Effects of experimental parameters on the growth of GaN nanowires on Ti-film/Si(1 0 0) and Ti-foil by molecular beam epitaxy

Gallium nitride (GaN) nanostructures are used in optoelectronic applications due to their unique optical and electronic properties. For some optoelectronic applications and potential photocatalytic systems, the growth of GaN nanowires on metallic substrates instead of expensive single crystalline semiconductors can be beneficial due to specific properties of metals. In this study, GaN nanowire systems were grown on 300 nm Ti-film/Si(1 0 0) and Ti-foil by plasma assisted molecular beam epitaxy (PA-MBE) and characterized in situ by Auger electron spectroscopy (AES) and ex situ by scanning electron microscopy (SEM). Effects of (i) the nature of substrate surface, (ii) Ga flux, and (iii) substrate temperature on the growth of GaN nanowires were investigated. Nearly vertical nanowires can be grown on Ti-films covered with amorphous TiOx or TiOxNy, which is formed during the nitridation process. To grow nearly vertical nanowires on Ti-foils, pre-nitridation of the substrate surface was found to be important. The orientation of GaN nanowires grown on nitridated Ti-foil is determined by the grain alignment of the original Ti-foil, however, GaN nanowires grown on nitridated Ti-foils are uniformly oriented to one direction within an individual grain, which is most likely due to the epitaxial relation between the nanowires and the underneath grains of the polycrystalline Ti-foils. Both the orientation and nanowire density vary on different grains

Switch in photocatalytic reaction selectivity: The effect of oxygen partial pressure on carbon-carbon bond dissociation over hydroxylated TiO₂(1 1 0) surfaces

Photocatalytic oxidation of ethanol over rutile TiO₂ (1 1 0) in the presence of O₂ have been studied with scanning tunneling microscopy and on-line mass spectrometry to elucidate the reaction mechanisms. The O₂ partial pressure has a direct impact on C–C bond cleavage, resulting in a shift of selectivity in gas phase products from acetaldehyde (dehydrogenation) to methyl radicals (C–C bond dissociation) with increasing pressure. This differs from the behavior of anatase TiO₂(1 0 1) single crystal, where at all investigated pressures negligible C–C bond dissociation occurs. The prevalence of the methyl radical species at high oxygen pressures is correlated with an increase in the surface population of an adsorbed species bound to Ti₅_c after the reaction, which are identified as formate moieties. Parallel XPS C1s, Ti2p and O1s further confirmed the assignment of surface population, by STM, to ethoxides at 300 K, in dark conditions (C1s at 286.7 and 285.4 eV attributed to –CHO₂–and–CH₃ groups respectively). After photoreaction, a large fraction of the surface was covered by formates (XPS C1 at 289.7 eV). This also correlated with the STM assignment where species spaced by 6 Å along the [0 0 1] direction and with a height of ca. 1.1 Å attributed to formates. Moreover the profile for CH₃ radical desorption in the gas phase as a function O₂ partial pressures correlated with the increasing surface population of formates. Analysis of the rate of methyl radical formation reveals fast and slow regimes, with photoreaction cross-sections between 10⁻¹⁷ cm² and 10⁻¹⁹ cm². The parallel channel of acetaldehyde production has a non-varying cross-section of ca. 2 × 10⁻¹⁹ cm². A schematic description of the two different reaction channels (dehydrogenation and C–C bond dissociation) is given and discussed

Up-conversion luminescence coupled to plasmonic gold nanorods for light harvesting and hydrogen production

The conversion of infrared light to visible-light which allows a larger fraction of sun-light to be used is needed to improve light-harvesting. In this work a tri-functional material composed of an up-converter (NaYF4–Yb–Tm), plasmonic gold nanorods and CdS was made photocatalytically active using 980 nm wavelength light for the reduction of H+ to H2.Peer ReviewedPostprint (author's final draft

