thomasaar

• Kang, Xiaolan; Reinertsen, Vilde Mari; Both, Kevin Gregor; Galeckas, Augustinas; Aarholt, Thomas & Prytz, Øystein [Show all 9 contributors for this article] (2022). Galvanic Restructuring of Exsolved Nanoparticles for Plasmonic and Electrocatalytic Energy Conversion. Small. ISSN 1613-6810. 18(29). doi: 10.1002/smll.202201106. Full text in Research Archive Show summary

  There is a growing need to control and tune nanoparticles (NPs) to increase their stability and effectiveness, especially for photo- and electrochemical energy conversion applications. Exsolved particles are well anchored and can be re-shaped without changing their initial location and structural arrangement. However, this usually involves lengthy treatments and use of toxic gases. Here, the galvanic replacement/deposition method is used, which is simpler, safer, and leads to a wealth of new hybrid nanostructures with a higher degree of tailorability. The produced NiAu bimetallic nanostructures supported on SrTiO3 display exceptional activity in plasmon-assisted photoelectrochemical (PEC) water oxidation reactions. In situ scanning transmission electron microscopy is used to visualize the structural evolution of the plasmonic bimetallic structures, while theoretical simulations provide mechanistic insight and correlate the surface plasmon resonance effects with structural features and enhanced PEC performance. The versatility of this concept in shifting catalytic modes to the hydrogen evolution reaction is demonstrated by preparing hybrid NiPt bimetallic NPs of low Pt loadings on highly reduced SrTiO3 supports. This powerful methodology enables the design of supported bimetallic nanomaterials with tunable morphology and catalytic functionalities through minimal engineering.

• Kang, Xiaolan; Chatzitakis, Athanasios; Aarholt, Thomas; Sun, Xinwei; Negri, Chiara & Norby, Truls (2022). Facet-engineered TiO<inf>2</inf>nanomaterials reveal the role of water-oxide interactions in surface protonic conduction. Journal of Materials Chemistry A. ISSN 2050-7488. 10(1), p. 218–227. doi: 10.1039/d1ta06075a. Full text in Research Archive Show summary

  Water adsorption and surface protonic conduction have been investigated at 25–400 °C in wet (H2O and D2O) atmospheres on nanocrystalline TiO2 hydrothermally grown to a predominance of different crystal facets. In situ Fourier transform infrared spectrometry shows that a portion of the water molecules in the first chemisorbed layer adsorbs dissociatively on the {001} and {100} surfaces, but molecularly on the {101} surface. The next layer of physisorbed molecular water is solid (ice)-like on the {001} and {100} surfaces, a fact that is attributed to relatively strong bonds to the surface terminating hydroxyls. On the other hand, it is looser, liquid-like on the {101} surface due to the lack of rigid hydroxyl groups to bond to. As relative humidity (RH) increases with decreasing temperature, additional water layers physisorb and eventually become liquid-like also on the {001} and {100} surfaces. The differences can be attributed to the different atomic arrangements and surface energies of the different crystal facets. Surface protonic conduction on {001} and {100} dominated samples exhibits high pre-exponentials, indicating a high concentration of sites for dissociative chemisorption, and high activation energies reflecting difficult proton migration in the rigid water layers on those surfaces. The surface protonic conduction on the {101} dominated sample reflects a lower coverage of dissociable water and lower activation enthalpies for protonic migration in the more loosely bonded water. This suggests that, more generally, surfaces with dissociative chemisorption may exhibit high surface protonic conductivity at the highest and lowest temperatures (corresponding to lowest and highest RH, respectively), while surfaces with molecular chemisorption may exhibit a relatively high conductivity (less deep minimum) at intermediate temperatures. H/D isotope effects show that the protonic conduction mechanism changes from Grotthuss to vehicle as the physisorbed water layers gain thickness and become liquid-like. The protonic conductivity decreased when the surface was terminated by more strongly bonded fluoride ions, believed to reflect that they block the sites for water adsorption.

