Athanasios Chatzitakis

• Grandcolas, Mathieu; Løvoll, Marte; Dahl, Øystein; Thøgersen, Annett; Svenum, Ingeborg-Helene & Wenckstern, Holger Von [Show all 7 contributors for this article] (2026). Rapid thermal processing of photoanodes for solar water splitting: status, challenges and opportunities. Solar Energy Materials and Solar Cells. ISSN 0927-0248. 309, p. 1–9. doi: 10.1016/j.solmat.2026.114728. Full text in Research Archive Show summary

  Rapid thermal processing (RTP) is gaining attention as an efficient alternative to conventional annealing of materials for photoelectrochemical (PEC) water splitting. By delivering ultrafast heating and precise temperature control, RTP introduces non-equilibrium conditions that enable metastable phase formation, defect engineering, and interfacial control, while preserving transparent conductive oxides (TCOs) essential for PEC architectures. These advantages improve crystallinity, activate dopants, and enhance interfacial quality without the drawbacks of prolonged furnace treatments. Recent studies report two- to fourfold increases in photocurrent density and negative onset potential shifts up to 300 mV for RTP-treated photoanodes, highlighting RTP's potential to accelerate PEC performance toward industrial benchmarks. Despite these advantages, RTP remains underutilized in PEC research, with challenges in process optimization, atmosphere control, and scalability still unresolved. This review summarizes current progress on RTP for several oxide photoanodes, compares its performance with that of conventional annealing processes, and discusses mechanistic insights. Future directions include hybrid thermal strategies, real-time diagnostics, and data-driven design to enable scalable PEC device manufacturing and sustainable solar hydrogen production.

• Petlund, Henrik; Tezel, Elif; Sypalyte, Goda; Molina, M. Asunción; Costley-Wood, Lucy & Szilagyi, Petra Agota [Show all 7 contributors for this article] (2025). Gas-Phase Electrocatalytic CO2 Reduction on Pd-Embedded UiO-67 Metal–Organic Framework Precatalysts Under Varying Relative Humidity Conditions. ChemSusChem. ISSN 1864-5631. p. 1–31. doi: 10.1002/cssc.202501929. Full text in Research Archive Show summary

  Electrocatalytic CO2 reduction (ECO2R) to high-value chemicals is a promising method to upcycle emitted CO2, but it is also a fascinating scientific challenge. Catalyst materials, as well as cell configurations, play a pivotal role in the efficacy and efficiency of the ECO2R reaction, which also dictates reaction pathways and product selectivity. In this work, we employ the isotopological Zr- and Ce-based UiO-67 metal–organic frameworks (MOFs) that contain Pd species in a zero-gap gas diffusion cathode electrode configuration, where the water content, i.e., relative humidity (RH) level, in the CO2 gas stream can be varied. We show that only UiO-67-based MOFs containing Pd embedded in their pores can produce syngas, while the product selectivity can be controlled by varying the RH levels in the gas stream. The pristine MOFs (precatalysts) undergo chemical and structural transformation during the ECO2R reaction, forming the active catalysts toward CO2 electroreduction to syngas. Our work highlights the effect of water content on the selectivity during ECO2R, but also the need for predictive catalyst design for effective electroreduction of CO2 to high-value chemicals.

• Bråtalien, Lars Botolv; Nilsen, Ola; Saha, Dipankar; Mohn, Chris Erik; Chatzitakis, Athanasios & Valldor, Björn Martin (2025). Layered anion ordering in oxide chloride Ca2TlO3Cl. Dalton Transactions. ISSN 1477-9226. 54(31), p. 11887–11894. doi: 10.1039/d5dt01111f. Full text in Research Archive Show summary

  Single crystals and polycrystals of the novel compound Ca2TlO3Cl can be obtained by gas-phase and solid-state reactions, respectively. The title compound's crystal structure is hexagonal (a = 3.8806(2) Å, c = 18.6168(7) Å, P63/mmc) as measured by single crystal X-ray diffraction. A Rietveld refinement on powder X-ray diffraction data confirms this crystal structure. The atomic arrangement can be described as a cation ordered anti-structure of ScAl3C3, with a homoleptically, five-fold oxygen coordinated Tl3+ that consitutes corner-sharing trigonal bipyramids. Between these Tl–O sheets, a Cl-layer is found, making the whole structure quasi-2D. Thermal expansion coefficients in the range 15–290 K, as estimated from X-ray powder diffraction data, reveal that the relative length of the crystallographic c-axis (α33) is more temperature dependent than that of the a-axis (α11) lengths. The brown color of the polycrystalline title compound was investigated by spectroscopic reflectometry measurements. These data suggest an optical band gap of either 1.4 eV (indirect) or about 2.4–2.7 eV (direct), depending on interpretation model. However, density funciton theory calculations deliver a band structure that indicates that indirect band gaps are most likely, but with a size of about 2.7 eV. This discrepancy is discussed as a result of further local excitation phenomena, like charge-transfers and crystal defects. Photo-electrochemical investigations suggest that the title compound has an n-type character and is electrochemically activated by solar light.

• Grandcolas, Mathieu; Thøgersen, Annett; Svenum, Ingeborg-Helene; Both, Kevin Gregor & Chatzitakis, Athanasios (2025). Tantalum nitride photoanodes: a promising future for photoelectrochemical water splitting? Current Opinion in Chemical Engineering. 48. doi: 10.1016/j.coche.2025.101127. Full text in Research Archive
• 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. Full text in Research Archive Show summary

  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.

• Petlund, Henrik; Faid, Alaa Yahia; Zhu, Junjie; Pokle, Anuj; Norby, Truls & Sunde, Svein [Show all 7 contributors for this article] (2024). Iron and Nickel Substituted Perovskite Cobaltites for Sustainable Oxygen Evolving Anodes in Alkaline Environment. ChemSusChem. ISSN 1864-5631. doi: 10.1002/cssc.202401403. Full text in Research Archive Show summary

  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.

