Emneord:
SMN,
Strukturfysikk
Publikasjoner
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Fernández, Javier García; Pokle, Anuj; Andreassen, Magnus; Prytz, Øystein; Grundmann, Marius & Wenckstern, Holger Von
(2026).
Ex Situ and In Situ Thermal Evolution and Crystallization of Amorphous (In,Ga)2O3.
ACS Applied Electronic Materials.
8(11),
s. 4582–4588.
doi:
10.1021/acsaelm.6c00284.
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The present study investigates the structural evolution of amorphous (In,Ga)2O3 thin films grown by pulsed laser deposition and thermal annealing under vacuum (ex situ) and in situ transmission electron microscopy (TEM). Ex situ annealing results showed that the crystallization starts close to the substrate interface at 500 °C with randomly oriented nanocrystals, nanovoids, and on top of that, columnar grains of cubic c-(In,Ga)2O3. For the in situ TEM studies, we observed the formation of a heterogeneous polycrystalline mixture of elemental indium (In), monoclinic β-Ga2O3, and a minor contribution of c-In2O3 when annealed at 400 °C. Further annealing at 700 °C led to gradual evaporation of indium, resulting in a β-Ga2O3-dominated sample with nanovoids. These findings provide insights into phase stability, evolution, and elemental redistribution, which are critical for optimizing (In,Ga)2O3 thin films for optoelectronic applications.
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Petlund, Henrik; Pokle, Anuj & Chatzitakis, Athanasios
(2025).
Suppression of surface amorphization during oxygen evolution reaction in A-site deficient perovskite oxides.
Journal of Physics: Energy.
7(2).
doi:
10.1088/2515-7655/adb2c0.
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Surface restructuring and the formation of amorphous layers during the initial stages of the oxygen evolution reaction (OER) is a common phenomenon in perovskite-based catalysts. It has also been observed that catalysts following the lattice oxygen mechanism (LOM) are prone to amorphization as the rates of lattice oxygen evolution and oxygen vacancy replenishment are not equal. This is accompanied by a significant A-site cation loss in the electrolyte, ultimately leading to the collapse of the crystalline phase and the appearance of an amorphous catalytic surface of a thickness of around 10 nm. Typically, the originally crystalline material is termed as precatalyst. In this work, we hypothesize that A-site deficient perovskite precatalysts of the family of BaxGdyLazCo2O3-δ (BGLCxyz), which operate under LOM, will have a reduced degree or no surface amorphization under OER conditions in alkaline media. For this reason, A-site deficient BGLC587 with a 20% deficiency is synthesized to validate our main hypothesis. Furthermore, thermal reduction of the A-site deficient BGLC587 precatalyst is expected, to form embedded nanoparticles on its surface through the process of exsolution, thus inducing higher surface area and improved electrocatalytic activity, while still suppressing surface amorphization.
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Gudmundsdottir, Jonina Björg; Thoreton, Vincent; Jiang, Bo; Pokle, Anuj; Vøllestad, Einar & Haugsrud, Reidar
[Vis alle 7 forfattere av denne artikkelen]
(2025).
Oxygen exchange kinetics of BaGd0.3La0.7Co2O6−δ with exsolved Co3O4 nanoparticles in dry and humid atmospheres.
Physical Chemistry, Chemical Physics - PCCP.
ISSN 1463-9076.
27(16),
s. 8467–8477.
doi:
10.1039/d4cp04791e.
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The oxygen exchange kinetics of BaGd0.3La0.7Co2O6−δ were investigated with pulsed isotope exchange (PIE) measurements in dry and humid atmospheres in the temperature range 400–600 °C in 0.005–0.21 bar O2. Synchrotron X-ray diffraction and scanning transmission electron microscopy (STEM) revealed exsolved Co3O4 nanoparticles dispersed on the surfaces that may contribute to the catalytic activity of the material towards the oxygen exchange reactions. The obtained oxygen exchange rate was 4.45 × 10−3 mol m−2 s−1 at 600 °C in 0.21 bar O2 with an activation energy of 0.76 eV. The rate determining step of the exchange reaction was determined to be dissociative adsorption of oxygen in both dry and humid atmospheres based on the individual rates of dissociative adsorption and incorporation, as well as pO2 dependencies of the oxygen exchange rate of around 1. The effect of water on the oxygen exchange rate was found to be dependent on the oxygen partial pressure, decreasing the rate at 0.21 bar O2 and 600 °C by a factor of approx. 2, while increasing the rate at 0.02 bar and 0.005 bar O2 by a similar amount. In situ thermogravimetric analysis was used to characterise the oxygen non-stoichiometry of the material throughout the oxygen exchange measurements.