• Both, Kevin Gregor; Reinertsen, Vilde Mari; Aarholt, Thomas; Jensen, Ingvild Julie Thue; Neagu, Dragos & Prytz, Øystein [Show all 8 contributors for this article] (2022). Ni-doped A-site excess SrTiO3 thin films modified with Au nanoparticles by a thermodynamically-driven restructuring for plasmonic activity. Catalysis Today. ISSN 0920-5861. 413. doi: 10.1016/j.cattod.2022.11.011. Full text in Research Archive Show summary

  Plasmonically active nanoparticles offer a promising pathway to extend the absorption range of photocatalysts. While not necessarily catalytically active themselves, these particles allow the absorption of lower energy photons in wide band gap photocatalysts. Here, we present A-site excess SrTiO3 thin films, doped with Ni, where through a subsequent exsolution process we created well-socketed Ni nanoparticles in the surface of SrTiO3. These were galvanically replaced by Au, resulting in well-socketed Au nanoparticles with variable size on the surface, depending on the galvanic replacement time. Photoelectrochemical measurements and electron energy loss spectroscopy revealed the improved photoresponse of the thin films by plasmonic activity of the nanoparticles. The energy of the plasmon peak suggests that the main improvement results from the injection of hot charge carriers. Our study opens new avenues for the design and synthesis of the next generation of photocatalytic materials.

• Zhu, Junjie; Gudmundsdottir, Jonina Björg; Strandbakke, Ragnar; Both, Kevin Gregor; Aarholt, Thomas & Carvalho, Patricia Almeida [Show all 12 contributors for this article] (2021). Double Perovskite Cobaltites Integrated in a Monolithic and Noble Metal-Free Photoelectrochemical Device for Efficient Water Splitting. ACS Applied Materials & Interfaces. ISSN 1944-8244. 13(17), p. 20313–20325. doi: 10.1021/acsami.1c01900. Full text in Research Archive Show summary

  Water photoelectrolysis has the potential to produce renewable hydrogen fuel, therefore addressing the intermittent nature of sunlight. Herein, a monolithic, photovoltaic (PV)-assisted water electrolysis device of minimal engineering and of low (in the μg range) noble-metal-free catalysts loading is presented for unassisted water splitting in alkaline media. An efficient double perovskite cobaltite catalyst, originally developed for high-temperature proton-conducting ceramic electrolyzers, possesses high activity for the oxygen evolution reaction in alkaline media at room temperatures too. Ba1–xGd1–yLax+yCo2O6−δ (BGLC) is combined with a NiMo cathode, and a solar-to-hydrogen efficiency of 6.6% in 1.0 M NaOH, under 1 sun simulated illumination for 71 h, is demonstrated. This work highlights how readily available earth-abundant materials and established PV methods can achieve high performance and stable and monolithic photoelectrolysis devices with potential for full-scale applications.

• Bonkerud, Julie; Zimmermann, Christian; Weiser, Philip Michael; Aarholt, Thomas; Verhoeven, Espen Førdestrøm & Vines, Lasse [Show all 8 contributors for this article] (2020). Fabrication and characterization of Schottky barrier diodes on rutile TiO2. Materials Research Express. 7(6). doi: 10.1088/2053-1591/ab9777. Full text in Research Archive
• Hansen, Per-Anders Stensby; Zikmund, Tomas; Yu, Ting; Kvalvik, Julie Nitsche; Aarholt, Thomas & Prytz, Øystein [Show all 8 contributors for this article] (2020). Single-step approach to sensitized luminescence through bulk-embedded organics in crystalline fluorides. Communications chemistry. 3. doi: 10.1038/s42004-020-00410-0. Full text in Research Archive Show summary

  Abstract Luminescent materials enable warm white LEDs, molecular tagging, enhanced optoelectronics and can improve energy harvesting. With the recent development of multi-step processes like down- and upconversion and the difficulty in sensitizing these, it is clear that optimizing all properties simultaneously is not possible within a single material class. In this work, we have utilized the layer-by-layer approach of atomic layer deposition to combine broad absorption from an aromatic molecule with the high emission yields of crystalline multi-layer lanthanide fluorides in a single-step nanocomposite process. This approach results in complete energy transfer from the organic molecule while providing inorganic fluoride-like lanthanide luminescence. Sm 3+ is easily quenched by organic sensitizers, but in our case we obtain strong fluoride-like Sm 3+ emission sensitized by strong UV absorption of terephthalic acid. This design allows combinations of otherwise incompatible species, both with respect to normally incompatible synthesis requirements and in controlling energy transfer and quenching routes.

