Vedran Jovic

Soft X-ray spectroscopic studies of the electronic structure of M:BiVO4 (M = Mo, W) single crystals

Bismuth vanadate, BiVO4, is a promising material for use as an anode in photoelectrochemical water splitting. However, its conversion efficiency is limited by poor bulk charge transport, which is via small-polarons. We report here the use of a suite of X-ray spectroscopic probes to determine the electronic structure of 0.3–0.6 at% M:BiVO4 (M = Mo or W). The results are interpreted in the context of current theories regarding the influence of doping on the existence of inter-band gap small-polaron states and their effect on the conversion efficiency of BiVO4. Preliminary X-ray absorption and emission measurements reveal that doping widens the band gap from 2.50 to 2.75 eV, whereas the indirect nature of the band gap remains unaffected. X-ray absorption spectroscopy verified that the doping levels did not affect the distorted tetrahedral environment of V5+ in BiVO4. For BiVO4 and W:BiVO4, V L3 resonant inelastic X-ray scattering showed energy loss features related to charge transfer from low lying valence metal/oxygen states to unoccupied V eg conduction band states. A 3.8 eV energy loss feature, coupled with small polaron-like peaks measured in valence band resonant photoelectron spectroscopy of M:BiVO4, point to the population of inter-band gap V 3d states of eg symmetry. The data reveals the existence of a band gap state in the absence of an applied bias in M:BiVO4, linked to small-polaron formation. We tentatively assign it as a deep trap state, which suggests that the improved conversion efficiency of M:BiVO4 relative to the undoped material is largely due to the increased carrier concentration in spite of increased carrier recombination rates.

Photocatalytic H2 Production from Ethanol over Au/TiO2 and Ag/TiO2

This paper compares the photocatalytic activities of Au/TiO2 (Au loadings 0-4 wt.%) and Ag/TiO2 photocatalysts (Ag loadings 0-4 wt.%) for H2 production from ethanol-water mixtures under UV irradiation. Au and Ag nanoparticles were deposited on commercially available Degussa P25 TiO2 (85% anatase, 15% rutile) using deposition-precipitation and liquid impregnation methods, respectively. TEM analyses showed the average noble metal nanoparticle size to be ~5 nm for the Au/TiO2 photocatalysts and ~3 nm for the Ag/TiO2 photocatalysts. Au/TiO2 photocatalysts showed a strong localised surface plasmon resonance (LSPR) at 570 nm characteristic for nanocrystalline Au. Complementary XRD studies confirmed that Au and Ag nanoparticles were present in metallic form. Photoluminescence measurements revealed that Au and Ag nanoparticles effectively suppress electron-hole recombination in TiO2, thereby enhancing the photocatalytic activity of TiO2 for hydrogen production. Au/TiO2 photocatalysts were far more active for H2 production from ethanol-water mixtures than Ag/TiO2 photocatalysts. A 1 wt.% Au/TiO2 photocatalyst yielded the highest H2 production rate (34 mmol g -1 h -1 ). Amongst the Ag/TiO2 photocatalysts, the 2 wt.% Ag/TiO2 sample was the most active (3.7 mmol g -1 h -1 ). Results are rationalised in terms of differences in the electronic properties of supported Au and Ag nanoparticles, with the former being near ideal for H2 production.

Observation of surface states on heavily indium-doped SnTe(111), a superconducting topological crystalline insulator

The topological crystalline insulator tin telluride is known to host superconductivity when doped with indium (Sn1-xInxTe), and for low indium content (x=0.04) it is known that the topological surface states are preserved. Here we present the growth, characterization, and angle resolved photoemission spectroscopy analysis of samples with much heavier In doping (up to x≈0.4), a regime where the superconducting temperature is increased nearly fourfold. We demonstrate that despite strong p-type doping, Dirac-like surface states persist.

