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Register a new userCompositional gradients in sputtered Ti-Au alloys: Site-selective Au-decoration of anodic TiO$_2$ nanotubes towards enhanced photocatalytic H$_2$ evolution
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Au nanoparticles at the TiO$_2$ surface can enhance the photocatalytic H$_2$ generation performances owing to their electron transfer co-catalytic ability. Key to maximize the co-catalytic effect is a fine control over Au nanoparticle size and placement on the photocatalyst, in relation to parameters such as the TiO$_2$ morphology, illumination wavelength and pathway, and light penetration depth in the photocatalyst. Here we present an approach for site-selective intrinsic-decoration of anodic TiO$_2$ nanotubes (TNs) with Au nanoparticles: we produce, by Ti and Au co-sputtering, Ti-Au alloy layers that feature compositional gradients across their thickness; these layers, when anodized under self-ordering electrochemical conditions, can form Au-decorated TNs where the Au nanoparticle density and placement vary according to the Au concentration profile in the metal alloy substrates. Our results suggest that, the Au co-catalyst placement strongly affects the photocatalytic H$_2$ evolution performance of the TNs layers. We demonstrate that, when growing Au-decorated TNs, the use of Ti-Au substrates with a suitable Au compositional gradient can lead to higher H$_2$ evolution rates compared to TNs classically grown with a homogenous co-catalyst decoration. As a side effect, a proper placement of the co-catalyst nanoparticles allows for reducing the amount of noble metal without dumping the H$_2$ evolution activity.
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Gold-decorated TiO$_2$ nanotubes were used for the photocatalytic abatement of Hg(II) in aqueous solutions. The presence of dewetted Au nanoparticles induces a strong enhancement of photocatalytic reduction and scavenging performances, with respect to naked TiO$_2$. In the presence of chlorides, a massive formation of Hg2Cl2 nanowires, produced from Au nanoparticles, was observed using highly Au loaded photocatalysts to treat a 10 ppm Hg(II) solution. EDS and XPS confirmed the nature of the photo-produced nanowires. In the absence of chlorides and/or at lower Hg(II) starting concentrations, the scavenging of mercury proceeds through the formation of Hg-Au amalgams. Solar light driven Hg(II) abatements up to 90% were observed after 24h. ICP-MS analysis revealed that the removed Hg(II) is accumulated on the photocatalyst surface. Regeneration of Hg-loaded exhaust photocatalysts was easily performed by anodic stripping of Hg(0) and Hg(I) to Hg(II). After four catalytic-regeneration cycles only a 10% decrease of activity was observed.
In this study, we investigate noble metal free photocatalytic water splitting on natural anatase single crystal facets and on wafer slices of the [001] plane before and after these surfaces have been modified by high pressure hydrogenation (HPH) and hydrogen ion-implantation. We find that on the natural, intact low index planes photocatalytic H$_2$ evolution (in absence of noble metal co-catalyst) can only be achieved when the hydrogenation treatment is accompanied by the introduction of crystal damage, such as simple scratching, miscut in the wafer or by implantation damage. X-ray reflectivity (XRR), Raman, and optical reflection measurements show that plain hydrogenation leads to a ~ 1 nm thick black titania surface layer without activity, while a colorless, density modified and ~ 7 nm thick layer with broken crystal symmetry is present in the ion implanted surface. These results demonstrate that i) the H-treatment of an intact anatase surface needs to be combined with defect formation for catalytic activation, and ii) activation does not necessarily coincide with the presence of black color.
Flat TiO$_2$ layers are deposited by magnetron sputtering on Ti/Si wafers. The TiO$_2$ surfaces are then sputter-coated with thin Au films of a nominal thickness of 0.5-10 nm that are converted by solid-state dewetting into Au nanoparticles of tuneable size and spacing; the Au nanoparticle size can be tuned over a broad range, i.e. ca. 3-200 nm. The Au-decorated TiO$_2$ surfaces enable plasmonic photo-electrochemical water splitting under visible light illumination (450-750 nm). The water splitting performance reaches a maximum for TiO$_2$ layers decorated with ~ 30 nm-sized Au particles. As expected, optical absorption measurements show a red shift of the plasmonic extinction band with increasing the Au nanoparticle size. However, the plasmonic photocurrent is found to peak at ~ 600 nm regardless of the size of the Au nanoparticles, i.e. the plasmonic photocurrent band position is size-independent. Such a remarkable observation can be ascribed to a hot electron injection cut-off effect.
We investigate the photocatalytic performance of nanocomposites prepared in a one-step process by liquid-phase exfoliation of graphite in the presence of TiO$_2$ nanoparticles (NPs) at atmospheric pressure and in water, without heating or adding any surfactant, and starting from low-cost commercial reagents. The nanocomposites show enhanced photocatalytic activity, degrading up to 40$%$ more pollutants with respect to the starting TiO$_2$-NPs. In order to understand the photo-physical mechanisms underlying this enhancement, we investigate the photo-generation of reactive species (trapped holes and electrons) by ultrafast transient absorption spectroscopy. We observe an electron transfer process from TiO$_2$ to the graphite flakes within the first picoseconds of the relaxation dynamics, which causes the decrease of the charge recombination rate, and increases the efficiency of the reactive species photo-production.
Au and Pt do not form homogeneous bulk alloys as they are thermodynamically not miscible. However, we show that anodic TiO$_2$ nanotubes (NTs) can in-situ be uniformly decorated with homogeneous AuPt alloy nanoparticles (NPs) during their anodic growth. For this, a metallic Ti substrate containing low amounts of dissolved Au (0.1 at%) and Pt (0.1 at%) is used for anodizing. The matrix metal (Ti) is converted to oxide while at the oxide/metal interface direct noble metal particle formation and alloying of Au and Pt takes place; continuously these particles are then picked up by the growing nanotube wall. In our experiments the AuPt alloy NPs have an average size of 4.2 nm and, at the end of the anodic process, are regularly dispersed over the TiO$_2$ nanotubes. These alloyed AuPt particles act as excellent co-catalyst in photocatalytic H2 generation - with a H2 production of 12.04 {mu}L h-1 under solar light. This represents a strongly enhanced activity as compared with TiO$_2$ NTs decorated with monometallic particles of Au (7 {mu}L h-1) or Pt (9.96 {mu}L h-1).
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