• Liu, Junliang; Li, Kexue; Sayers, James; Aarholt, Thomas; He, Guanze & Hulme, Helen [Show all 14 contributors for this article] (2020). Characterisation of deuterium distributions in corroded zirconium alloys using high-resolution SIMS imaging. Acta Materialia. ISSN 1359-6454. 200, p. 581–596. doi: 10.1016/j.actamat.2020.09.040. Full text in Research Archive Show summary

  Hydrogen diffusion through the oxide grown on Zr alloys by aqueous corrosion processes plays a critical role in determining the rate of hydrogen pickup (HPU) which can result in embrittlement of fuel cladding and limit the burnup of the nuclear fuel it encapsulates. Mapping the hydrogen/deuterium distributions in these oxide layers, especially in the barrier layer close to the metal/oxide interface, is a powerful way to understand the mechanism of both oxidation and hydrogen pickup. Here we have characterised by high-resolution SIMS analysis the deuterium distribution in oxide layers on a series of Zr alloys, including autoclave-oxidised Zircaloy-4, Zr-1Nb and Zr-2.5Nb alloys, and in-flux and out-of-flux corroded Zr-2.5Nb samples. Pre-transition Zircaloy-4 samples show a high deuterium trapping ratio in the oxide and a higher diffusion coefficient than in oxides on the Nb-containing samples. Neutron irradiation increases the deuterium diffusion coefficient, the deuterium concentration in the oxide and the pickup fraction in Zr-2.5 Nb samples. Comparative NanoSIMS and EDX/SEM analysis demonstrates that the deuterium is not preferentially trapped at second phase particles in the oxides on any of the alloys studied, but there is direct evidence for trapping at the surfaces of small oxide cracks especially in Zircaloy-4 samples. The high resolution mapping of these hot-spots in 3D can provide unique information on the mechanisms of hydrogen uptake, and suggests that the development of interconnected porosity in the oxide may be the critical rate-determining mechanism that controls HPU in the aqueous corrosion of zirconium alloys in water-cooled reactors.

• Aarholt, Thomas; Frodason, Ymir Kalmann & Prytz, Øystein (2020). Imaging defect complexes in scanning transmission electron microscopy: Impact of depth, structural relaxation, and temperature investigated by simulations. Ultramicroscopy. ISSN 0304-3991. 209. doi: 10.1016/j.ultramic.2019.112884. Full text in Research Archive
• Kexue, Li; Aarholt, Thomas; Liu, Junliang; Hulme, Helen; Garner, Alistair & Preuss, Michael [Show all 8 contributors for this article] (2019). 3D-characterization of deuterium distributions in zirconium oxide scale using high-resolution SIMS. Applied Surface Science. ISSN 0169-4332. 464, p. 311–320. doi: 10.1016/j.apsusc.2018.09.101. Full text in Research Archive Show summary

  In pressurised water reactors (PWRs), fuel rods are clad with zirconium alloy tubes chosen for their low neutron capture cross section and good oxidation resistance. Understanding cladding-water corrosion reactions at 280–350 °C and the correlated hydrogen pickup into these Zr alloys is crucial to the safe operation of PWRs and to increasing the burnup of the fuel. Here we describe a method based on 3D mapping by high resolution SIMS to measure the distribution of deuterium in oxidised Zircaloy-4 alloy samples. Two analysis directions, depth-profiling and cross-sectional, were used to ensure we understand possible imaging artefacts during sputtering of the complex microstructure in these samples. The topography of the sputtering carter and sputtering rate have been calibrated by Focused Ion Beam (FIB)/Scanning Electron Microscopy (SEM) analysis. The deuterium diffusion coefficients at 360 °C is calculated suing the depth profile in both samples. The results show that the 3D deuterium distribution can be successfully measured in isotopically spiked samples, but that care has to be taken to understand the effect of deuterium outgassing and surface diffusion during Cs+ primary ion bombardment, which results in a degraded lateral resolution, which distorts the apparent deuterium distribution. The detection method in this paper can be a useful tool in the analysis of the distribution of hydrogenic species in zirconium fuel cladding materials in service, and can also provide other engineering materials.