Direct Visualization of a Gold Nanoparticle Electron Trapping Effect

A new atomic-scale anisotropy in the photoreaction of surface carboxylates on rutile TiO2(110) induced by gold clusters is found. STM and DFT+U are used to study this phenomenon by monitoring the photoreaction of a prototype hole-scavenger molecule, benzoic acid, over stoichiometric (s) s-TiO2, Au9/s-TiO2, and reduced (r) Au9/r-TiO2. STM results show that benzoic acid adsorption displaces a large fraction of Au clusters from the terraces toward their edges. DFT calculations explain that Au9 clusters on stoichiometric TiO2 are distorted by benzoic acid adsorption. The influence of sub-monolayers of Au on the UV/visible photoreaction of benzoic acid was explored at room temperature, with adsorbate depletion taken as a measure of activity. The empty sites, observed upon photoexcitation, occurred in elongated chains (2 to 6 molecules long) in the [11̅0] and [001] directions. A roughly 3-fold higher depletion rate is observed in the [001] direction. This is linked to the anisotropic conduction of excited electrons along [001], with subsequent trapping by Au clusters leaving a higher concentration of holes and thus an increased decomposition rate. To our knowledge this is the first time that atomic-scale directionality of a chemical reaction is reported upon photoexcitation of the semiconductor

Gold Cluster Coverage Effect on H2 Production over Rutile TiO2(110)

Unlike catalytic reactions thermally driven by metal nanoparticles, reaction rates in photocatalysis do not scale with either the density of nanoparticles or their size. Because of the complexity of multicomponent photocatalysts in powder form, this lack of correlation, routinely observed for decades, is still not understood. In order to explore this phenomenon, H2 production from ethanol over Au clusters with different coverages deposited on single-crystal rutile TiO2(110) were studied by scanning tunneling microscopy and online mass spectrometry. There is a nonlinear increase of the H2 production with increasing gold coverage. The key determining factor appears to be the Au intercluster distance. Increasing this distance resulted in an increase in the normalized production. These results are explained in terms of competition between clusters for excited electrons to reduce H+ (of surface OH groups) to H2. It was possible to determine the proportionality factor between the hydrogen production and the number of absorbed photons. A slope close to 1 is found, which is in line with the "current doubling effect" in electrocatalysis. Moreover, pump probe transient absorption spectroscopy measurements were conducted. The results show that excited electrons transfer from the conduction band of TiO2 to Au particles within the first picoseconds after UV excitation. The fact that Au metal intercluster distances directly affect the reaction rate indicates that there is an optimum arrangement between the metal and the semiconductor that could potentially be achieved by nanostructuring

Mechanism of Ethanol Photooxidation on Single-Crystal Anatase TiO 2

Despite the proven properties of the anatase phase of TiO2 related to photocatalysis, detailed mechanistic information regarding a photooxidation reaction has not yet been derived from single-crystal studies. In this work, we have studied the photooxidation of ethanol (as a prototype hole-scavenger organic molecule) adsorbed on the anatase TiO2(101) surface by STM and online mass spectrometry to determine the adsorbate species in the dark and under UV illumination in the presence of O2 and to extract kinetic reaction parameters under photoexcitation. The reaction rate for the photooxidation of ethanol to acetaldehyde was found to depend on the O2 partial pressure and surface coverage, where the order of the reaction with respect to O2 is close to 0.15. Carbon–carbon bond dissociation leading to the formation of CH3 radicals in the gas phase was found to be a minor pathway, which is contrary to the case of the rutile TiO2(110) single crystal. Our STM images distinguished two types of surface adsorbates upon ethanol exposure that can be attributed to its molecular and dissociative modes. A mixed adsorption is also supported by our DFT calculations, in which we determined similar energies of adsorption (Eads) for the molecular (1.11 eV) and dissociative (0.93 eV) modes. Upon UV exposure at (and above) 3 × 10–8 mbar O2, a third species was identified on the surface as a reaction product that can be tentatively attributed to acetate/formate species on the basis of C 1s XPS results. The kinetics of the initial oxidation steps were evaluated using the STM and mass spectrometry data

Up-conversion luminescence coupled to plasmonic gold nanorods for light harvesting and hydrogen production

The conversion of infrared light to visible-light which allows a larger fraction of sun-light to be used is needed to improve light-harvesting. In this work a tri-functional material composed of an up-converter (NaYF4–Yb–Tm), plasmonic gold nanorods and CdS was made photocatalytically active using 980 nm wavelength light for the reduction of H+ to H2.Peer Reviewe