Performance evaluation of Pd/TiO 2 and Pt/TiO 2 photocatalysts for hydrogen production from ethanol-water mixtures

This study compares the photocatalytic activity of Pd/TiO 2 and Pt/TiO 2 photocatalysts (metal loadings 0.5 wt.% and 2 wt.%) for H 2 production from ethanol-water mixtures under UV excitation. Cationic Pd(II) and Pt(IV)/Pt(II) species were deposited on Degussa P25 TiO 2 (85% anatase, 15% rutile) via the deposition-precipitation with urea method. Subsequent H 2 treatment at 350oC reduced the adsorbed cationic species to predominantly Pd 0 and Pt 0 , respectively. TEM analyses of the Pd/TiO 2 and Pt/TiO 2 photocatalysts revealed supported Pd and Pt nanoparticles of size <3 nm. XPS studies confirmed that the platinum was present primarily in metallic form, whereas palladium was present as both Pd 0 and PdO or Pd(OH) 2 . Photoluminescence measurements demonstrated that Pd and Pt nanoparticles suppress electron-hole pair recombination in TiO 2 by acting as electron acceptors. The Pt/TiO 2 and Pd/TiO 2 photocatalysts exhibited high activity and stability for H 2 production, with the optimum metal loadings being 0.5 wt.% for Pt (H 2 production rate = 41 mmol h –1 g –1 ) and 2 wt.% for Pd (H 2 production rate = 34 mmol h –1 g –1 ). Results confirm that the presence of zero valent Pd or Pt is critical for efficient H 2 production, and that both Pt/TiO 2 and Pd/TiO 2 photocatalysts are promising materials for solar H 2 production from biofuels.

How Indium Nitride Senses Water

The unique electronic band structure of indium nitride InN, part of the industrially significant III-N class of semiconductors, offers charge transport properties with great application potential due to its robust n-type conductivity. Here, we explore the water sensing mechanism of InN thin films. Using angle-resolved photoemission spectroscopy, core level spectroscopy, and theory, we derive the charge carrier density and electrical potential of a two-dimensional electron gas, 2DEG, at the InN surface and monitor its electronic properties upon in situ modulation of adsorbed water. An electric dipole layer formed by water molecules raises the surface potential and accumulates charge in the 2DEG, enhancing surface conductivity. Our intuitive model provides a novel route toward understanding the water sensing mechanism in InN and, more generally, for understanding sensing material systems beyond InN.

Improving solar hydrogen production with photonic band gap materials

Hydrogen production from renewable sources is the most promising method to secure energy for future generations.1 One approach to hydrogen production is photocatalysis, which requires the interaction between sunlight (photons) and a semiconducting material.2 Accordingly, the semiconducting material needs to maximally absorb photons, have a high work function metal layer to act as a Schottky barrier3—creating a reservoir of electrons to reduce hydrogen cations to molecular hydrogen— and have, in some cases, another metal oxide layer for fast oxidation of oxygen anions to molecular oxygen.4 Photoexcitation of a semiconductor material creates electrons and holes—charge carriers—that recombine on a timescale much faster than that required for the electron transfer reaction to occur.5 That is, we need to increase the lifetime of the charge carriers to make use of them in catalysis. Hence, photonic band gap (PBG) materials6 designed to have a periodic 3D crystal structure with unit cell dimensions close to the wavelength of the light7 have a key role in photocatalysis. The design of semiconductor photocatalysts with electronic band gaps (EBGs) that coincide with their PBGs ‘theoretically’ suppresses electron/hole recombination, a result of the lack of corresponding optical energy levels. This feature would make them ideal for photocatalysis and, therefore, the combination of slow photons at the edge of the PBG (increasing light absorption) and the inhibition of spontaneous light emission at the desired wavelength is poised to significantly increase photocatalytic reaction rates. Here, we describe our recent work with gold/titanium dioxide (Au/TiO2) PBG semiconductor materials.8, 9 Figure 1. Synthetic routes (top), transmission electron microscopy images (bottom left), and x-ray diffraction data (bottom right) of gold/titanium dioxide (Au/TiO2) photonic band gap semiconductor materials. CC: Colloidal crystals. HAuCl4.3H2O: Hydrated chloroauric acid.

Orbital orientation mapping of V2O5 thin films

We report the effects of growth methods on the orbital orientation in vanadium pentoxide (V2O5) thin films, an important factor to consider when selecting growth techniques for highly selective catalysts and devices. Thermal evaporation and sol-gel methods were used to synthesize the V2O5 films. The surface morphology, roughness, and orientation of the films were characterized by atomic force microscopy and x-ray diffraction. Surface electronic properties and oxidation states were assessed by x-ray photoemission spectroscopy. Polarized x-ray absorption spectroscopy demonstrated that the thermally evaporated film [which was in the (001) orientation] exhibited greater anisotropy than the (100) oriented sol-gel film. The observed increase in anisotropy agrees well with computational findings which revealed that more vanadyl bonds are present at the surface of the thermally evaporated film than at the surface of the sol-gel film. The same computational study also found that the orientation of these bonds is m...