• Sky, Thomas Neset; Johansen, Klaus Magnus H; Frodason, Ymir Kalmann; Aarholt, Thomas; Riise, Heine Nygard & Prytz, Øystein [Show all 8 contributors for this article] (2019). Diffusion of indium in single crystal zinc oxide: a comparison between group III donors. Semiconductor Science and Technology. ISSN 0268-1242. 34(2), p. 1–6. doi: 10.1088/1361-6641/aafa4c. Full text in Research Archive
• Zhan, Wei; Venkatachalapathy, Vishnukanthan; Aarholt, Thomas; Kuznetsov, Andrej & Prytz, Øystein (2018). Band gap maps beyond the delocalization limit: correlation between optical band gaps and plasmon energies at the nanoscale. Scientific Reports. 8(848), p. 1–7. doi: 10.1038/s41598-017-18949-9. Full text in Research Archive Show summary

  Recent progresses in nanoscale semiconductor technology have heightened the need for measurements of band gaps with high spatial resolution. Band gap mapping can be performed through a combination of probe-corrected scanning transmission electron microscopy (STEM) and monochromated electron energy-loss spectroscopy (EELS), but are rare owing to the complexity of the experiments and the data analysis. Furthermore, although this method is far superior in terms of spatial resolution to any other techniques, it is still fundamentally resolution-limited due to inelastic delocalization of the EELS signal. In this work we have established a quantitative correlation between optical band gaps and plasmon energies using the Zn1−xCd x O/ZnO system as an example, thereby side-stepping the fundamental resolution limits of band gap measurements, and providing a simple and convenient approach to achieve band gap maps with unprecedented spatial resolution.

• Pedrazzini, S; Child, DJ; Aarholt, Thomas; Ball, C; Dowd, M & Girling, A [Show all 11 contributors for this article] (2018). On the Effect of Environmental Exposure on Dwell Fatigue Performance of a Fine-Grained Nickel-Based Superalloy. Metallurgical and Materials Transactions A. ISSN 1073-5623. 49(9), p. 3908–3922. doi: 10.1007/s11661-018-4752-7. Full text in Research Archive Show summary

  The influence of sulfur contamination on the corrosion-fatigue behavior of a polycrystalline superalloy used in aero-engines is considered. Samples tested under a variety of environmental conditions (including exposures to air, SOx gas, and salt) are characterized through a suite of high-resolution characterization methods, including transmission electron microscopy (TEM), secondary ion mass spectroscopy (nanoSIMS), and atom probe tomography (APT). The primary effect of sulfur contamination is to accelerate the crack growth rate by altering the failure mechanism. The SIMS and TEM analyses indicate Cr-Ti sulfide particle formation at grain boundaries ahead of and around oxidized cracks. The APT analysis suggests that these particles then oxidize as the crack propagates and are enveloped in chromia. The chromia is surrounded by a continuous layer of alumina within the cracks. All of the sulfur detected was confined within the particles, with no elemental segregation found at grain boundaries.

• Granerød, Cecilie Skjold; Bilden, Sindre Rannem; Aarholt, Thomas; Yao, Yu-Feng; Yang, C. C. & Look, David C. [Show all 9 contributors for this article] (2018). Direct observation of conduction band plasmons and the related Burstein-Moss shift in highly doped semiconductors: A STEM-EELS study of Ga-doped ZnO. Physical review B (PRB). ISSN 2469-9950. 98(11). doi: 10.1103/PhysRevB.98.115301. Full text in Research Archive Show summary

