Quasi-homogenous photocatalysis of quantum-sized Fe-doped TiO2_{2} in optically transparent aqueous dispersions

In this study, the preparation of anatase TiO2 nanocrystals via a facile non-aqueous sol–gel route and their characterization are reported. The 3–4 nm particles are readily dispersable in aqueous media and show excellent photoreactivity in terms of rhodamine B degradation. The catalytic performance can be further increased considerably by doping with iron and UV-light irradiation as a pre-treatment. The effect of surface ligands (blocked adsorption sites, surface defects etc.) on the photoreactivity was thoroughly probed using thermogravimetric analysis combined with mass spectrometry. Photoelectrochemical characterization of thin-film electrodes made from the same TiO2 nanocrystals showed the opposite trend to the catalytic experiments, that is, a strong decrease in photocurrent and quantum efficiency upon doping due to introduction of shallow defect states

Fabrication of nanocrystalline high-entropy oxide CoNiFeCrMnOₓ thin film electrodes by dip-coating for oxygen evolution electrocatalysis

The variation of calcination conditions allows for adjusting the oxidation states and coordination sites of near-surface elements in sol–gel-derived high-entropy oxides. Optimized synthesis conditions resulted in fabrication of nanocrystalline CoNiFeCrMnOₓ thin films with 1–2 nm pores, showing low OER overpotentials of 258 mV vs. RHE at 10 mA cm⁻²

Sol‐Gel‐Derived Ordered Mesoporous High Entropy Spinel Ferrites and Assessment of Their Photoelectrochemical and Electrocatalytic Water Splitting Performance

The novel material class of high entropy oxides with their unique and unexpected physicochemical properties is a candidate for energy applications. Herein, it is reported for the first time about the physico‐ and (photo‐) electrochemical properties of ordered mesoporous (CoNiCuZnMg)Fe₂O₄ thin films synthesized by a soft‐templating and dip‐coating approach. The A‐site high entropy ferrites (HEF) are composed of periodically ordered mesopores building a highly accessible inorganic nanoarchitecture with large specific surface areas. The mesoporous spinel HEF thin films are found to be phase‐pure and crack‐free on the meso‐ and macroscale. The formation of the spinel structure hosting six distinct cations is verified by X‐ray‐based characterization techniques. Photoelectron spectroscopy gives insight into the chemical state of the implemented transition metals supporting the structural characterization data. Applied as photoanode for photoelectrochemical water splitting, the HEFs are photostable over several hours but show only low photoconductivity owing to fast surface recombination, as evidenced by intensity‐modulated photocurrent spectroscopy. When applied as oxygen evolution reaction electrocatalyst, the HEF thin films possess overpotentials of 420 mV at 10 mA cm⁻² in 1 m KOH. The results imply that the increase of the compositional disorder enhances the electronic transport properties, which are beneficial for both energy applications

Quasi-homogenous photocatalysis of quantum-sized Fe-doped TiO₂ in optically transparent aqueous dispersions

In this study, the preparation of anatase TiO₂ nanocrystals via a facile non-aqueous sol–gel route and their characterization are reported. The 3–4 nm particles are readily dispersable in aqueous media and show excellent photoreactivity in terms of rhodamine B degradation. The catalytic performance can be further increased considerably by doping with iron and UV-light irradiation as a pre-treatment. The effect of surface ligands (blocked adsorption sites, surface defects etc.) on the photoreactivity was thoroughly probed using thermogravimetric analysis combined with mass spectrometry. Photoelectrochemical characterization of thin-film electrodes made from the same TiO₂ nanocrystals showed the opposite trend to the catalytic experiments, that is, a strong decrease in photocurrent and quantum efficiency upon doping due to introduction of shallow defect states

Resolving a structural issue in cerium-nickel-based oxide

CeNiO3 has been reported in the literature in the last few years as a novel LnNiO3 compound with promising applications in different catalytic fields, but its structure has not been correctly reported so far. In this research, CeNiO3 (RB1), CeO2 and NiO have been synthesized in a nanocrystalline form using a modified citrate aqueous sol-gel route. A direct comparison between the equimolar physical mixture (n(CeO2) : n(NiO) = 1:1) and a compound RB1 was made. Their structural differences were investigated by laboratory powder X-ray diffraction (PXRD), selected area electron diffraction (SAED), transmission electron microscopy (TEM) with energy-dispersive X-ray spectroscopy detector (EDS), and Raman spectroscopy. The surface of the compounds was analyzed by X-ray photoelectron spectroscopy (XPS), while the thermal behaviour was explored by thermogravimetric analysis (TGA). Their magnetic properties were also investigated with the aim of exploring the differences between these two compounds. There were clear differences between the physical mixture of CeO2 + NiO and RB1 presented by all of these employed methods. Synchrotron methods, such as atomic pair distribution function analysis (PDF), X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) were used to explore the structure of RB1 in more detail. Three different models for the structural solution of RB1 were proposed. One structural solution proposes that RB1 is a single-phase pyrochlore compound (Ce2Ni2O7) while the other two solutions suggest that RB1 is a two-phase system of either CeO2 + NiO or Ce1–xNixO2 and NiO

