Metal oxides are generally very stable in aqueous solutions and cheap, but their photochemical activity is usually limited by poor charge carrier separation. Here we show that this problem can be solved by introducing a gradient dopant concentration in the metal oxide film, thereby creating a distributed n þ -n homojunction. This concept is demonstrated with a lowcost, spray-deposited and non-porous tungsten-doped bismuth vanadate photoanode in which carrier-separation efficiencies of up to 80% are achieved. By combining this state-ofthe-art photoanode with an earth-abundant cobalt phosphate water-oxidation catalyst and a double-or single-junction amorphous Si solar cell in a tandem configuration, stable shortcircuit water-splitting photocurrents of B4 and 3 mA cm À 2 , respectively, are achieved under 1 sun illumination. The 4 mA cm À 2 photocurrent corresponds to a solar-to-hydrogen efficiency of 4.9%, which is the highest efficiency yet reported for a stand-alone water-splitting device based on a metal oxide photoanode.
We unravel for the first time the origin of the poor carrier transport properties of BiVO 4 , a promising metal oxide photoanode for solar water splitting. Time-resolved microwave conductivity (TRMC) measurements reveal an (extrapolated) carrier mobility of ∼4 × 10 −2 cm 2 V −1 s −1 for undoped BiVO 4 under ∼1 sun illumination conditions, which is unusually low for a photoanode material. The poor carrier mobility is compensated by an unexpectedly long carrier lifetime of 40 ns. This translates to a relatively long diffusion length of 70 nm, consistent with the high quantum efficiencies reported for BiVO 4 photoanodes. Tungsten (W) doping is found to strongly decrease the carrier mobility by introducing intermediate-depth donor defects as carrier traps. At the same time, the increased carrier density improves the overall photoresponse, which confirms that bulk electronic conductivity is one of the main performance bottlenecks for BiVO 4 .
Hydrogen-air mixtures are highly flammable. Hydrogen sensors are therefore of paramount importance for timely leak detection during handling. However, existing solutions do not meet the stringent performance targets set by stakeholders, while deactivation due to poisoning, for example by carbon monoxide, is a widely unsolved problem. Here we present a plasmonic metal-polymer hybrid nanomaterial concept, where the polymer coating reduces the apparent activation energy for hydrogen transport into and out of the plasmonic nanoparticles, while deactivation resistance is provided via a tailored tandem polymer membrane. In concert with an optimized volume-to-surface ratio of the signal transducer uniquely offered by nanoparticles, this enables subsecond sensor response times. Simultaneously, hydrogen sorption hysteresis is suppressed, sensor limit of detection is enhanced, and sensor operation in demanding chemical environments is enabled, without signs of long-term deactivation. In a wider perspective, our work suggests strategies for next-generation optical gas sensors with functionalities optimized by hybrid material engineering.
Thin films of the high-temperature superconductor YBa 2 Cu 3 O 7 ؊ ␦ exhibit both a large critical current (the superconducting current density generally lies between 10 11 and 10 12 A m −2 at 4.2 K in zero magnetic field) and a decrease in such currents with magnetic field that point to the importance of strong vortex pinning along extended defects 1,2 . But it has hitherto been unclear which types of defect-dislocations, grain boundaries, surface corrugations and anti-phase boundaries-are responsible. Here we make use of a sequential etching technique to address this question. We find that both edge and screw dislocations, which can be mapped
Bismuth vanadate (BiVO4) is a promising semiconductor material for photoelectrochemical water splitting showing good visible light absorption and a high photochemical stability. To improve the performance of BiVO4, it is of key importance to understand its photophysics upon light absorption. Here we study the carrier dynamics of BiVO4 prepared by the spray pyrolysis method using broadband transient absorption spectroscopy (TAS), in thin films as well as in a photoelectrochemical (PEC) cell under water-splitting conditions. The use of a dual-laser setup consisting of electronically synchronized Ti:sapphire amplifiers enable us to measure the femtosecond to microsecond time scales in a single experiment. On the basis of this data, we propose a model of carrier dynamics that includes relaxation and trapping rates for electrons and holes. Hole trapping occurs in multiple phases, with the majority of the photogenerated holes being trapped with a time constant of 5 ps and a small fraction of this hole trapping taking place within the instrument response of 120 fs. The induced absorption band that represents the trapped holes is modulated by an oscillation of 63 cm–1, which is assigned to the coupling of holes to a phonon mode. We find electrons to undergo a relaxation with a time constant of 40 ps, followed by deeper trapping on the 2.5 ns time scale. On time scales longer than 10 ns, trap-limited recombination that follows a power law is found, spanning time scales up to microseconds. Finally, we observe no spectral or kinetic differences by applying a bias voltage to the PEC cell, indicating that the effect of a voltage and the charge transfer processes between BiVO4 and the electrolyte occurs on longer time scales. Our results therefore provide new insights into the carrier dynamics of BiVO4 and further expand the application window of TAS as an analytical tool for photoanode materials.
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