  The combination of high optical transparency and low electrical resistivity has made transparent conductive oxides (TCOs) a key technology in many optoelectronic applications. Furthermore, the study of TCOs yields insight into many fundamental parameters of semiconductors. For example, the high charge carrier concentration results in an apparent shift in the band gap, the so-called Burstein-Moss shift, in addition to plasmonic resonances in the near infrared regime. While both effects are related to the carrier concentration and band structure, their lateral distribution and interaction with boundary conditions such as interfaces and surfaces are difficult to assess, and a direct observation of the local distribution has remained elusive. Here we employ electron energy-loss spectroscopy in scanning transmission electron microscopy (STEM-EELS) for direct observation of spatially resolved plasmonic resonances and Burstein-Moss shift in gallium doped zinc oxide (GZO), one of the most widely used TCOs. A 25 nm thick GZO film with a carrier concentration of 7× 10 20 cm − 3 has been grown epitaxially on a nominally undoped ZnO film. The GZO film shows a renormalized Burstein-Moss shift of ∼ 0.5 eV, in accordance with that expected from Hall effect measurements. The plasma resonance of the conduction band electrons is located at 0.82 eV in the bulk GZO, and with a substantial increase in intensity towards the sample surface, where a surface plasmon energy of 0.54 eV is observed. Hence we have directly measured differences in optical properties between the two films, the local variation across the GZO film has been studied, and we are also directly observing the difference between bulk and surface properties of GZO. © 2018 American Physical Society

• Sønsteby, Henrik Hovde; Aarholt, Thomas; Prytz, Øystein; Fjellvåg, Helmer & Nilsen, Ola (2018). First Complex Oxide Superconductor by Atomic Layer Deposition. Chemical Communications. ISSN 1359-7345. 54(59), p. 8253–8256. doi: 10.1039/c8cc04998j. Full text in Research Archive

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• Kang, Xiaolan; Reinertsen, Vilde Mari; Both, Kevin Gregor; Galeckas, Augustinas; Aarholt, Thomas & Prytz, Øystein [Show all 9 contributors for this article] (2022). Galvanic restructuring of exsolved nanoparticles for plasmonic and electrocatalytic energy conversion. Full text in Research Archive
• Kang, Xiaolan; Reinertsen, Vilde Mari; Both, Kevin Gregor; Galeckas, Augustinas; Aarholt, Thomas & Prytz, Øystein [Show all 9 contributors for this article] (2022). Galvanic Restructuring of Exsolved Nanoparticles for Plasmonic and Electrocatalytic Energy Conversion (Small 29/2022 Inside back cover feature). 18(29), p. 2270156–2270156. Full text in Research Archive
• Chatzitakis, Athanasios; Zhu, Junjie; Gudmundsdottir, Jonina Björg; Strandbakke, Ragnar; Both, Kevin Gregor & Aarholt, Thomas [Show all 12 contributors for this article] (2022). A monolithic and noble metal-free photoelectrochemical device of minimal engineering for efficient, unassisted water splitting. Full text in Research Archive
• Kang, Xiaolan; Reinertsen, Vilde Mari; Both, Kevin Gregor; Galeckas, Augustinas; Aarholt, Thomas & Prytz, Øystein [Show all 9 contributors for this article] (2022). Exsolved nanoparticles, galvanically restructured for tunable photo-electrocatalytic energy conversion. Full text in Research Archive
• Both, Kevin Gregor; Reinertsen, Vilde Mari; Kang, Xiaolan; Aarholt, Thomas; Neagu, Dragos & Prytz, Øystein [Show all 8 contributors for this article] (2022). Improved Photoelectrochemical Performance of SrTiO3 by Plasmonically Active Au Nanoparticles. Full text in Research Archive
• Aarholt, Thomas; Both, Kevin Gregor; Reinertsen, Vilde Mari & Prytz, Øystein (2021). Surface plasmon investigations by STEM-EELS mapping of Au/Ni nanoparticles on STO. Full text in Research Archive Show summary