Sol-gel-derived Ordered Mesoporous High Entropy Spinel Ferrites and Assessment of their Photoelectrochemical and Electrocatalytic Water Splitting Performance

For driving the (photo-) electrocatalytic water splitting reaction both efficient and photostable absorber materials and electrocatalysts are needed in order to make the technology economically competitive. The novel material class of high entropy oxides with their unique and unexpected physicochemical properties is a potential candidate for energy applications. Herein, we report for the first time about the physico- and (photo-) electrochemical properties of ordered mesoporous (CoNiCuZnMg)Fe2O4 thin films synthesized by a soft-templating and dip-coating approach. The high entropy ferrites (HEF) are composed of 15 ‒ 18 nm sized and periodically ordered mesopores building a highly accessible inorganic nanoarchitecture with specific surface areas up to 170 m2/g. The mesoporous HEF thin films crystallize in the cubic spinel structure and were found to be crack-free on the meso- and macroscale. The formation of the spinel structure hosting six distinct cations was verified by means of gracing incidence X-ray diffraction, X-ray photoelectron spectroscopy, time-of-flight secondary ion mass spectrometry, and transmission electron microscopy accompanied with energy dispersive X-ray spectroscopy. Photoelectron spectroscopy gave insight into chemical state of the implemented transition metals supporting the structural characterization data. Analyzed as photoanode for photoelectrochemical water splitting, the HEFs showed only low photoconductivity owing to fast surface recombination as suggested by intensity-modulated photocurrent spectroscopy. When applied as oxygen evolution reaction electrocatalyst, the HEF thin films possess overpotentials of 420 mV vs. RHE at 10 mA/cm2 in 1 M KOH. The results imply that the increase of the configurational disorder within the spinel structure enhances the electronic transport properties. The evaluation of the energy band alignment by Mott-Schottky analysis allows for an estimation which redox reactions can be driven, showing that the materials are theoretically capable of promoting overall water splitting

Mesoporous CuFe2O4 Photoanodes for Solar Water Oxidation: Impact of Surface Morphology on the Photoelectrochemical Properties

To make photoelectrochemical water splitting with metal oxide absorbers an economically viable technology for the production of green hydrogen, further improvements of solar-to-hydrogen conversion efficiency as well as photoelectrode stability have to be accomplished. One step towards optimized photoelectrodes is nanostructuring of the metal oxide absorbers addressing the generally short minority charge carrier diffusion lengths. In this work, mesoporous CuFe2O4 (CFO) thin film photoanodes were prepared by a sol-gel chemistry-based dip-coating and soft-templating strategy. The mesoporous CFOs were fabricated with distinct pore morphologies in order to study the impact of pore ordering and surface structure on the photoelectrochemical properties. The degree of pore ordering and geometry was varied by using different structure-directing copolymer surfactants, which were poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (Pluronic F-127), polyisobutylene-block-poly(ethyleneoxide) (PIB-PEO), and poly(ethylene-co-butylene)-block-poly(ethylene oxide) (Kraton liquid-PEO, KLE). The distinctly mesostructured CFO materials were characterised by means of scanning electron (SEM) and transmission electron microscopies (TEM), grazing-incidence X-ray diffraction (GIXRD), and X-ray photoelectron spectroscopy (XPS). The structural and electronic properties were correlated with the photoelectrochemical water and sulfite oxidation scans. The non-ordered, Pluronic F-127 templated CFO showed the highest photocurrent density of 0.2 mA/cm2 at 1.3 V vs. RHE for sulfite oxidation, but the least photocurrent density (1.5 µA/cm2) for water oxidation. This can be understood on the basis of the high surface area which promotes hole transfer to sulfite (a fast hole acceptor), but retards oxidation of water (a slow hole acceptor) due to electron-hole recombination at the defective surface. These results are confirmed by intensity-modulated photocurrent spectroscopy (IMPS). These yield the lowest surface recombination rate for the ordered KLE-based mesoporous CFO thin films, which retain spherical pore shapes at the surface resulting in fewer surface defects and thus in the highest water oxidation activity. Vibrating Kelvin probe surface photovoltage spectroscopy (VKP-SPS) on CFO films in contact with aqueous NaIO4 solution confirms these results and further reveals the presence of a detrimental Schottky junction at the FTO-CFO interface. Lastly, the electronic band diagram of the CFO photoanodes was investigated via X-ray photoelectron spectroscopy providing electronic structure arguments about the photoelectrochemical reactions the material is capable of driving. Post-use XPS data suggest that the concentration of hydroxyl groups, oxygen vacancies and adsorbed water/oxygen has indeed changed during photoelectrochemical operation, however the chemical state of the surface metal cations remained unaffected