• Nambi, Ashwin; Chatzitakis, Athanasios; Olsbye, Unni; Hjelm, Johan; Zhao, Yujie & Kaiser, Andreas (2024). Gas-phase electrochemical CO2 reduction on silver-copper BTC MOF in a zero-gap membrane electrode assembly. Electrochimica Acta. ISSN 0013-4686. 506. doi: 10.1016/j.electacta.2024.144763. Full text in Research Archive Show summary

  Bimetallic AgCu-BTC MOFs, derived from Cu-BTC (HKUST-1), have been synthesized by fast co-precipitation method and evaluated for CO2 reduction reaction (CO2RR) using humidified CO2 gas in a zero-gap configuration. Three compositions with varying Ag content – AgCu-1 (9.4 at.%), AgCu-2 (12.5 at.%), and AgCu-3 (16.5 at.%) – were characterized by SEM-EDS, PXRD, FTIR, and XPS. These structural analyzes confirmed the formation of a low-crystalline AgCu-BTC MOF with Ag+1 ions coordinated to the BTC framework. Electrochemical tests (CV and LSV) revealed that AgCu-3 MOF, with the highest Ag content (16.5 at.%), exhibited a more positive onset potential at -0.65 V vs Ag/AgCl compared to HKUST-1 MOF, indicating enhanced CO2RR activity, owing to Ag incorporation. Constant potential (CP) experiments showed that AgCu-3 MOF predominantly produced CO and H2, achieving faradaic efficiency of approximately 65 % for CO production and 5 % for H2 production. Moreover, lowering the relative humidity (RH%) in the inlet CO2 gas stream from 80% to 20% increased CO production by 2-fold from 6.2 µmol s-1 cm-2 to 13 µmol s-1 cm-2, simultaneously suppressing the hydrogen evolution reaction (HER). This reduction in humidity resulted in an increased local concentration of CO2 at the catalyst site, leading to an enhanced CO2RR rate. However, substantial morphological changes have been observed in the AgCu-3 MOF after the CP experiment under humid CO2 reduction conditions.

• Both, Kevin Gregor; Neagu, Dragos; Prytz, Øystein; Norby, Truls & Chatzitakis, Athanasios (2024). Exsolution of Ni nanoparticles in A-site excess STO films. Nanoscale Advances. doi: 10.1039/d4na00213j. Full text in Research Archive Show summary

  Exsolution is a technique to create metal nanoparticles embedded within a matrix. The phenomenon has previously predominantly been studied in A-site deficient and stoichiometric perovskite powders. Here, we present a systematic study of an A-site excess perovskite oxide based on SrTiO3 thin films, doped with nickel and exsolved under different conditions. The study aims to shed light on particle formation in these novel systems, including the effects of (i) the thin film thickness, (ii) pre-exsolution annealing in an oxidative atmosphere, (iii) a reductive atmosphere during the exsolution step, and (iv) exsolution time on the particle size and particle density. Our results indicate that exsolution occurs quickly, forming nanoparticles both on the surface and in the bulk of the host perovskite. The findings indicate that pre-annealing in an ambient atmosphere leads to fewer but larger exsolved particles compared to samples without pre-annealing. Consequently, while crystallization of the thin film occurs in both atmospheres, the simultaneous crystallization of the thin film and formation of the nanoparticles leads to a smaller apparent average radius. Moreover, we present evidence that metal particles can be found beyond the originally doped region. These findings are a step towards realizing tunable functional materials using exsolution to create metallic nanostructures within a thin film in a predictable manner.

• Sannes, Johnny Andreas; Chatzitakis, Athanasios; Andersen, Niels Højmark; Nilsen, Ola; Frøen, Emil Herman & Valldor, Björn Martin (2024). Characterization and Solid-State UV-Vis Investigations of Photoelectrocatalytically Active La5Cl7[TeO3]4, a Mixed Anion Compound with Alternating 2D Layers of Oxygen and Chlorine. Inorganic Chemistry. ISSN 0020-1669. 63(40), p. 18632–18641. doi: 10.1021/acs.inorgchem.4c02392. Full text in Research Archive Show summary

  An oxide chloride, La5Cl7[TeO3]4, was synthesized using the conventional high-temperature solid-state synthesis technique in an inert atmosphere. This compound possesses a novel crystal structure that can be described with the triclinic space group P1̅ (No. 2) and unit cell parameters: a = 7.2634(3) Å, b = 8.1241(3) Å, c = 9.1993(3) Å, α = 79.373(1)°, β = 83.599(1)°, and γ = 82.511(1)°. The preference of Te(IV) to coordinate to oxygen and direct its lone pair toward the lower charged chlorine results in 2D layers of both oxygen and chlorine, alternating along the crystallographic b-direction. Homoleptic coordination, solely to oxygen, and heteroleptic coordination to oxygen and chlorine are observed for lanthanum, forming layers connected through edge-sharing polyhedra. In the crystal structure, two distinct tellurium positions are observed, with three close Te–O distances, emphasizing an active lone pair. The compound has been investigated by solid-state UV–vis measurements, and a band gap of 3.44 eV has been determined by DFT calculations. Detailed photoelectrochemical measurements clearly indicate that the title compound is photoelectrocatalytically active, showing an n-type behavior. Raman spectroscopy confirms that complex tellurite ions are present in the crystal structure; several observed bands can be assigned to Te–O stretching, reflecting the relatively low crystallographic symmetry of the title compound.

