Impact of surface defects on LaNiO3 perovskite electrocatalysts for the oxygen evolution reaction

Perovskite oxides are regarded as promising electrocatalysts for water splitting due to their cost-effectiveness, high efficiency and durability in the oxygen evolution reaction (OER). Despite these advantages, a fundamental understanding of how critical structural parameters of perovskite electrocatalysts influence their activity and stability is lacking. Here, we investigate the impact of structural defects on OER performance for representative LaNiO3 perovskite electrocatalysts. Hydrogen reduction of 700¿°C calcined LaNiO3 induces a high density of surface oxygen vacancies, and confers significantly enhanced OER activity and stability compared to unreduced LaNiO3; the former exhibit a low onset overpotential of 380 mV at 10 mA¿cm-2 and a small Tafel slope of 70.8 mV¿dec-1. Oxygen vacancy formation is accompanied by mixed Ni2+/Ni3+ valence states, which quantum-chemical DFT calculations reveal modify the perovskite electronic structure. Further, it reveals that the formation of oxygen vacancies is thermodynamically more favourable on the surface than in the bulk; it increases the electronic conductivity of reduced LaNiO3 in accordance with the enhanced OER activity that is observed.Peer ReviewedPostprint (author's final draft

Highly catalytically active CeO2-x-based heterojunction nanostructures with mixed micro/meso-porous architectures

Achieving high densities of accessible active sites in catalysts, which depend principally on the architecture of nanostructures, is critical to obtain enhanced performance. The present work introduces a template-free, high-yield, and flexible approach to fabricate 3D, mesoporous, CeO2-x nanostructures in centimeter-scale that are comprised of extremely thin holey 2D nanosheets. The method involves conversion of a stacked, 2D, Ce-based coordination polymer by controlling the removal kinetics of organic species. The resultant polycrystalline 2D-3D CeO2-x exhibits a large density of defects as well as outstanding surface areas of 251 m2 g-1. This mesoporous nanomaterial yields superior CO conversion performance (T90% = 148°C). Further improvements in catalysis were attained by synthesis CeO2-x -based transition metal oxides (TMOs) hetero-nanostructures, for which structural analyses and first principles simulations revealed active sites associated with the TMOs. This versatile fabrication technique delivers new pathways to engineer nanostructures and advance their functionalities for catalysis.Peer ReviewedPostprint (author's final draft

Aqueous and surface chemistries of photocatalytic Fe-doped CEO<inf>2</inf> nanoparticles

© 2017 by the authors; licensee MDPI, Basel, Switzerland. The present work describes the effects of water on Fe-doped nanoparticulate CeO2, produced by flame spray pyrolysis, which is a critical environmental issue because CeO2 is not stable in typical atmospheric conditions. It is hygroscopic and absorbs ~29 wt % water in the bulk when exposed to water vapor but, more importantly, it forms a hydrated and passivating surface layer when immersed in liquid water. In the latter case, CeO2 initially undergoes direct and/or reductive dissolution, followed by the establishment of a passivating layer calculated to consist of ~69 mol % solid CeO2·2H2O and ~30 mol % gelled Ce(OH)4. Under static flow conditions, a saturated boundary layer also forms but, under turbulent flow conditions, this is removed. While the passivating hydrated surface layer, which is coherent probably owing to the continuous Ce(OH)4 gel, would be expected to eliminate the photoactivity, this does not occur. This apparent anomaly is explained by the calculation of (a) the thermodynamic stability diagrams for Ce and Fe; (b) the speciation diagrams for the Ce4+-H2O, Ce3+-H2O, Fe3+-H2O, and Fe2+-H2O systems; and (c) the Pourbaix diagrams for the Ce-H2O and Fe-H2O systems. Furthermore, consideration of the probable effects of the localized chemical and redox equilibria owing to the establishment of a very low pH (<0) at the liquid-solid interface also is important to the interpretation of the phenomena. These factors highlight the critical importance of the establishment of the passivating surface layer and its role in photocatalysis. A model for the mechanism of photocatalysis by the CeO2 component of the hydrated phase CeO2·2H2O is proposed, explaining the observation of the retention of photocatalysis following the apparent alteration of the surface of CeO2 upon hydration. The model involves the generation of charge carriers at the outer surface of the hydrated surface layer, followed by the formation of radicals, which decompose organic species that have diffused through the boundary layer, if present

