Kevin Gregor Both

• 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.

• 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
• Touni, Aikaterini; Liu, Xin; Kang, Xiaolan; Almeida Carvalho, Patricia; Diplas, Spyridon & Both, Kevin Gregor [Show all 8 contributors for this article] (2021). Galvanic Deposition of Pt Nanoparticles on Black TiO2 Nanotubes for Hydrogen Evolving Cathodes. ChemSusChem. ISSN 1864-5631. 14(22), p. 4993–5003. doi: 10.1002/cssc.202101559. Full text in Research Archive Show summary

  A galvanic deposition method for the in-situ formation of Pt nanoparticles (NPs) on top and inner surfaces of high-aspect-ratio black TiO2-nanotube electrodes (bTNTs) for true utilization of their total surface area has been developed. Density functional theory calculations indicated that the deposition of Pt NPs was favored on bTNTs with a preferred [004] orientation and a deposition mechanism occurring via oxygen vacancies, where electrons were localized. High-resolution transmission electron microscopy images revealed a graded deposition of Pt NPs with an average diameter of around 2.5 nm along the complete nanotube axis (length/pore diameter of 130 : 1). Hydrogen evolution reaction (HER) studies in acidic electrolytes showed comparable results to bulk Pt (per geometric area) and Pt/C commercial catalysts (per mg of Pt). The presented novel HER cathodes of minimal engineering and low noble metal loadings (μg cm−2 range) achieved low Tafel slopes (30–34 mV dec−1) and high stability in acidic conditions. This study provides important insights for the in-situ formation and deposition of NPs in high-aspect-ratio structures for energy applications.

• Zhu, Junjie; Gudmundsdottir, Jonina Björg; Strandbakke, Ragnar; Both, Kevin Gregor; Aarholt, Thomas & Almeida Carvalho, Patricia [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.

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• 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.
• 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.
• Both, Kevin Gregor; Reinertsen, Vilde Mari; Kang, Xiaolan; Neagu, Dragos; Prytz, Øystein & Norby, Truls [Show all 7 contributors for this article] (2022). Thin Film Exsolution of Metal Nanoparticles and Their Galvanic Restructuring for Plasmonically Enhanced Photocatalytic Activity.
• 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.
• Elgvin, Cana; Kjeldby, Snorre Braathen; Both, Kevin Gregor; Nguyen, Phuong Dan & Prytz, Øystein (2022). Optical Properties of Zinc Ferrite Nanoparticles Embedded in Zinc Oxide Thin Films Investigated by STEM, EELS and CL.
• 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). Small. ISSN 1613-6810. 18(29).
• 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 .
• Aarholt, Thomas; Both, Kevin Gregor; Reinertsen, Vilde Mari & Prytz, Øystein (2021). Surface plasmon investigations by STEM-EELS mapping of Au/Ni nanoparticles on STO. 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.

• Elgvin, Cana; Nguyen, Phuong Dan; Both, Kevin Gregor; Vines, Lasse & Prytz, Øystein (2021). Crystallographically oriented ZnFe2O4 nanoparticles in ZnO thin films .
• Both, Kevin Gregor; Chatzitakis, Athanasios; Bergum, Kristin; Hansen, Per-Anders Stensby; Prytz, Øystein & Norby, Truls (2023). Plasmonically Enhanced Photocatalysis: Synthesis, Physical Properties, and Applications. University of Oslo Livrary. ISSN 1501-7710. Full text in Research Archive Show summary

  In this work, plasmonic metal nanoparticles (MNPs) are utilized to improve the photoelectrochemical (PEC) response of strontium titanate (STO). These MNPs were introduced by either direct exsolution, i.e., nickel (Ni), copper (Cu), iron (Fe), ruthenium (Ru), and silver (Ag), or by galvanically replacing exsolved less noble MNPs, i.e., Ni by Gold (Au), or Cu for Ag. Au, Ag, and Cu were the materials chosen with significant plasmonic activity; Fe, Ru, Pt, and Ni were used to make MNPs with minimal plasmonic response. Two different stoichiometries of STO were synthesized. One, La-doped A-site deficient STO (La0.6Sr0.2Ti0.9Ni0.1O3–x), was exclusively doped with Ni and utilized as powder samples. The other stoichiometry was A-site excess STO (Sr1.07Ti0.93M0.07O3±δ, where M is the dopant) was doped with various metals. These excess perovskites were studied in thin film and powder forms. A-site excess STO thin films were deposited by pulsed laser deposition on silicon substrates. The as-deposited thin films appeared nanocrystalline or amorphous until the exsolution process was engaged. The exsolution step was studied explicitly for these A-site excess STO thin films where the formation of MNPs occurred not only at or near the thin film surface but also on grain interfaces and in bulk. Moreover, the dopant diffused significantly during the process. While the size of the template particles depended on the exsolution conditions, the galvanic replacement reaction determined the shapes and sizes of the newly formed MNPs. The replacement time and the form (thin film/powder) of STO influenced the results, both completely replaced particles and partially replaced particles with complex structures were obtained. Additionally, more prolonged galvanic replacement reactions lead to larger particles. In turn, the specific shape of the plasmonic MNPs determined the localized surface plasmon resonance band shape and peak position. Overall, exsolution leads to well-socketed MNPs, a property seemingly inherited by the MNPs created by galvanic replacement. Well-socketed MNPs are extremely difficult to obtain by any other technique and have a favorable localized surface plasmon resonance peak shift. The PEC response revealed that reducing STO first decreases the material’s response. Reducing it further, however, increases the PEC response significantly. Au MNPs increase the PEC performance until the MNPs reach a specific size and subsequently decrease the PEC performance when growing more prominent. This work highlights the ease by which well-socketed plasmonic MNPs can be created, some impossible to synthesize by another technique, and how different reaction conditions can change the shape and size of the MNPs, ultimately tuning the localized surface plasmon resonance band shape and peak position. The method of exsolution and galvanic replacement reaction was generalized by utilizing different elements, implying that the tuning of catalytic activity depends on the choice of elements and reaction conditions.

• Køller, Egil Sperstad; Norby, Truls; Bjørheim, Tor Svendsen & Both, Kevin Gregor (2021). The Effect of Exsolved Nanoparticles on the Thermoelectric Properties of SrTiO3. Universitetet i Oslo.
• Both, Kevin Gregor; Norby, Truls Eivind; Bjørheim, Tor Svendsen & Løvvik, Ole Martin (2019). Cu2O – ZnO p-n Junctions: Thermodynamics, Structure, and Properties – an Experimental and Theoretical Study. Universitetet i Oslo.

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