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Petlund, Henrik; Faid, Alaa Yahia; Zhu, Junjie; Pokle, Anuj; Norby, Truls & Sunde, Svein
[Vis alle 7 forfattere av denne artikkelen]
(2024).
Iron and Nickel Substituted Perovskite Cobaltites for Sustainable Oxygen Evolving Anodes in Alkaline Environment.
ChemSusChem.
ISSN 1864-5631.
doi:
10.1002/cssc.202401403.
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Perovskite oxides have great flexibility in their elemental composition, which is accompanied by large adjustability in their electronic properties. Herein, we synthesized twelve perovskite oxide-based catalysts for the oxygen evolution reaction (OER) in alkaline media. The catalysts are based on the parent oxide perovskite Ba0.5Gd0.8La0.7Co2O6-δ (BGLC587) and are synthesized through the sol-gel citrate synthesis route. To reduce the demand on cobalt (Co), but also increase the intrinsic catalytic activity of BGLC587 for the OER, we substitute Co on the B-site with certain amounts of Fe and Ni, synthesizing catalysts of the general formula Ba0.5Gd0.8La0.7Co2-x-yFexNiyO6-δ. A plethora of physicochemical and electrochemical methods suggest that an Fe content between 30 % and 70 % increases the intrinsic catalytic activity of BGLC587, while Tafel slopes in combination with in-situ Raman spectroscopy suggest the rate determining step is likely a proton-exchange reaction, progressing possibly through the lattice oxygen mechanism (LOM). We apply one of the optimized, Co-substituted perovskites in a monolithic, photovoltaic (PV)-driven electrolysis cell and we achieve an initial solar-to-hydrogen (STH) conversion efficiency of 10.5 % under one sun solar simulated illumination.
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Fernandez, Javier Garcia; Kjeldby, Snorre Braathen; Zeng, L. J.; Azarov, Alexander; Pokle, Anuj & Nguyen, Phuong Dan
[Vis alle 10 forfattere av denne artikkelen]
(2024).
In situ atomic-resolution study of transformations in double polymorph γ/β-Ga2O3 structures.
Materials Advances.
5,
s. 3824–3831.
doi:
10.1039/D3MA01011B.
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Disorder-induced formation of metastable Ga2O3 polymorphs as well as the recovery of the stable state upon annealing attract attention because of the fundamental novelty and rapidly increasing interest in the use of Ga2O3 in practical applications. In this study, double polymorph γ/β-Ga2O3 structures fabricated by the radiation-induced disorder approach were used as a starting point for systematic in situ annealing electron microscopy experiments. We show that, under the conditions of the TEM in situ annealing, double γ/β-Ga2O3 polymorph structures remained stable up to 300 °C, when onsets of the γ-to-β transformation become traceable, leading to a prominent γ- and β-mixture already at 500 °C. Interestingly, the recrystallization of the β-Ga2O3 occurs throughout the whole γ-film and the preferential alignments at the newly emerging γ/β-interfaces are different from that of the initial γ/β-interface formed as a result of the disorder-induced ordering. The alignments of the two polymorphs are maintained as a function of temperature – with a reduction in the volumetric ratio of γ-domains for increasing annealing temperature. Finally, at 1100 °C, γ-Ga2O3 fully transforms into β-Ga2O3, without dominating crystallographic relationships or preferred orientations, indicating that energy barriers are not any longer implied limiting factors, because of a sufficiently high thermal energy supply. Thus, these TEM in situ measurements enable a new level of accuracy for assessing polymorphic transformations in Ga2O3.
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Johnsen, Hennie Marie; Pokle, Anuj; Filtvedt, Werner Olav; Hiorth, Marianne; Klaveness, Jo & Sjåstad, Anja Olafsen
(2024).
From silicon to silica: a green chemistry approach for hollow sphere nanoparticle formation.
Nanoscale Advances.
6,
s. 6196–6204.
doi:
10.1039/d4na00586d.