Multiwalled carbon nanotubes modified with MoO2 nanoparticles for voltammetric determination of the pesticide oxyfluorfen

A glassy carbon electrode was functionalized by MoO2 nanoparticle–decorated multiwalled carbon nanotubes (MWCNTs) and examined as a working electrode in oxyfluorfen (OXY) detection by differential pulse stripping voltammetry (DPSV). Measurement parameters were as follows: initial potential − 0.1 V, end potential + 0.5 V, accumulation potential − 0.15 V, accumulation time 80 s, and scan rate 50 mV s−1. A stripping potential of + 0.315 V vs. Ag/AgCl was employed. The pPesticide oxyfluorfen was determined in model samples by DPSV with good reproducibility (RSD <2.4%) in the concentration range 2.5 to 34.5 ng mL−1, with r = 0.99 and a limit of detection of 1.5 ng mL−1. These results are in the same range as those of HPLC/DAD, which is used as the comparative method. Recovery for OXY determination in a real river water sample was 102%. Analyses in Briton-Robinson buffer has shown to be pH dependent with the best response at pH 6.0. Structural characterization of MoO2-MWCNT by Raman spectroscopy, field emission scanning electron microscopy, high-resolution transmission electron microscopy, and X-ray crystallography revealed a preserved MWCNT structure decorated with firmly attached clusters of MoO2 nanoparticles

Voltammetric sensor based on Pt nanoparticles suported MWCNT for determination of pesticide clomazone in water samples

Novel electrochemical sensor based on Pt supported multiwalled carbon nanotubes is used for determination of pesticide clomazone in aqueous media via differential pulse stripping voltammetry (DPSV). Since clomazone is stable and readily soluble in water, it is often found in water sources. Hence, its determination in the environment is of utmost importance. Herein, clomazone is determined in 0.1 M phosphate buffer solution at pH 7.0 in the concentration range of 0.61–20.56 ng cm−3, with LOQ = 0.61 and LOD = 0.38. These results are in the same range with HPLC/DAD, which is used as comparative method. It is shown that DPSV is a facile and efficient way for determination of clomazone in contrast to precise but field-impractical HPLC. Mechanistic approach in explaining electrode processes is correlated to structural aspects of the synthesized sensor. HRTEM data reveals a uniform distribution of Pt nanoparticles on the MWCNT support as a source of crucial, structural and electronic changes. Furthermore, characterisation of Raman results indicates the existence of structural defects, which is believed to be the leading reason for improvement in sensing response. © 2019 Taiwan Institute of Chemical Engineer

Design strategies for ceria nanomaterials: untangling key mechanistic concepts

The morphologies of ceria nanocrystals play an essential role in determining their redox and catalytic performances in many applications, yet the effects of synthesis variables on the formation of ceria nanoparticles of different morphologies and their related growth mechanisms have not been systematised. The design of these morphologies is underpinned by a range of fundamental parameters, including crystallography, optical mineralogy, the stabilities of exposed crystallographic planes, CeO2-x stoichiometry, phase equilibria, thermodynamics, defect equilibria, and the crystal growth mechanisms. These features are formalised and the key analytical methods used for analysing defects, particularly the critical oxygen vacancies, are surveyed, with the aim of providing a source of design parameters for the synthesis of nanocrystals, specifically CeO2-x. However, the most important aspect in the design of CeO2-x nanocrystals is an understanding of the roles of the main variables used for synthesis. While there is a substantial body of data on CeO2-x morphologies fabricated using low cerium concentrations ([Ce]) under different experimental conditions, the present work fully maps the effects of the relevant variables on the resultant CeO2-x morphologies in terms of the commonly used raw materials [Ce] (and [NO3-] in Ce(NO3)3·6H2O) as feedstock, [NaOH] as precipitating agent, temperature, and time (as well as the complementary vapour pressure). Through the combination of consideration of the published literature and the generation of key experimental data to fill in the gaps, a complete mechanistic description of the development of the main CeO2-x morphologies is illustrated. Further, the mechanisms of the conversion of nanochains into the two variants of nanorods, square and hexagonal, have been elucidated through crystallographic reasoning. Other key conclusions for the crystal growth process are the critical roles of (1) the formation of Ce(OH)4 crystallite nanochains as the precursors of nanorods and (2) the disassembly of the nanorods into Ce(OH)4 crystallites and NO3--assisted reassembly into nanocubes (and nanospheres) as an unrecognised intermediate stage of crystal growth.Peer ReviewedPostprint (author's final draft

