Electrochemical Properties of Transparent Conducting Films of Tantalum-Doped Titanium Dioxide

Highly conducting, optically transparent Ta-doped TiO2 (anatase) thin films are grown on ordinary soda-lime glass substrate by pulsed-laser deposition. They exhibit quasi-reversible cyclic voltammograms of Fe(CN)63-/4- and dimethylviologen redox couples, mimicking the electrochemical activity of F-doped SnO2 (FTO). Hence, our Ta-doped titania films can prospectively replace FTO, e.g. in dye-sensitized and perovskite solar cells. However, these films are idle for the Ru(bpy)32+ oxidation, which is attributed to the space-charge barrier. The flatband potentials are near the values for undoped reference films and/or for the pristine anatase single-crystal electrode. Our films show photoelectrochemical activity upon irradiation with UV light at potentials positive to flatband. The photocurrents decrease proportionally to the increase of Ta-content. The Li-insertion ability analogously decreases with the increasing Ta-content which is attributed to the positive charge of Ta5+ cations which occupy the Ti4+ sites in anatase lattice. Consistent with the quasi-metallic nature of our films, the Li-extraction peak in cyclic voltammograms shows no cut at larger potentials

Dye-sensitization of boron-doped diamond foam: Champion photoelectrochemical performance of diamond electrodes under solar light illumination

Diamond foams composed of hollow spheres of polycrystalline boron-doped diamond are chemically modified with two donor-acceptor type molecular dyes, BT-Rho and CPDT-Fur, and tested as electrode materials for p-type dye-sensitized solar cells with an aqueous electrolyte solution containing methyl viologen as a redox mediator. Reference experiments with flat polycrystalline diamond electrodes evidence full blocking of the methyl viologen redox reaction by these dyes, whereas only partial blocking is observed for the diamond foams. This is ascribed to sp(2)-carbon impurities in the foam, viz. trans-polyacetylene and graphite-like carbon. Cathodic photocurrents under solar light illumination are about 3 times larger on foam electrodes compared to flat diamond. Long-term (1-2 days) illumination of the sensitized foam electrodes with chopped light at 1 sun intensity causes an increase of the cathodic photocurrent density to ca. 15-22 mu A cm(-2). These photocurrent densities represent the largest values reported so far for dye-sensitized diamond electrodes. The photoelectrochemical activation of the sensitized diamond electrodes is accompanied with characteristic changes of the dark voltammogram of the MV2+/MV+ redox couple and with gradual changes of the IPCE spectra

Diamond functionalization with light-harvesting molecular wires : improved surface coverage by optimized Suzuki cross-coupling conditions

Donor-acceptor type light-harvesting molecular wires are covalently attached to a boron-doped diamond surface via a combination of diazonium electrografting and Suzuki cross-coupling. For the Suzuki reaction, various catalytic systems are compared with respect to their imposed surface coverage. Combining 2-dicyclohexylphosphino-2,6-dimethoxybiphenyl (SPhos) and Pd(0), the diamond coverage improves considerably (by 98%) as compared to the standard tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)(4)) catalyst. As the energy levels between the molecular chromophores and the diamond film align well, the sophisticated functionalized diamond surfaces present a first step towards the development of fully carbon-based devices for light to electricity conversion.Funding Agencies|Special Research Fund of Hasselt University; Research Foundation Flanders (FWO) [G.0555.10N]; EU FP7 Collaborative Project "MOLESOL" [256617]; Swedish Research Council Linnaeus grant; Grant Agency of the Czech Republic [13-37383S]</p

Ochrana hematitových elektrod pomocí krycích TiO2 vrstev vytvořených ALD

Iron (III) oxide, in the form of hematite (alpha-Fe2O3), is a n-type semiconductor which is photoactive in the visible spectral region. Therefore, use in photoelectrocatalysis and photoassisted water electrolysis may be suggested. For such implementations, stability of contacts with liquid phases is mandatory. Hematite is stable in alkaline media but less stable in acidic media. For the first time the coverage of porous photoactive Sn doped hematite by thin capping layers of TiO2, deposited by Atomic Layer Deposition (ALD) and its impact on photocurrent and chemical stability of hematite is shown. The nominal thicknesses of the TiO2 ALD coatings were 0.5, 2 and 7.5 nm. The presence of the TiO2 coatings was evidenced by X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy (HR-TEM) and scanning TEM coupled with energy dispersive X-ray (EDX) spectroscopy. HR-TEM analyses revealed that the TiO2 capping layers were amorphous and conformal. Exposure of uncovered hematite layers to 1 M sulfuric acid led to a nominal dissolution rate of 0.23 nm/h which was halved when a TiO2 ALD coating (7.5 nm thin) was applied. Due to mismatch of the valence band positions of the two semiconductors, photocurrents were strongly diminished as the capping layer thickness was increased. Post calcination of as deposited ALD films on hematite resulted in an increase of photocurrent, which only exceeded photocurrents of pristine hematite when the ALD thickness was not more than 0.5 nm.Oxid železitý ve formě hematitu (α-Fe2O3) je polovodič n-typu a je fotoaktivní ve viditelné spektrální oblasti. Proto lze využít ve fotoelektrokatalýze a fotoasistovaném rozkladu vody. Pro tyto využití je důležitá jeho stabilita v kontaktu s kapalnou fází. Hematit je stabilní v alkalickém prostředí, ale méně stabilní v kyselém prostředí. V této práci poprvé ukazueme pokrytí porézního Sn-dopovaného hematitu tenkými krycími vrstvami TiO2, deponovanými depozicí atomárních vrstev (ALD) a dopad těchto vrstev na chemickou stabilitu a stabilitu fotoproudů. Nominální tlouštky ALD TiO2 vrstev byly 0.5, 2 a 7.5 nm. Přítomnost TiO2 vrstev byla detekována pomocí rentgenové fotoelektronové spektroskopie, transmisní elektronové mikroskopie s vysokým rozlišením (HR-TEM) a skenovací TEM s energiově disperzním detektorem (EDX). HR-TEM analýzy odhalili, že TiO2 vrstavy byly rovnoměrné a zárověn amorfní. Jejich expozice nepokrytého hematitu v 1M kyselině sírové způsobila rozpouštění o rychlosti 0.23 nm/h, přičemž tato hodnota poklesla o polovinu při pokrytí TiO2 vrstvou (7.5 nm). Díky energetické nevyrovnanosti valenčních pásů obou polovodičů docházelo ke graduálnímu poklesu vygenerovaných fotoproudů s rostoucí tloušťkou krycí vrstvy. Následné žíhání deponovaných hematitů zvýšilo fotoproudy, které byly v případě nejtenčí krycí vrstvy (0.5 nm) dokonce vyšší než fotoproudy samotného hematitu