Mesoporous CuFe₂O₄ Photoanodes for Solar Water Oxidation: Impact of Surface Morphology on the Photoelectrochemical Properties

Metal oxide‐based photoelectrodes for solar water splitting often utilize nanostructures to increase the solid‐liquid interface area. This reduces charge transport distances and increases the photocurrent for materials with short minority charge carrier diffusion lengths. While the merits of nanostructuring are well established, the effect of surface order on the photocurrent and carrier recombination has not yet received much attention in the literature. To evaluate the impact of pore ordering on the photoelectrochemical properties, mesoporous CuFe₂O₄ (CFO) thin film photoanodes were prepared by dip‐coating and soft‐templating. Here, the pore order and geometry can be controlled by addition of copolymer surfactants poly(ethylene oxide)‐block‐poly(propylene oxide)‐block‐poly(ethylene oxide) (Pluronic® F‐127), polyisobutylene‐block‐poly(ethylene oxide) (PIB‐PEO) and poly(ethylene‐co‐butylene)‐block‐poly(ethylene oxide) (Kraton liquid™‐PEO, KLE). The non‐ordered CFO showed the highest photocurrent density of 0.2 mA/cm² at 1.3 V vs. RHE for sulfite oxidation, but the least photocurrent density for water oxidation. Conversely, the ordered CFO presented the best photoelectrochemical water oxidation performance. These differences can be understood on the basis of the high surface area, which promotes hole transfer to sulfite (a fast hole acceptor), but retards oxidation of water (a slow hole acceptor) due to electron‐hole recombination at the defective surface. This interpretation is confirmed by intensity‐modulated photocurrent (IMPS) and vibrating Kelvin probe surface photovoltage spectroscopy (VKP‐SPS). The lowest surface recombination rate was observed for the ordered KLE‐based mesoporous CFO, which retains spherical pore shapes at the surface resulting in fewer surface defects. Overall, this work shows that the photoelectrochemical energy conversion efficiency of copper ferrite thin films is not just controlled by the surface area, but also by surface order

On the structural evolution of nanoporous optically transparent CuO photocathodes upon calcination for photoelectrochemical applications.

Copper oxides are promising photocathode materials for solar hydrogen production due to their narrow optical band gap energy allowing broad visible light absorption. However, they suffer from severe photocorrosion upon illumination, mainly due to copper reduction. Nanostructuring has been proven to enhance the photoresponse of CuO photocathodes; however, there is a lack of precise structural control on the nanoscale upon sol-gel synthesis and calcination for achieving optically transparent CuO thin film photoabsorbers. In this study, nanoporous and nanocrystalline CuO networks were prepared by a soft-templating and dip-coating method utilizing poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (Pluronic® F-127) as a structure-directing agent, resulting for the first-time in uniformly structured, crack-free, and optically transparent CuO thin films. The photoelectrochemical properties of the nanoporous CuO frameworks were investigated as a function of the calcination temperature and film thickness, revealing important information about the photocurrent, photostability, and photovoltage. Based on surface photovoltage spectroscopy (SPV), the films are p-type and generate up to 60 mV photovoltage at 2.0 eV (0.050 mW cm-2) irradiation for the film annealed at 750 °C. For these high annealing temperatures, the nanocrystalline domains in the thin film structure are more developed, resulting in improved electronic quality. In aqueous electrolytes with or without methyl viologen (as a fast electron acceptor), CuO films show cathodic photocurrents of up to -2.4 mA cm-2 at 0.32 V vs. RHE (air mass (AM) 1.5). However, the photocurrents were found to be entirely due to photocorrosion of the films and decay to near zero over the course of 20 min under AM 1.5 illumination. These fundamental results on the structural and morphological development upon calcination provide a direction and show the necessity for further (surface) treatment of sol-gel derived CuO photocathodes for photoelectrochemical applications. The study demonstrates how to control the size of nanopores starting from mesopore formation at 400 °C to the evolution of macroporous frameworks at 750 °C