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Herein we report on an environmentally friendly and scalable production route for hollow silica spheres (HSSs). The process is based on close to 100% conversion of non-crystalline solid Si nanoparticles ([D with combining macron] = 40 ± 9 nm) in mild alkaline solutions (pH ≤ 9.0) at ambient temperature. The Si nanoparticles are prepared using the centrifugal chemical vapor deposition (cCVD) method. Combining transmission electron microscopy (TEM) imaging and nanoparticle size analysis with hydrogen evolution data, elemental mapping, and nitrogen adsorption for surface area measurement, we show for the first time experimental data that document a Kirkendall type Si-to-HSS formation process. Our understanding is that the Si nanoparticles exposed to air form a SiO2 film, which is stable in the mild alkaline environment. Silicon from the Si nanoparticles is transported through the thin SiO2 film and is reacting with H2O/OH− species on the particle surface or in the already thickened SiO2 shell to form silicic acid that in turn rapidly gets converted to a sol–gel to continue the growing of the silica shell. We foresee that this green chemistry approach can be utilized for HSS preparation for use in batteries, insulation materials and drug delivery.
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Silva, Valmir Rodrigo da; Fjellvåg, Øystein Slagtern; Pokle, Anuj; Hauback, Bjørn Christian & Deledda, Stefano
(2024).
Effect of Carbon Addition and Mechanical Activation on FeNi Alloys for Permanent Magnet Applications.
Metals.
14(10).
doi:
10.3390/met14101125.
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Tetrataenite is a promising candidate for rare earth-free permanent magnets due to its low cost and intrinsic magnetic properties. This work investigates the effect of combined milling at liquid nitrogen temperatures (cryomilling) and the addition of carbon as an interstitial element for promoting the formation of tetrataenite. Crystal structure, microstructure, and magnetic properties are investigated to understand the influence of mechanical processing and compositional modifications. No unambiguous evidence of the ordered phase of tetrataenite is found in the structural characterization. However, using Scanning Transmission Electron Microscopy (STEM) and powder X-ray diffraction (PXD) analyses, the occurrence of both twinning and stacking faults resulting from the high-energy milling process is observed, which is a relevant factor for identifying tetrataenite in FeNi alloys. The probability of a stacking fault and twinning occurring for a carbon-free FeNi sample before annealing is found to be 2% and 1.4%, respectively. After annealing, the stacking fault probability decreased to 1.2%, while that of twinning was 1.4%. By increasing the carbon concentration to 5 at.%, the stacking faults and twinning probabilities decrease slightly to 1.2% and 1.3%, respectively. The occurrence of stacking faults combined with small crystallite sizes was a hindering factor in identifying the presence of tetrataenite.
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Redekop, Evgeniy; Cordero-Lanzac, Tomás; Salusso, Davide; Pokle, Anuj; Øien-Ødegaard, Sigurd & Sunding, Martin Fleissner
[Vis alle 11 forfattere av denne artikkelen]
(2023).
Zn Redistribution and Volatility in ZnZrOx Catalysts for CO2 Hydrogenation.
Chemistry of Materials.
ISSN 0897-4756.
35(24),
s. 10434–10445.
doi:
10.1021/acs.chemmater.3c01632.
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ZnO–ZrO2 mixed oxide (ZnZrOx) catalysts are widely studied as selective catalysts for CO2 hydrogenation into methanol at high-temperature conditions (300–350 °C) that are preferred for the subsequent in situ zeolite-catalyzed conversion of methanol into hydrocarbons in a tandem process. Zn, a key ingredient of these mixed oxide catalysts, is known to volatilize from ZnO under high-temperature conditions, but little is known about Zn mobility and volatility in mixed oxides. Here, an array of ex situ and in situ characterization techniques (scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDX), transmission electron microscopy (TEM), powder X-ray diffraction (PXRD), X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), Infrared (IR)) was used to reveal that Zn2+ species are mobile between the solid solution phase with ZrO2 and segregated and/or embedded ZnO clusters. Upon reductive heat treatments, partially reversible ZnO cluster growth was observed above 250 °C and eventual Zn evaporation above 550 °C. Extensive Zn evaporation leads to catalyst deactivation and methanol selectivity decline in CO2 hydrogenation. These findings extend the fundamental knowledge of Zn-containing mixed oxide catalysts and are highly relevant for the CO2-to-hydrocarbon process optimization.
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Genz, Nina; Kallio, Antti-Jussi; Oord, Ramon; Krumeich, Frank; Pokle, Anuj & Prytz, Øystein
[Vis alle 10 forfattere av denne artikkelen]
(2022).
