nanorod forms a plasmonic core that concentrates light inside of a nanotubular anodic TiO2 shell. Following the ubiquitous element approach aimed at providing an enhanced device functionality without the usage of noble or rare earth elements, we utilized only inexpensive Ti to create a complex Ti@TiO2 nanostructure with an enhanced UV and Vis photocatalytic activity that emerges from the interplay between the surface plasmon resonance in the Ti core, Vis light absorption in the Ti-rich oxide layer at the Ti/TiO2 interface and UV light absorption in the nanotubular TiO2 shell.

From Extended Nanofluidics to an Autonomous Solar-Light-Driven Micro Fuel-Cell Device

Autonomous micro/nano mechanical, chemical, and biomedical sensors require persistent power sources scaled to their size. Realization of autonomous micro-power sources is a challenging task, as it requires combination of wireless energy supply, conversion, storage, and delivery to the sensor. Herein, we realized a solar-light-driven power source that consists of a micro fuel cell (μFC) and a photocatalytic micro fuel generator (μFG) integrated on a single microfluidic chip. The μFG produces hydrogen by photocatalytic water splitting under solar light. The hydrogen fuel is then consumed by the μFC to generate electricity. Importantly, the by-product water returns back to the photocatalytic μFG via recirculation loop without losses. Both devices rely on novel phenomena in extended-nano-fluidic channels that ensure ultra-fast proton transport. As a proof of concept, we demonstrate that μFG/μFC source achieves remarkable energy density of ca. 17.2u2005mWhu2009cm-2 at room temperature.