The quenching effect of uranyl species in the photoluminescence emission and visible-light-driven water dissociation activity of CdS and TiO2 photocatalysts.

Anchoring of uranyl species (2-4 mol%) led to the complete quenching of photoluminescence emission and the visible-light-driven water photodissociation activity of TiO2 (Degussa-P25) and a hydrothermally synthesized CdS photocatalyst. Photophysical measurements revealed a fast relaxation and the transfer of photogenerated electrons/energy from the TiO2 or CdS substrate to the acceptor uranyl moieties. Besides the position of flat band levels, significant overlap of the absorption/emission spectra of the host matrix and the surface-bound UO2(2+) species is responsible for this reverse energy transfer from a wide bandgap semiconductor to a material with a smaller band-to-band energy gap.

NiS1.97: A New Efficient Water Oxidation Catalyst for Photoelectrochemical Hydrogen Generation.

NiS1.97, a sulfur-deficient dichalcogenide, in nanoscale form, is shown to be a unique and efficient photoelectrochemical (PEC) catalyst for H2 generation by water splitting. Phase pure NiS1.97 nanomaterial is obtained by converting nickel oxide into sulfide by controlled sulfurization method, which is otherwise difficult to establish. The defect states (sulfur vacancies) in this material increase the carrier density and in turn lead to favorable band line-up with respect to redox potential of water, rendering it to be an effective photoelectrochemical catalyst. The material exhibits a remarkable PEC performance of 1.25 mA/cm(2) vs NHE at 0.68 V in neutral pH, which is almost 1000 times superior as compared with that of the stoichiometric phase of NiS2. The latter is well-known to be a cocatalyst but not as a primary PEC catalyst.

Water electrolysis with a conducting carbon cloth: subthreshold hydrogen generation and superthreshold carbon quantum dot formation.

A conducting carbon cloth, which has an interesting turbostratic microstructure and functional groups that are distinctly different from other ordered forms of carbon, such as graphite, graphene, and carbon nanotubes, was synthesized by a simple one-step pyrolysis of cellulose fabric. This turbostratic disorder and surface chemical functionalities had interesting consequences for water splitting and hydrogen generation when such a cloth was used as an electrode in the alkaline electrolysis process. Importantly, this work also gives a new twist to carbon-assisted electrolysis. During electrolysis, the active sites in the carbon cloth allow slow oxidation of its surface to transform the surface groups from COH to COOH and so forth at a voltage as low as 0.2 V in a two-electrode system, along with platinum as the cathode, instead of 1.23 V (plus overpotential), which is required for platinum, steel, or even graphite anodes. The quantity of subthreshold hydrogen evolved was 24 mL cm(-2) h(-1) at 1 V. Interestingly, at a superthreshold potential (>1.23 V+overpotential), another remarkable phenomenon was found. At such voltages, along with the high rate and quantity of hydrogen evolution, rapid exfoliation of the tiny nanoscale (5-7 nm) units of carbon quantum dots (CQDs) are found in copious amounts due to an enhanced oxidation rate. These CQDs show bright-blue fluorescence under UV light.