Operando Laboratory-Based Multi-Edge X-Ray Absorption Near-Edge Spectroscopy of Solid Catalysts.
Angewandte Chemie International Edition.
ISSN 1433-7851.
61(48).
doi:
10.1002/anie.202209334.
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Laboratory-based X-ray absorption spectroscopy (XAS) and especially X-ray absorption near-edge structure (XANES) offers new opportunities in catalyst characterization and presents not only an alternative, but also a complementary approach to precious beamtime at synchrotron facilities. We successfully designed a laboratory-based setup for performing operando, quasi-simultaneous XANES analysis at multiple K-edges, more specifically, operando XANES of mono-, bi-, and trimetallic CO2 hydrogenation catalysts containing Ni, Fe, and Cu. Detailed operando XANES studies of the multielement solid catalysts revealed metal-dependent differences in the reducibility and re-oxidation behavior and their influence on the catalytic performance in CO2 hydrogenation. The applicability of operando laboratory-based XANES at multiple K-edges paves the way for advanced multielement catalyst characterization complementing detailed studies at synchrotron facilities.
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Petlund, Henrik; Geo, Sarah; Velluva, Abhijai; Pokle, Anuj & Chatzitakis, Athanasios
(2025).
nanoGe - MATSUSFall25 - The dual role of A-site deficiency in perovskite oxides: Exsolution of metal nanoparticles and suppression of surface amorphization.
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Catalysts enable important chemical transformations for industrial applications, as well as for emerging technologies for the decarbonization of our societies, such as water electrolysis and the production of green hydrogen. Very often, catalysts experience structural and compositional changes during operating conditions, most notably the oxygen evolution reaction (OER), that hinder their ratio...
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Geo, Sarah; Pokle, Anuj; Velluva, Abhijai; Dayaghi, Amir Masoud & Chatzitakis, Athanasios
(2025).
Suppressing surface amorphization in BSCF-based perovskites through A-site deficiency for use as anodes in water electrolysis.
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The development of efficient and stable anodes for water electrolysis is critical to advancing sustainable hydrogen production. Among perovskite oxides, BSCF82 (Ba₀.₅Sr₀.₅Co₀.₈Fe₀.₂O₃-δ) has emerged as a promising candidate due to its favourable balance of electronic conductivity, oxygen mobility, and catalytic activity. However, BSCF82 suffers from serious degradation during the oxygen evolution reaction (OER) process due to surface amorphization caused by the leeching of A-site ions (Ba²⁺ and Sr²⁺)[1]. Previously, we showed that A-site deficiency in Ba0.5Gd0.8La0.7Co2O6 can suppress it surface amorphization during OER activity and in the present study, we generalize this observation by employing a similar strategy in BSCF82[2]. By creating controlled A-site vacancies in the Ba₀.₅-xSr₀.₅-xCo₀.₈Fe₀.₂O₃-δ perovskite structure, we achieved improvement in its structural stability under OER conditions in 1 M KOH. High-resolution transmission electron microscopy revealed that A-site deficient compositions exhibited reduced surface amorphization and preserved crystalline surface structures after electrochemical testing in 1 M KOH. This work provides new insights into mitigating surface amorphization in perovskite catalysts through defect engineering and presents a promising strategy for developing durable electrocatalysts for renewable energy applications.
Acknowledgements: This work has received financial support from the European Union HORIZON Research and Innovation Actions under grant agreement ID 101122323 (REFINE).
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Fernandez, Javier Garcia; Pokle, Anuj; Andreassen, Magnus; Prytz, Øystein; Grundmann, Marius & Wenckstern, Holger Von
(2025).
In situ and ex situ thermal evolution, crystallization and optical properties of Amorphous (In,Ga)2O3.
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Petlund, Henrik; Pokle, Anuj; Norby, Truls & Chatzitakis, Athanasios
(2025).
Front Cover: Iron and Nickel Substituted Perovskite Cobaltites for Sustainable Oxygen Evolving Anodes in Alkaline Environment (ChemSusChem 6/2025).
ChemSusChem.
ISSN 1864-5631.
doi:
10.1002/cssc.202580601.
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Finstad, Terje Gunnar; Mayandi, Jeyanthinath; Soltani, Nayereh; Pokle, Anuj; Løvvik, Ole Martin & Carvalho, Patricia Almeida
(2023).
TiFe2Si Thin Films.
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Publisert
5. okt. 2021 09:14
- Sist endret
15. sep. 2023 14:32