• Liu, Zhengrong; Zhou, Zilin; Yang, Jiaming; Sun, Yueyue; Yin, Chaofan & Li, Ruhuan [Show all 9 contributors for this article] (2024). Design of a B-Site Co-Free Multielement Perovskite Oxide as Oxygen Electrode for Efficient CO2 Solid Oxide Electrolysis Cells. ACS Applied Energy Materials. doi: 10.1021/acsaem.4c02000. Full text in Research Archive Show summary

  The traditional oxygen electrode in solid oxide electrolysis cells (SOECs), (La,Sr)(Co,Fe)O3 (LSCF), suffers from high cost, evaporation at high temperatures, and societal aspects of the use of Co. In this work, a Co-free B-site multielement (so-called high-entropy) perovskite oxide, La0.6Sr0.4Cu0.2Fe0.2Ti0.2Mn0.2Ni0.2O3‑δ (LSCuFTMN), has been synthesized and successfully applied as a novel oxygen electrode. X-ray photoelectron spectroscopy (XPS) data indicate that the multiple transition elements in the B-site exist in various valence states, leading to a spatially variable electron structure. Electrochemical measurements of LSCuFTMN suggest that the material exhibits extraordinary catalytic activity and stability under the studied working atmospheres and a decrease in polarization resistance by 24% compared to LSCF. By distribution of relaxation time (DRT) analysis, LSCuFTMN possesses better mass and charge transfer performance than traditional LSCF. An SOEC with LSCuFTMN as the oxygen electrode has been assembled and tested, and a current density of 1.2 A cm−2 is obtained at 2.0 V and 800 °C in electrolysis of pure CO2, higher by nearly 50% compared to LSCF. The faradaic efficiency is over 95%. No clear recession is observed in the long term stability test. It is evident that multication − so-called high-entropy − oxides could be promising materials for improving the working performance of SOECs.

• Dahl, Øystein; Sunding, Martin Fleissner; Killi, Veronica Anne-Line Kathrine; Svenum, Ingeborg-Helene; Grandcolas, Mathieu & Andreassen, Magnus [Show all 10 contributors for this article] (2023). Interrogation of the Interfacial Energetics at a Tantalum Nitride/Electrolyte Heterojunction during Photoelectrochemical Water Splitting by Operando Ambient Pressure X-ray Photoelectron Spectroscopy. ACS Catalysis. 13(17), p. 11762–11770. doi: 10.1021/acscatal.3c02423. Full text in Research Archive Show summary

  Photoelectrochemical (PEC) water electrolysis is an important energy conversion (power-to-chemical) method, providing a solution to the intermittent nature of solar energy. However, as PEC systems usually suffer from low operational stability, they are seriously lagging in up-scaled demonstrations and viability. PEC systems are based on semiconductor/liquid interfaces, which have been extensively studied by experiments and theory, but there is a significant knowledge gap in the energetics of such interfaces during operation. In this work, operando ambient pressure X-ray photoelectron spectroscopy (AP-XPS) has been used to characterize the electrical and spectroscopic properties of a pristine Ta3N5 photoelectrode and a Ta3N5/NiOx protection/passivation layer system, which stabilizes an otherwise quickly corroding pristine photoelectrode. We directly observed Fermi-level pinning of Ta3N5 within the applied potential window under both dark and illumination conditions, detrimental to the performance and stability of the photoelectrode. Interestingly, in the Ta3N5/NiOx protection/passivation layer system, the Fermi level gets unpinned under illumination, allowing quasi-Fermi-level splitting and sustaining a significant PEC performance as well as high stability.

• 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.

• Sun, Xinwei; Vøllestad, Einar; Rørvik, Per Martin; Prodinger, Sebastian; Kalantzopoulos, Georgios & Chatzitakis, Athanasios [Show all 7 contributors for this article] (2022). Surface protonic conductivity in chemisorbed water in porous nanoscopic CeO2. Applied Surface Science. ISSN 0169-4332. 611, p. 1–12. doi: 10.1016/j.apsusc.2022.155590. Full text in Research Archive Show summary

  CeO2 surfaces play decisive roles in heterogeneous catalysis of important processes. Here, we investigate adsorption and dissociation of water and migration of protons on internal surfaces of nanoscopic porous CeO2. Sorption and thermogravimetry confirm literature suggestions that the surface is hydrogenated to Ce3+ ions and protons H+. The following chemisorption is dissociative, yet weak, and physisorption sets in only at the very highest relative humidities, reflecting hydrophobic behaviour. We link sample conductivities to surface protonic conductances via a brick layer model and show that behaviours at high, intermediate, and low temperatures with, respectively, positive, close to zero, and negative apparent activation energies and pH2O^(1/2), pH2O^1, and pH2O^(3/2) dependences, can be attributed to different models of migration all within the chemisorbed layer, without contribution from physisorbed water. While CeO2 may special in this respect due to the effect of the hydrogenated surface, we believe the extended models of transport in the chemisorbed layer may apply also to other oxides. Unsaturated chemisorption may play an important role for CeO2 as catalyst in that the surface is left available for reactant molecules, still with availability of dissociated and mobile protons in the chemisorbed layer and electronic defects by Ce3+ in the surface.

• Liu, Xin; Risbakk, Sanne; Carvalho, Patricia Almeida; Yang, Mingyi; Backe, Paul Hoff & Bjørås, Magnar [Show all 8 contributors for this article] (2022). Immobilization of FeFe-hydrogenase on black TiO2 nanotubes as biocathodes for the hydrogen evolution reaction. Electrochemistry communications. ISSN 1388-2481. 135. doi: 10.1016/j.elecom.2022.107221. Full text in Research Archive Show summary

  Hydrogenases are attractive biocatalysts for utilization in electrochemical devices as potential replacement for Pt in hydrogen evolving electrodes. In this work, we investigate the immobilization of ferredoxin tagged FeFe-hydrogenase (Fd-HydA1) on black TiO2 nanotubes (bTNTs), with uniform nanotube opening diameters of 140 nm. By utilizing an immunogold labelling method, we show that the enzymes attach on the top surface of the bTNTs film rather than on the inner nanotube walls, reflecting the difficulty to insert enzymes into high aspect-ratio nanomaterials for O2-shielding. Nevertheless, cyclic voltammetry demonstrates direct electron transfer between Fd-HydA1 and bTNTs for the hydrogen evolution reaction (HER) in neutral media. This work provides new insight towards design of new nanostructured electrodes for enzyme immobilization.

