Photogeneration of Hydrogen from Water by Hybrid Molybdenum Sulfide Clusters Immobilized on Titania

Two new hybrid molybdenum(IV) Mo3S7 cluster complexes derivatized with diimino ligands have been prepared by replacement of the two bromine atoms of [Mo3S7Br6]2− by a substituted bipyridine ligand to afford heteroleptic molybdenum(IV) Mo3S7Br4(diimino) complexes. Adsorption of the Mo3S7 cores from sample solutions on TiO2 was only achieved from the diimino functionalized clusters. The adsorbed Mo3S7 units were reduced on the TiO2 surface to generate an electrocatalyst that reduces the overpotential for the H2 evolution reaction by approximately 0.3 V (for 1 mA cm−2) with a turnover frequency as high as 1.4 s−1. The nature of the actual active molybdenum sulfide species has been investigated by X-ray photoelectron spectroscopy. In agreement with the electrochemical results, the modified TiO2 nanoparticles show a high photocatalytic activity for H2 production in the presence of Na2S/Na2SO3 as a sacrificial electron donor system.

Sol–gel copper chromium delafossite thin films as stable oxide photocathodes for water splitting

Significant effort is being devoted to the study of photoactive electrode materials for artificial photosynthesis devices. In this context, photocathodes promoting water reduction, based on earth-abundant elements and possessing stability under illumination, should be developed. Here, the photoelectrochemical behavior of CuCrO2 sol–gel thin film electrodes prepared on conducting glass is presented. The material, whose direct band gap is 3.15 eV, apparently presents a remarkable stability in both alkaline and acidic media. In 0.1 M HClO4 the material is significantly photoactive, with IPCE values at 350 nm and 0.36 V vs. RHE of over 6% for proton reduction and 23% for oxygen reduction. This response was obtained in the absence of charge extraction layers or co-catalysts, suggesting substantial room for optimization. The photocurrent onset potential is equal to 1.06 V vs. RHE in both alkaline and acidic media, which guarantees the combination of the material with different photoanodes such as Fe2O3 or WO3, potentially yielding bias-free water splitting devices.

Development of exfoliated layered stannosilicate for hydrogen adsorption

The use of hydrogen as an energy vector leads to the development of materials with high hydrogen adsorption capacity. In this work, a new layered stannosilicate, UZAR-S3, is synthesized and delaminated, producing UZAR-S4. UZAR-S3, with the empirical formula Na4SnSi5O14·3.5H2O and lamellar morphology, is a layered stannosilicate built from SnO6 and SiO4 polyhedra. The delamination process used here comprises three stages: protonation with acetic acid, swelling with nonylamine and the delamination itself with an HCl/H2O/ethanol solution. UZAR-S4 is composed of sheets a few nanometers thick with a high aspect ratio and a surface area of 236 m2/g, twenty times higher than that of UZAR-S3. At −196 °C for UZAR-S4, H2 adsorption reached remarkable values of 3.7 and 4.2 wt% for 10 and 40 bar, respectively, the latter value giving a high volumetric H2 storage capacity of 26.2 g of H2/L.

Desiccant capability of organic xerogels: Surface chemistry vs porous texture

Resorcinol-Formaldehyde xerogels are organic polymers that can be easily tailored to have specific properties. These materials are composed of carbon, hydrogen and oxygen, and have a surface that is very rich in oxygen functionalities, and is therefore very hydrophilic. Their most interesting feature is that they may have the same chemical composition but a different porous texture. Consequently, the influence of porous characteristics, such as pore volume, surface area or pore size can be easily assessed. In this work, a commonly used desiccant, silica gel, is compared with organic xerogels to determine their rate and capacity of water adsorption, and to evaluate the role of surface chemistry versus porous texture. It was found that organic xerogels showed a higher rate of moisture adsorption than silica gel. Pore structure also seems to play an important role in water adsorption capacity. The OX-10 sample, whose porosity was mainly composed of micro-mesoporosity displayed a water adsorption capacity two times greater than that of the silica gel, and three times higher than that of the totally macroporous xerogel OX-2100. The presence of feeder pores (mesopores) that facilitate the access to the hydrophilic surface was observed to be the key factor for a good desiccant behaviour. Neither the total pore volume nor the high surface area (i.e. high microporosity) of the desiccant sample, is as important as the mesopore structure.

