The role of carbon dots - derived underlayer in hematite photoanodes.

Hematite is a promising candidate as photoanode for solar-driven water splitting, with a theoretically predicted maximum solar-to-hydrogen conversion efficiency of ∼16%. However, the interfacial charge transfer and recombination greatly limits its activity for photoelectrochemical water splitting. Carbon dots exhibit great potential in photoelectrochemical water splitting for solar to hydrogen conversion as photosensitisers and co-catalysts. Here we developed a novel carbon underlayer from low-cost and environmental-friendly carbon dots through a facile hydrothermal process, introduced between the fluorine-doped tin oxide conducting substrate and hematite photoanodes. This led to a remarkable enhancement in the photocurrent density. Owing to the triple functional role of carbon dots underlayer in improving the interfacial properties of FTO/hematite and providing carbon source for the overlayer as well as the change in the iron oxidation state, the bulk and interfacial charge transfer dynamics of hematite are significantly enhanced, and consequently led to a remarkable enhancement in the photocurrent density. The results revealed a substantial improvement in the charge transfer rate, yielding a charge transfer efficiency of up to 80% at 1.25 V vs. RHE. In addition, a significant enhancement in the lifetime of photogenerated electrons and an increased carrier density were observed for the hematite photoanodes modified with a carbon underlayer, confirming that the use of sustainable carbon nanomaterials is an effective strategy to boost the photoelectrochemical performance of semiconductors for energy conversion.

Carbon Nitride Materials as Efficient Catalyst Supports for Proton Exchange Membrane Water Electrolyzers.

Carbon nitride materials with graphitic to polymeric structures (gCNH) were investigated as catalyst supports for the proton exchange membrane (PEM) water electrolyzers using IrO2 nanoparticles as oxygen evolution electrocatalyst. Here, the performance of IrO2 nanoparticles formed and deposited in situ onto carbon nitride support for PEM water electrolysis was explored based on previous preliminary studies conducted in related systems. The results revealed that this preparation route catalyzed the decomposition of the carbon nitride to form a material with much lower N content. This resulted in a significant enhancement of the performance of the gCNH-IrO2 (or N-doped C-IrO2) electrocatalyst that was likely attributed to higher electrical conductivity of the N-doped carbon support.

One-Step Synthesis, Structure, and Band Gap Properties of SnO2 Nanoparticles Made by a Low Temperature Nonaqueous Sol-Gel Technique.

Because of its electrically conducting properties combined with excellent thermal stability and transparency throughout the visible spectrum, tin oxide (SnO2) is extremely attractive as a transparent conducting material for applications in low-emission window coatings and solar cells, as well as in lithium-ion batteries and gas sensors. It is also an important catalyst and catalyst support for oxidation reactions. Here, we describe a novel nonaqueous sol-gel synthesis approach to produce tin oxide nanoparticles (NPs) with a low NP size dispersion. The success of this method lies in the nonhydrolytic pathway that involves the reaction between tin chloride and an oxygen donor, 1-hexanol, without the need for a surfactant or subsequent thermal treatment. This one-pot procedure is carried out at relatively low temperatures in the 160-260 °C range, compatible with coating processes on flexible plastic supports. The NP size distribution, shape, and dislocation density were studied by powder X-ray powder diffraction analyzed using the method of whole powder pattern modeling, as well as high-resolution transmission electron microscopy. The SnO2 NPs were determined to have particle sizes between 3.4 and 7.7 nm. The reaction products were characterized using liquid-state 13C and 1H nuclear magnetic resonance (NMR) that confirmed the formation of dihexyl ether and 1-chlorohexane. The NPs were studied by a combination of 13C, 1H, and 119Sn solid-state NMR as well as Fourier transform infrared (FTIR) and Raman spectroscopy. The 13C SSNMR, FTIR, and Raman data showed the presence of organic species derived from the 1-hexanol reactant remaining within the samples. The optical absorption, studied using UV-visible spectroscopy, indicated that the band gap (E g) shifted systematically to lower energy with decreasing NP sizes. This unusual result could be due to mechanical strains present within the smallest NPs perhaps associated with the organic ligands decorating the NP surface. As the size increased, we observed a correlation with an increased density of screw dislocations present within the NPs that could indicate relaxation of the stress. We suggest that this could provide a useful method for band gap control within SnO2 NPs in the absence of chemical dopants.

Erratum: One-Step Synthesis, Structure, and Band Gap Properties of SnO2 Nanoparticles Made by a Low Temperature Nonaqueous Sol-Gel Technique.

[This corrects the article DOI: 10.1021/acsomega.8b02122.].

Fe-N-Doped Carbon Capsules with Outstanding Electrochemical Performance and Stability for the Oxygen Reduction Reaction in Both Acid and Alkaline Conditions.

High surface area N-doped mesoporous carbon capsules with iron traces exhibit outstanding electrocatalytic activity for the oxygen reduction reaction in both alkaline and acidic media. In alkaline conditions, they exhibit more positive onset (0.94 V vs RHE) and half-wave potentials (0.83 V vs RHE) than commercial Pt/C, while in acidic media the onset potential is comparable to that of commercial Pt/C with a peroxide yield lower than 10%. The Fe-N-doped carbon catalyst combines high catalytic activity with remarkable performance stability (3500 cycles between 0.6 and 1.0 V vs RHE), which stems from the fact that iron is coordinated to nitrogen. Additionally, the newly developed electrocatalyst is unaffected by the methanol crossover effect in both acid and basic media, contrary to commercial Pt/C. The excellent catalytic behavior of the Fe-N-doped carbon, even in the more relevant acid medium, is attributable to the combination of chemical functions (N-pyridinic, N-quaternary, and Fe-N coordination sites) and structural properties (large surface area, open mesoporous structure, and short diffusion paths), which guarantees a large number of highly active and fully accessible catalytic sites and rapid mass-transfer kinetics. Thus, this catalyst represents an important step forward toward replacing Pt catalysts with cheaper alternatives. In this regard, an alkaline anion exchange membrane fuel cell was assembled with Fe-N-doped mesoporous carbon capsules as the cathode catalyst to provide current and power densities matching those of a commercial Pt/C, which indicates the practical applicability of the Fe-N-carbon catalyst.

Anion order in perovskite oxynitrides.

Transition-metal oxynitrides with perovskite-type structures are an emerging class of materials with optical, photocatalytic, dielectric and magnetoresistive properties that may be sensitive to oxide-nitride order, but the anion-ordering principles were unclear. Here we report an investigation of the representative compounds SrMO(2)N (M = Nb, Ta) using neutron and electron diffraction. This revealed a robust 1O/2(O(0.5)N(0.5)) partial anion order (up to at least 750 °C in the apparently cubic high-temperature phases) that directs the rotations of MO(4)N(2) octahedra in the room-temperature superstructure. The anion distribution is consistent with local cis-ordering of the two nitrides in each octahedron driven by covalency, which results in disordered zigzag M-N chains in planes within the perovskite lattice. Local structures for the full range of oxynitride perovskites are predicted and a future challenge is to tune properties by controlling the order and dimensionality of the anion chains and networks.