• 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.

• Touni, Aikaterini; Liu, Xin; Kang, Xiaolan; Papoulia, Chrysanthi; Pavlidou, Eleni & Lambropoulou, Dimitra [Show all 9 contributors for this article] (2022). Methanol Oxidation at Platinum Coated Black Titania Nanotubes and Titanium Felt Electrodes. Molecules. ISSN 1431-5157. 27(19), p. 1–15. doi: 10.3390/molecules27196382. Full text in Research Archive Show summary

  Optimized Pt-based methanol oxidation reaction (MOR) anodes are essential for commercial direct methanol fuel cells (DMFCs) and methanol electrolyzers for hydrogen production. High surface area Ti supports are known to increase Pt catalytic activity and utilization. Pt has been deposited on black titania nanotubes (bTNTs), Ti felts and, for comparison, Ti foils by a galvanic deposition process, whereby Pt(IV) from a chloroplatinate solution is spontaneously reduced to metallic Pt (at 65 °C) onto chemically reduced (by CaH2) TNTs (resulting in bTNTs), chemically etched (HCl + NaF) Ti felts and grinded Ti foils. All Pt/Ti-based electrodes prepared by this method showed enhanced intrinsic catalytic activity towards MOR when compared to Pt and other Pt/Ti-based catalysts. The very high/high mass specific activity of Pt/bTNTs (ca 700 mA mgPt−1 at the voltammetric peak of 5 mV s−1 in 0.5 M MeOH) and of Pt/Ti-felt (ca 60 mA mgPt−1, accordingly) make these electrodes good candidates for MOR anodes and/or reactive Gas Diffusion Layer Electrodes (GDLEs) in DMFCs and/or methanol electrolysis cells.

• 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.

• Xu, Kaiqi; Chatzitakis, Athanasios Eleftherios; Backe, Paul Hoff; Ruan, Qiushi; Tang, Junwang & Rise, Frode [Show all 8 contributors for this article] (2021). In situ cofactor regeneration enables selective CO2 reduction in a stable and efficient enzymatic photoelectrochemical cell. Applied Catalysis B: Environmental. ISSN 0926-3373. 296. doi: 10.1016/j.apcatb.2021.120349. Full text in Research Archive Show summary

  Mimicking natural photosynthesis by direct photoelectrochemical (PEC) reduction of CO2 to chemicals and fuels requires complex cell assemblies with limitations in selectivity, efficiency, cost, and stability. Here, we present a breakthrough cathode utilizing an oxygen tolerant formate dehydrogenase enzyme derived from clostridium carboxidivorans and coupled to a novel and efficient in situ nicotinamide adenine dinucleotide (NAD+/NADH) regeneration mechanism through interfacial electrochemistry on g-C3N4 films. We demonstrate stable (20 h) aerobic PEC CO2-to-formate reduction at close to 100 % faradaic efficiency and unit selectivity in a bio-hybrid PEC cell of minimal engineering with optimized Ta3N5 nanotube photoanode powered by simulated sunlight with a solar to fuel efficiency of 0.063 %, approaching that of natural photosynthesis.

• 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.

• Kang, Xiaolan; Berberidou, Chrysanthi; Galeckas, Augustinas; Bazioti, Kalliopi; Sagstuen, Einar & Norby, Truls Eivind [Show all 8 contributors for this article] (2021). Visible Light Driven Photocatalytic Decolorization and Disinfection of Water Employing Reduced TiO2 Nanopowders. Catalysts. 11(2). doi: 10.3390/catal11020228. Full text in Research Archive Show summary

  Defect-engineering of TiO2 can have a major impact on its photocatalytic properties for the degradation of persisting and non-biodegradable pollutants. Herein, a series of intrinsic and extrinsic defects are induced by post annealing of crystalline TiO2 under different reducing atmospheres. A detailed optoelectronic characterization sheds light on the key characteristics of the defect-engineered TiO2 nanopowders that are linked to the photocatalytic performance of the prepared photocatalysts. The photodegradation of a model dye, malachite green, as well as the inactivation of bacterial endospores of the Geobacillus stearothermophilus species were studied in the presence of the developed catalysts under visible light illumination. Our results indicate that a combination of certain defects is necessary for the improvement of the photocatalytic process for water purification and disinfection under visible light.

• Kang, Xiaolan; Chaperman, Larissa; Galeckas, Augustinas; Ammar, Souad; Mammeri, Fayna & Norby, Truls [Show all 7 contributors for this article] (2021). Water Vapor Photoelectrolysis in a Solid-State Photoelectrochemical Cell with TiO2 Nanotubes Loaded with CdS and CdSe Nanoparticles. ACS Applied Materials & Interfaces. ISSN 1944-8244. 13(39), p. 46875–46885. doi: 10.1021/acsami.1c13047. Full text in Research Archive Show summary

  In this study, polyol-made CdS and CdSe crystalline nanoparticles (NPs) are loaded by impregnation on TiO2 nanotube arrays (TNTAs) for solar-simulated light-driven photoelectrochemical (PEC) water vapor splitting. For the first time, we introduce a safe way to utilize toxic, yet efficient photocatalysts by integration in solid-state PEC (SSPEC) cells. The enabling features of SSPEC cells are the surface protonic conduction mechanism on TiO2 and the use of polymeric electrolytes, such as Nafion instead of liquid ones, for operation with gaseous reactants, like water vapor from ambient humidity. Herein, we studied the effects of both the operating conditions in gaseous ambient atmospheres and the surface modifications of TNTAs-based photoanodes with well-crystallized CdS and CdSe NPs. We showed 3.6 and 2.5 times increase in the photocurrent density of defective TNTAs modified with CdS and CdSe, respectively, compared to the pristine TNTAs. Electrochemical impedance spectroscopy and structural characterizations attributed the improved performance to the higher conductivity induced by intrinsic defects as well as to the enhanced electron/hole separation at the TiO2/CdS heterojunction under gaseous operating conditions. The SSPEC cells were evaluated by cycling between high relative humidity (RH) (80%) and low RH levels (40%), providing direct evidence of the effect of RH and, in turn, adsorbed water, on the cell performance. Online mass spectrometry indicated the corresponding difference in the H2 production rate. In addition, a complete restoration of the SSPEC cell performance from low to high RH levels was also achieved. The presented system can be employed in off-grid, water depleted, and air-polluted areas for the production of hydrogen from renewable energy and provides a solution for the safe use of toxic, yet efficient photocatalysts.