Improving the photoactivity of bismuth vanadate thin film photoanodes through doping and surface modification strategies

Currently, one of the most attractive and desirable ways to solve the energy challenge is harvesting energy directly from the sunlight through the so-called artificial photosynthesis. Among the ternary oxides based on earth–abundant metals, bismuth vanadate has recently emerged as a promising photoanode. Herein, BiVO4 thin film photoanodes have been successfully synthesized by a modified metal-organic precursor decomposition method, followed by an annealing treatment. In an attempt to improve the photocatalytic properties of this semiconductor material for photoelectrochemical water oxidation, the electrodes have been modified (i) by doping with La and Ce (by modifying the composition of the BiVO4 precursor solution with the desired concentration of the doping element), and (ii) by surface modification with Au nanoparticles potentiostatically electrodeposited. La and Ce doping at concentrations of 1 and 2 at% in the BiVO4 precursor solution, respectively, enhances significantly the photoelectrocatalytic performance of BiVO4 without introducing important changes in either the material structure or the electrode morphology, according to XRD and SEM characterization. In addition, surface modification of the electrodes with Au nanoparticles further enhances the photocurrent as such metallic nanoparticles act as co-catalysts, promoting charge transfer at the semiconductor/solution interface. The combination of these two complementary ways of modifying the electrodes has resulted in a significant increase in the photoresponse, facilitating their potential application in artificial photosynthesis devices.

Structural and photoelectrochemical properties of porous TiO2 nanofibers decorated with Fe2O3 by sol-flame

The hybrid structure of Fe2O3 nanoparticles/TiO2 nanofibers (NFs), combines the merits of large surface areas of TiO2 NFs and absorption in ultraviolet light–visible light range. This structure can be used for many applications such as photoelectrochemical water splitting and photo-catalysis. Here, a sol-flame method is used for depositing Fe2O3 on TiO2 NFs that were prepared by hydrothermal on Ti sheets. The obtained materials were characterized by XRD, SEM, UV/Vis diffuse reflectance, Raman, and XPS. The results revealed the formation of rutile and anatase crystalline phases together with Fe2O3. This process moves the absorption threshold of TiO2 NFs support into visible spectrum range and enhances the photocurrent in comparison to bare TiO2 NFs, although no hole scavenger was used. The impedance measurement at low and high frequencies revealed an increase in series resistance and a decrease in resistance of charge transfer with sol-flame treatment time. A mechanism for explaining the charge transfer in these TiO2 NFs decorated with Fe2O3 nanoparticles was proposed.

Modification of Hematite Electronic Properties with Trimethyl Aluminum to Enhance the Efficiency of Photoelectrodes

The electronic properties of hematite were investigated by means of synchrotron radiation photoemission (SR-PES) and X-ray absorption spectroscopy (XAS). Hematite samples were exposed to trimethyl aluminum (TMA) pulses, a widely used Al-precursor for the atomic layer deposition (ALD) of Al2O3. SR-PES and XAS showed that the electronic properties of hematite were modified by the interaction with TMA. In particular, the hybridization of O 2p states with Fe 3d and Fe 4s4p changed upon TMA pulses due to electron inclusion as polarons. The change of hybridization correlates with an enhancement of the photocurrent density due to water oxidation for the hematite electrodes. Such an enhancement has been associated with an improvement in charge carrier transport. Our findings open new perspectives for the understanding and utilization of electrode modifications by very thin ALD films and show that the interactions between metal precursors and substrates seem to be important factors in defining their electronic and photoelectrocatalytic properties.