  Metallic nanoparticles have traditionally had sharp absorption peaks between 1 and 5 eV due to the excitation of surface plasmons. Localized surface plasmon resonance (LSPR) is the coherent oscillation of quasi-free electrons excited by the electromagnetic fields of a photon or fast electron1,2. Since the absorption peak energies are in the range of visible light, surface plasmons present a potential pathway for absorbing energy to drive photoelectrochemical reactions or contribute to photovoltaic applications. For driving photoelectrochemical reactions, one difficulty is placing photoactive nanoparticles near the catalyst responsible for a chosen reaction, for example, water splitting. The ideal semiconductor for water splitting has a bandgap of 1.9 – 2.3 eV, although semiconductors in this range are seldom stable during photocatalytic water splitting3. Utilizing plasmonics in wide bandgap semiconductors is a promising alternative, avoiding the stability issue3. In this work, we present a STEM-EELS study of Au nanoparticles created by galvanic replacement of Ni nanoparticles grown by exsolution of a PLD-deposited thin-film of A-site excess strontium titanate (Sr1.07Ti 0.93Ni0.07O 3-δ) (STO). STO is an indirect, wide bandgap (3.2 eV) perovskite, studied as a promising photocatalyst for water splitting 4. Ni can easily substitute the Ti in the STO, making it a suitable candidate for exsolution, while Au cannot substitute Ti5. Simultaneously, Ni nanoparticles barely show any plasmonic activities, while Au particles are second only to Ag nanoparticles6. The presented manufacturing technique results in nanoparticles of gold and nickel freestanding, but socketed, on the surface and embedded in the bulk, with diameters between 50 and 100 nm, which are well-suited for plasmonic applications. As a starting point, STEM-EELS was performed with a monochromated 300kV STEM probe with ZeroLoss Peak (ZLP) full-width at half-maximum of 110 meV on an FEI Titan G2. A peculiarly shaped gold nanocluster was studied. After removing the ZLP, the spectrum image was linearly fitted with Gaussians centered at 1.46, 1.95, and 2.40 eV. The 2.40 eV peak is a well-known Au surface plasmon, whereas the remaining two are investigated in conjunction with modeling approaches. An RGB composite of the fit results can be seen in figure 1. Spectra extracted from the coloured regions can be seen in figure 2.

• Kjeldby, Snorre Braathen; Azarov, Alexander; Aarholt, Thomas; Prytz, Øystein & Vines, Lasse (2021). Defect-annealing in Si+-implanted β-Ga2O3. Full text in Research Archive
• Aarholt, Thomas (2020). Transmission Electron Microscopy for Advanced Functional Materials. Full text in Research Archive
• Aarholt, Thomas (2020). Imaging Nanomaterials with Transmission Electron Microscopy. Full text in Research Archive
• Aarholt, Thomas (2020). International Webinar on Sample Preparation for TEM and Data Analysis. Full text in Research Archive
• Aarholt, Thomas (2019). Å se atomer. Full text in Research Archive
• Haug, Kristian; Nguyen, Phuong Dan; Karlsen, Ole Bjørn; Aarholt, Thomas; Bazioti, Kalliopi & Prytz, Øystein (2019). Zinc ferrite spinel embedded in ZnO matrix for solar applications. Full text in Research Archive
• Aarholt, Thomas (2019). Såpeboblens fysikk. Full text in Research Archive
• Aarholt, Thomas; Frodason, Ymir Kalmann & Prytz, Øystein (2019). The impact of local relaxation on defect-complex STEM contrast. Full text in Research Archive
• Aarholt, Thomas; Sky, Thomas Neset; Nguyen, Phuong Dan & Prytz, Øystein (2018). Low-kV EELS band gapmeasurements on indium monolayer structures in ZnO. Full text in Research Archive
• Hansen, Per-Anders Stensby; Zikmund, Tomas; Yu, Ting; Kvalvik, Julie Nitsche; Aarholt, Thomas & Prytz, Øystein [Show all 8 contributors for this article] (2018). Aromatic-fluoride nanocomposite materials by atomic layer deposition. Full text in Research Archive
• Aarholt, Thomas (2017). Statistical analysis for noise reduction and phase separation in spectroscopy techniques. Full text in Research Archive
• Aarholt, Thomas (2017). Searching for single-defects by GPU-accelerated STEM simulation. Full text in Research Archive
• Aarholt, Thomas (2017). Smart Align - an acquisition tool for scanning microscopy. Full text in Research Archive
• Aarholt, Thomas (2017). Searching for single-defects by GPU-accelerated STEM simulation. Full text in Research Archive
• Aarholt, Thomas & Lobato, Ivan (2017). MULTEM Simulation Workshop. Full text in Research Archive
• Aarholt, Thomas (2017). Searching for single-defects by GPU-accelerated STEM Simulation. Full text in Research Archive

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