We present a modular approach to the synthesis of nanostructured catalysts for photochemical splitting of water into hydrogen and oxygen. The catalysts are built from exfoliated, semiconducting niobate nanosheets derived from the layered perovskite HCa 2 Nb 3 O 10 . The latter is a catalyst for photochemical evolution of hydrogen from water under UV irradiation. After chemical modification with 3-aminopropyltrimethoxysilane (APS), IrO 2 or Pt particles can be attached to the nanosheets to produce various two-component nanostructures that were fully characterized with transmission electron microscopy and ultraviolet and infrared spectroscopy. Cyclic voltammetry was used to determine the onset potentials for O 2 and H 2 evolution. At pH ) 14, the observed values are in the range +0.61 to +1.24 V (NHE, water oxidation) and -1.36 to -1.62 V (NHE, water reduction). Under UV irradiation, all catalysts evolve hydrogen from water without any sign of deactivation for 5 h. The highest quantum efficiency of 3.49% is observed for a structure with Pt directly grown onto the nanosheets. No O 2 is evolved, which we attribute to the adsorption of O 2 to the catalyst surface. For Pt-[HCa 2 Nb 3 O 10 ], this process starts to shut down H 2 evolution after 9 h of constant irradiation, but the activity can be restored to >60% by evacuating the catalyst dispersion and purging it with Ar. Catalysts assembled from preformed citrate-coated Pt nanoparticles are slightly less active for H 2 evolution and so are catalysts that use the linker aminoethyl-aminoundecanetrimethoxysilane (AEAUS) instead of APS. The activity of IrO 2 -APS-[Ca 2 Nb 3 O 10 ] is lowest among two component catalysts, near the activities of the pure or APSmodified nanosheets. On the basis of XPS data, IrO 2 in this catalyst undergoes photochemical reduction to Ir(0) upon UV irradiation.
Using low energy electron microscopy, Au on Ge(111) is determined to follow a Stranski–Krastanov growth mode consisting of a single layer up to one monolayer (ML), followed by three-dimensional Au–Ge alloy droplets. Near 600 °C, we report the first observation of a reversible first-order phase transition that occurs from the (3 × 3)R30° phase to a (1 × 1) phase, which has a coverage of 0.367 ML. The transition gradually occurs through a coexistence region with a temperature range of about 2 °C and weakly depends on coverage, varying from 640 °C at 1 ML down to 580 °C at 0.8 ML. The phase transition is accompanied by phase fluctuations of small domains or the fluctuations of phase boundaries of large domains. At coverage >1 ML and above 250 °C, the 3D droplets move with stick-slip hopping behavior that has previously been explained by dissolution of Ge at step edges into the alloy droplet, which then comes to concentration and thermal equilibrium via the island motion.
The growth of the (3 Â 1) and (ͱ3 Â ͱ3)R30 phases of Ag on Ge(111) on substrates at temperatures from 540 to 660 C is characterized with low energy electron microscopy (LEEM) and low energy electron diffraction (LEED). From 540 C to the Ag desorption temperature of 575 C, LEEM images show that growth of the (3 Â 1) phase begins at step edges. Upon completion of the (3 Â 1) phase, the (ͱ3 Â ͱ3)R30 phase is observed with a dendritic growth morphology that is not much affected by steps. For sufficiently high deposition rates, Ag accumulates on the sample above the desorption temperature. From 575 to 640 C, the growth proceeded in a manner similar to that at lower temperatures, with growth of the (3 Â 1) phase to completion, followed by growth of the (ͱ3 Â ͱ3)R30 phase. Increasing the substrate temperature to 660 C resulted in only (3 Â 1) growth. In addition, for samples with Ag coverage less than 0.375ML, LEEM and LEED images were used to follow a reversible phase transformation near 575 C, between a mixed high coverage phase of [(4 Â 4) þ (3 Â 1)] and the high temperature, lower coverage (3 Â 1) phase.
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