• Touni, Aikaterini; Liu, Xin; Kang, Xiaolan; Carvalho, Patricia Almeida; 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.

• Andersen, Håkon; Xu, Kaiqi; Malyshkin, Dmitry; Strandbakke, Ragnar & Chatzitakis, Athanasios Eleftherios (2020). A highly efficient electrocatalyst based on double perovskite cobaltites with immense intrinsic catalytic activity for water oxidation. Chemical Communications. ISSN 1359-7345. 56(7), p. 1030–1033. doi: 10.1039/c9cc08765f. Full text in Research Archive
• Barakezehi, Marjan; Montazer, Majid; Sharif, Farhad; Norby, Truls Eivind & Chatzitakis, Athanasios Eleftherios (2020). MOF-modified polyester fabric coated with reduced graphene oxide/polypyrrole as electrode for flexible supercapacitors. Electrochimica Acta. ISSN 0013-4686. 336, p. 135743–135743. doi: 10.1016/j.electacta.2020.135743. Full text in Research Archive Show summary

  The interest in wearable supercapacitors has been flourishing in both the scientific and industrial fields. Here, daily-used polyester fabric (PET) is modified by using an Al-based MOF, namely MIL-53(Al), via a layer-by-layer assembly method at room temperature. Then, reduced graphene oxide is deposited on the PET/MOF surface through dip coating in a graphene oxide suspension, followed by chemical reduction. Ultimately, pyrrole is polymerized on the surface of the PET/MOF/rGO surface by an in situ chemical polymerization process. The obtained composite is used as electrode achieving an areal capacitance of 510 mF cm−2 at a scanning rate of 1 mV s−1 in an aqueous H2SO4 electrolyte. The optimized electrode is employed in a symmetrical solid-state-supercapacitor device, which delivers a volumetric capacitance of 3.5 F cm−3, energy density of 64 μW h cm−3 and power density of 0.6 mW cm−3. The symmetrical device shows a good cycling stability even after 12 months storage in ambient conditions retained 85% of the initial capacitance after 1000 voltammetric cycles.

• Getz, Marit Norderhaug; Chatzitakis, Athanasios Eleftherios; Liu, Xin; Carvalho, Patricia Almeida; Bjørheim, Tor Svendsen & Norby, Truls Eivind (2020). Voids in walls of mesoporous TiO2 anatase nanotubes by controlled formation and annihilation of protonated titanium vacancies. Materials Chemistry and Physics. ISSN 0254-0584. 239. doi: 10.1016/j.matchemphys.2019.121953. Full text in Research Archive Show summary

  Amorphous TiO2 nanotubes (TNTs) grown anodically on Ti metal in aqueous electrolytes show a crystallization behaviour strongly dependent on the atmosphere used during annealing at high temperature. In wet oxidizing conditions, the amorphous TNTs walls transform into relatively large and well crystallized anatase-like domains permeated by prismatic voids. On the other hand, crystallization of the amorphous nanotubes under dry reducing conditions induces nanocrystalline aggregates that do not show prismatic voids and exhibit different electronic properties. Supported by density functional theory calculations, it is argued that the formation or absence of voids can be understood in terms of formation and condensation of protonated titanium vacancies. Tunable morphology through defect chemical engineering as such, enables TNTs with increased surface area and catalytic activity, which find potential application in supercapacitors, sensors, photocatalysis, photoelectrochemistry, and dye-sensitized solar cells.

• Sun, Xinwei; Xu, Kaiqi; Chatzitakis, Athanasios Eleftherios & Norby, Truls Eivind (2020). Photocatalytic generation of gas phase reactive oxygen species from adsorbed water: Remote action and electrochemical detection. Journal of Environmental Chemical Engineering. doi: 10.1016/j.jece.2020.104809. Full text in Research Archive Show summary

  The improvement of indoor environments is of great importance as it can significantly improve human health, comfort and productivity. Herein, different forms of TiO2 nanorods were used as the photocatalyst for generation of reactive oxygen species (ROS) in a gas phase photoreactor under controlled humidity. Several parameters were investigated by monitoring the remote decolourisation of Methylene Blue (MB) embedded in a Nafion film. A decolourisation of 26% under 80% relative humidity was observed when the MB film was 0.5 cm away from the photocatalyst. The length and ratio of light/dark intervals have major impacts on the efficiency of the gas phase photocatalytic process, which we link to the amount of water adsorbed on the photocatalyst, as the source for hydroxyl radicals. Furthermore, the photocatalytic production of ROS was quantified through a polyaniline electrochemical sensor and a rate of 1 · 10*12 of ROS molecules s−1 was estimated. This study contributes to the efficacy of the gas phase photocatalytic method in air decontamination, for the development of efficient air cleaning devices.