Transparent and Flexible Mn1−x−y(CexLay)O2−δ Ultrathin-Film Device for Highly-Stable Pseudocapacitance Application

Control over the fabrication of state-of-the-art portable pseudocapacitors with the desired transparency, mechanical flexibility, capacitance, and durability is challenging, but if resolved will have fundamental implications. Here, defect-rich Mn1−x−y(CexLay)O2−δ ultrathin films with controllable thicknesses (5–627 nm) and transmittance (≈29–100%) are fabricated via an electrochemical chronoamperometric deposition using a aqueous precursor derived from end-of-life nickel-metal hydride batteries. Due to percolation impacts on the optoelectronic properties of ultrathin films, a representative Mn1−x−y(CexLay)O2−δ film with 86% transmittance exhibits an outstanding areal capacitance of 3.4 mF cm−2, mainly attributed to the intercalation/de-intercalation of anionic O2− through the atomic tunnels of the stratified Mn1−x−y(CexLay)O2−δ crystallites. Furthermore, the Mn1−x−y(CexLay)O2−δ thin-film device exhibits excellent capacitance retention of ≈90% after 16 000 cycles. Such stability is associated with intervalence charge transfer occurring among interstitial Ce/La cations and Mn oxidation states within the Mn1−x−y(CexLay)O2−δ structure. The energy and power densities of the transparent flexible Mn1−x−y(CexLay)O2−δ full-cell pseudocapacitor device, is measured to be 0.088 μWh cm−2 and 843 µW cm−2, respectively. These values show insignificant changes under vigorous twisting and bending to 45–180° confirming these value-added materials are intriguing alternatives for size-sensitive energy storage devices.</p

Self-adhesive flexible patches of oxide heterojunctions with tailored band alignments for electrocatalytic H2O2generation

The new class of oxide heterojunctions with mixed dimensionalities holds great promise for energy and environmental applications. However, the existing fabrication strategies typically involve low-yield and multistep processes leading to the formation of powders that necessitate binding agents when used for electrochemical applications; thereby, the durability and performance of the resultant electrode may be adversely impacted. To address these challenges, the present work first reports a high-temperature counter-current gas flow technique for rapid fabrication (5–10 min) of centimetre-size, self-adhesive, free-standing 3D patches made of ZnO-based woven nanowires. Furthermore, the high applicability of the method was shown by layer-by-layer assembly of the ZnO and layers of 0D heteroatoms including Bi2O3, CdO, SnO2, and carbon forming stratified oxide heterojunction (SOH) nanostructures with midgap states within their electronic bandgap region. This work is innovative in that the ZnO and the fabricated SOHs are synthesised through a sustainable and large-scale method based on microrecycling of waste materials. The engineering of the electronic band positions can modify the functionality of the SOH patches by optimising the potentials required for catalytic reactions. As a representative, the SOH nanostructures were tested for anodic electrocatalytic water oxidation to H2O2. The results showed that the ZnO–CdO patch has the lowest overpotential of 150 mV and outstanding stability at 2.33 V vs. RHE. Furthermore, the results of first-principles density functional theory (DFT) calculations (i) confirmed realignments of the band position due to the formation of midgap states, and (ii) revealed that significant improvements in the electrocatalytic H2O2 performance can be achieved with overpotentials as low as 0.19 and 0.31 V for ZnO–CdO and ZnO–Bi2O3, respectively. This work offers an ultrafast fabrication strategy to synthesise binder-free SOH nanostructures with an engineered electronic structure that can be an alternative to state-of-the-art noble metal electrocatalysts such as Pt.</p