• Chaguetmi, Samiha; Chaguetmi, Salem; Sobti, Nadjah; Belkahla, Hanène; Chaperman, Larissa & Chatzitakis, Athanasios Eleftherios [Show all 9 contributors for this article] (2020). Enhancement of the photoelectrochemical properties of TiO2 nanofibers supported on Ti sheets by polyol-made CdSe quantum-dots impregnation. Materials Letters. ISSN 0167-577X. 273. doi: 10.1016/j.matlet.2020.127934. Full text in Research Archive
• Xu, Kaiqi; Chatzitakis, Athanasios Eleftherios; Risbakk, Sanne; Yang, Mingyi; Backe, Paul Hoff & Grandcolas, Mathieu [Show all 8 contributors for this article] (2019). High performance and toxicity assessment of Ta3N5 nanotubes for photoelectrochemical water splitting. Catalysis Today. ISSN 0920-5861. 361. doi: 10.1016/j.cattod.2019.12.031. Full text in Research Archive Show summary

  In this work, Co-based cocatalysts are electrodeposited on mesoporous Ta3N5 nanotubes. The electrodeposition time is varied and the optimized photoelectrode reaches a photocurrent density of 6.3 mA/cm2 at 1.23 V vs. SHE, under simulated solar illumination of 1 Sun, in 1 M NaOH. The best performing electrode, apart from the high photocurrent density, shows improved stability under intense photoelectrochemical water splitting conditions. The dual function of the cocatalyst to improve not only the photoelectrochemical performance, but also the stability, is highlighted. Moreover, we adopted a simple protocol to assess the toxicity of Co and Ta contained nanostructured materials (representing used photoelectrodes) employing the human cell line HeLa S3 as target cells.

• Chatzitakis, Athanasios Eleftherios & Longela, Simon Makusudi (2019). Electrochemical characterisation of bovine serum albumin adsorption on phenolic- coated nickel-titanium. Journal of Engineering Science and Technology Review. ISSN 1791-9320. 12(2), p. 1–8. doi: 10.25103/jestr.122.01. Full text in Research Archive
• Xu, Kaiqi; Chatzitakis, Athanasios Eleftherios; Vøllestad, Einar; Ruan, Qiushi; Tang, Junwang & Norby, Truls Eivind (2019). Hydrogen from wet air and sunlight in a tandem photoelectrochemical cell. International Journal of Hydrogen Energy. ISSN 0360-3199. 44(2), p. 587–593. doi: 10.1016/j.ijhydene.2018.11.030. Full text in Research Archive Show summary

  A solid-state photoelectrochemical (SSPEC) cell is an attractive approach for solar water splitting, especially when it comes to monolithic device design. In a SSPEC cell the electrodes distance is minimized, while the use of polymer-based membranes alleviates the need for liquid electrolytes, and at the same time they can separate the anode from the cathode. In this work, we have made and tested, firstly, a SSPEC cell with a Pt/C electrocatalyst as the cathode electrode, under purely gaseous conditions. The anode was supplied with air of 80% relative humidity (RH) and the cathode with argon. Secondly, we replaced the Pt/C cathode with a photocathode consisting of 2D photocatalytic g-C3N4, which was placed in tandem with the photoanode (tandem-SSPEC). The tandem configuration showed a three-fold enhancement in the obtained photovoltage and a steady-state photocurrent density. The mechanism of operation is discussed in view of recent advances in surface proton conduction in absorbed water layers. The presented SSPEC cell is based on earth-abundant materials and provides a way towards systems of artificial photosynthesis, especially for areas where water sources are scarce and electrical grid infrastructure is limited or nonexistent. The only requirements to make hydrogen are humidity and sunlight.

• Grandcolas, Mathieu; Wabende, Brian; Yang, Juan; Mei, Sen; Xu, Kaiqi & Norby, Truls Eivind [Show all 7 contributors for this article] (2019). Preparation of TiO2 rutile nanorods decorated with cobalt oxide nanoparticles for solar photoelectrochemical activity. Materials Letters. ISSN 0167-577X. 247, p. 1–3. doi: 10.1016/j.matlet.2019.03.087. Full text in Research Archive Show summary

  The present paper investigates the preparation of titanium dioxide (TiO2) nanorods decorated with cobalt oxide (Co3O4) nanoparticles and its application as solar photoelectrocatalyst. TiO2 rutile nanorods have been prepared on conductive glass via a hydrothermal reaction in acidic media, shaped as squared rods with an average length of 1.7 µm and thicknesses between 50 and 120 nm. Co3O4 nanoparticles with an average diameter of 30 nm were synthesized using a simple precipitation method. Spin coating the nanoparticles on the TiO2 nanorods shows enhanced photocurrents under simulated solar light up to 1.3 mA/cm2 at 1.6 V vs. SHE.

• Chatzitakis, Athanasios Eleftherios & Sartori, Sabrina (2019). Recent Advances in the Use of Black TiO2 for Production of Hydrogen and Other Solar Fuels. ChemPhysChem. ISSN 1439-4235. 20(10), p. 1272–1281. doi: 10.1002/cphc.201801094. Full text in Research Archive Show summary

  Black TiO2 has emerged as one of the most promising photocatalysts recently discovered. The reason behind its catalytic activity is considered to be due to the presence of defects and Ti3+ species at the surface of black TiO2 nanostructures, which are crucial for its diverse applications. Moreover, disordered/crystalline surface layers and bulk regions have been identified and appear to influence the intrinsic properties of the material. Here, we present the latest studies on the use of black TiO2 for metal free hydrogen production, as well as for CO2 photoreduction and N2 photofixation. After highlighting the structure/property relations, we conclude with some critical questions and suggest further topics of research in order to better understand the underlying mechanisms of light absorption in black TiO2, especially towards solar fuels production.

• Liu, Xin; Carvalho, Patricia Almeida; Getz, Marit Norderhaug; Norby, Truls Eivind & Chatzitakis, Athanasios Eleftherios (2019). Black Anatase TiO2 Nanotubes with Tunable Orientation for High Performance Supercapacitors. Journal of Physical Chemistry C. ISSN 1932-7447. 126(36), p. 21391–21940. doi: 10.1021/acs.jpcc.9b05070. Full text in Research Archive Show summary

  Black TiO2 is a versatile material that finds many applications in the field of photocatalysis but also in energy storage devices, such as supercapacitors. In this work, a new method is developed for the synthesis of black TiO2 in the form of nanotubes. Amorphous TiO2 nanotubes (TNTs) are annealed and reduced in the presence of CaH2, and by a simple manipulation, we are able to obtain control over the preferred crystal orientation. The reduced TNTs show high power and energy densities, as well as unprecedented stability under intensive charge/discharge cycling. The oriented reduced TNTs show metallic-like behavior with improved conductivity when compared to the polycrystalline analogue. This work contributes to the development of highly robust and efficient oxides for electrode material in supercapacitors.

• Barakezehi, Marjan; Montazer, Majid; Sharif, Farhad; Norby, Truls Eivind & Chatzitakis, Athanasios Eleftherios (2019). A textile-based wearable supercapacitor using reduced graphene oxide/polypyrrole composite. Electrochimica Acta. ISSN 0013-4686. 305, p. 187–196. doi: 10.1016/j.electacta.2019.03.058. Full text in Research Archive
• Sun, Xinwei; Simonsen, Stian Christopher; Norby, Truls Eivind & Chatzitakis, Athanasios Eleftherios (2019). Composite Membranes for High Temperature PEM Fuel Cells and Electrolysers: A Critical Review. Membranes. 9(7). doi: 10.3390/membranes9070083. Full text in Research Archive
• Xu, Kaiqi; Chatzitakis, Athanasios Eleftherios; Jensen, Ingvild Julie Thue; Grandcolas, Mathieu & Norby, Truls Eivind (2019). Ta3N5/Co(OH)x composites as photocatalysts for photoelectrochemical water splitting. Photochemical and Photobiological Sciences. ISSN 1474-905X. 18(4), p. 837–844. doi: 10.1039/c8pp00312b. Full text in Research Archive Show summary

  Ta3N5 nanotubes (NTs) were obtained from nitridation of Ta2O5 NTs, which were grown directly on Ta foil through a 2-step anodization procedure. With Co(OH)x decoration, a photocurrent density as high as 2.3 mA cm−2 (1.23 V vs. NHE) was reached under AM1.5G simulated solar light; however, the electrode suffered from photocorrosion. More stable photoelectrochemical (PEC) performance was achieved by first loading Co(OH)x, followed by loading cobalt phosphate (Co–Pi) as double co-catalysts. The Co(OH)x/Co–Pi double co-catalysts may act as a hole storage layer that slows down the photocorrosion caused by the accumulated holes on the surface of the electrode. A “waggling” appearance close to the “mouth” of Ta2O5 NTs was observed, and may indicate structural instability of the “mouth” region, which breaks into segments after nitridation and forms a top layer of broken Ta3N5 NTs. A unique mesoporous structure of the walls of the Ta3N5 NTs, which is reported here the first time, is also a result of the nitridation process. We believe that the mesoporous structure makes it difficult for the nanotubes to be fully covered by the co-catalyst layer, hence rationalizing the remaining degradation by photocorrosion.

• Sun, Xinwei; Xu, Kaiqi; Fleischer, Christian; Liu, Xin; Grandcolas, Mathieu & Strandbakke, Ragnar [Show all 9 contributors for this article] (2018). Earth-abundant electrocatalysts in proton exchange membrane electrolyzers. Catalysts. 8:657(12), p. 1–41. doi: 10.3390/catal8120657. Full text in Research Archive Show summary

  In order to adopt water electrolyzers as a main hydrogen production system, it is critical to develop inexpensive and earth-abundant catalysts. Currently, both half-reactions in water splitting depend heavily on noble metal catalysts. This review discusses the proton exchange membrane (PEM) water electrolysis (WE) and the progress in replacing the noble-metal catalysts with earth-abundant ones. The efforts within this field for the discovery of efficient and stable earth-abundant catalysts (EACs) have increased exponentially the last few years. The development of EACs for the oxygen evolution reaction (OER) in acidic media is particularly important, as the only stable and efficient catalysts until now are noble-metal oxides, such as IrOx and RuOx. On the hydrogen evolution reaction (HER) side, there is significant progress on EACs under acidic conditions, but there are very few reports of these EACs employed in full PEM WE cells. These two main issues are reviewed, and we conclude with prospects for innovation in EACs for the OER in acidic environments, as well as with a critical assessment of the few full PEM WE cells assembled with EACs.

• Fleischer, Christian; Chatzitakis, Athanasios Eleftherios & Norby, Truls Eivind (2018). Intrinsic photoelectrocatalytic activity in oriented, photonic TiO2 nanotubes. Materials Science in Semiconductor Processing. ISSN 1369-8001. 88, p. 186–191. doi: 10.1016/j.mssp.2018.08.009. Full text in Research Archive Show summary

  In this work, we grow oriented photonic TiO2 nanotubes (TNTs) with a well-established anodization method and their intrinsic catalytic and electronic properties are studied by means of (photo)electrochemical methods. The oriented photonic TNTs are assessed by the RctCdl (ΩF) product, which is a measure of the intrinsic photoelectrocatalytic activity (IPA) of the material. The RctCdl product decouples any surface area effects and it can be applied at any potential and accompanied reaction without knowing the actual electrochemically active surface area (EASA). Moreover, electrochemical impedance spectroscopy (EIS) is used in order to investigate the frequency dispersion in the single frequency Mott-Schottky (MS) measurement. The choice of frequency and the frequency dispersion in MS measurements in nanostructured and of high surface area semiconductors are rarely addressed and can result in erroneous values of two important parameters, i.e. flat band position and donor or acceptor densities. This work provides further insights into important electrochemical aspects and catalytic properties of high surface area nanomaterials for photoelectrochemical applications.

• Chatzitakis, Athanasios Eleftherios; Grandcolas, Mathieu; Xu, Kaiqi; Mei, Sen; Yang, Juan & Jensen, Ingvild Julie Thue [Show all 8 contributors for this article] (2017). Assessing the photoelectrochemical properties of C, N, F codoped TiO2 nanotubes of different lengths. Catalysis Today. ISSN 0920-5861. 287, p. 161–168. doi: 10.1016/j.cattod.2016.11.040. Full text in Research Archive Show summary

  The aim of this work has been the photoelectrochemical (PEC) study of nanostructured photoanodes based on TiO2. Highly ordered and well adhered TiO2 nanotubes (TNTs) of different lengths (∼2–20 μm) were prepared in a two-step process in ethylene glycol solutions containing fluorides, and detailed XPS analysis showed that they have become co-doped with C, N and F. PEC measurements revealed that increasing surface area is not followed by increase in the photoconversion efficiency, but rather that an optimal balance between electroactive surface area (ESA) and charge carrier concentration exists. TNTs of around 10 μm show the optimum incident photon-to-current efficiency (IPCE) of ∼33% and an overall photoconversion efficiency of ∼6.3% under UV illumination of 4 mW cm−2 light intensity. Finally, Mott-Schottky analysis revealed significant frequency dispersion of the estimated space charge layer capacitance, which renders the accurate estimation of the flatband position and charge carrier concentration unreliable. On the other hand, more realistic charge carrier concentrations can be obtained by normalizing the capacitance per ESA.

• Longela, Simon M & Chatzitakis, Athanasios Eleftherios (2017). In vitro corrosion behaviour of phenolic coated nickel-titanium surfaces. Biosurface and Biotribology. 6(1), p. 25–30. doi: 10.1016/j.bsbt.2017.10.002. Full text in Research Archive
• Xu, Kaiqi; Chatzitakis, Athanasios Eleftherios & Norby, Truls (2017). Solid-state photoelectrochemical cell with TiO2 nanotubes for water splitting. Photochemical and Photobiological Sciences. ISSN 1474-905X. 16(1), p. 10–16. doi: 10.1039/c6pp00217j. Full text in Research Archive Show summary

  We have fabricated and tested a photoelectrochemical (PEC) cell where the aqueous electrolyte has been replaced by a proton conducting hydrated Nafion® polymer membrane. The membrane was sandwiched between a TiO2-based photoanode and a Pt/C-based cathode. The performance was tested with two types of photoanode electrodes, a thermally prepared TiO2 film on Ti foil (T-TiO2) and a nanostructured TiO2 films in the form of highly ordered nanotubes (TNT) of different lengths. Firstly, photovoltammetry experiments were conducted under asymmetric conditions, where the anode was immersed in deionized water, while the cathode was kept in ambient air. The results showed a high incident photon-to-current efficiency (IPCE) of 19% under unassisted conditions (short-circuit, 0 V vs. cathode) with short TNT (ca. 1 μm) under 4 mW cm−2 illumination with UV-A rich light. Secondly, the deionized water was replaced by 0.5 M Na2SO4 and now the performance was higher with longer nanotubes, assigned to increased ionic conductivity inside the tubes. An unassisted (0 V) IPCE of 33% was achieved with nanotubes of ca. 8 μm. The presented solid-state PEC cell minimizes the electrode distance and volume of the device, and provides a way towards compact practical applications in solar water splitting.

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• Svenum, Ingeborg-Helene; Dahl, Øystein; Sunding, Martin Fleissner; Killi, Veronica Anne-Line Kathrine; Grandcolas, Mathieu & Andreassen, Magnus [Show all 10 contributors for this article] (2026). Operando insight into Fermi-level (un)pinning at Ta3N5/NiOx/electrolyte interfaces during photoelectrochemical water splitting. Full text in Research Archive
• Jafari, Kimia; Kane, Evan; Soriano, Micah; Chatzitakis, Athanasios & Neagu, Dragos (2025). nanoGe - MATSUSFall25 - From lattice to surface: exsolved nanoparticles in Cu-doped perovskites for alkaline oxygen evolution. Full text in Research Archive Show summary

  In alkaline water electrolysis (AWE), the oxygen evolution reaction (OER) is a key limiting electrochemical reaction because of its complex reaction pathway through a four-electron transfer process, and high overpotential. This results in sluggish kinetics and increased energy consumption, due to hindering efficiency and scalability of hydrogen production, thus posing stringent catalyst require...

• Soriano, Micah; Jafari, Kimia; Velluva, Abhijai; Zhang, Xiaolei; Chatzitakis, Athanasios & Neagu, Dragos (2025). nanoGe - MATSUSFall25 - Developing Sustainable Cobalt-doped OER Catalysts Using Exsolution. Full text in Research Archive Show summary

  Perovskite oxides offer a sustainable platform for catalyzing the oxygen evolution reaction (OER) in alkaline water electrolysis through the use of earth-abundant and cost-effective alternatives to platinum group metals (PGM). While iridium and ruthenium-based anodes demonstrate the highest activity towards the OER, cobalt oxides offer an opportunity to achieve similar performance with fewer ec...

• 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. Full text in Research Archive Show summary

  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...

• 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. Full text in Research Archive Show summary

  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).

• 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. Full text in Research Archive
• Chatzitakis, Athanasios (2024). Double perovskite oxides with reduced cobalt content as catalysts for alkaline water electrolysis. Full text in Research Archive
• Chatzitakis, Athanasios (2024). Novel catalysts design for gas phase electrocatalytic CO2 reduction. Full text in Research Archive
• Dahl, Øystein; Sunding, Martin Fleissner; Thøgersen, Annett; Killi, Veronica Anne-Line Kathrine; Svenum, Ingeborg-Helene & Grandcolas, Mathieu [Show all 10 contributors for this article] (2023). Direct observation of interfacial energetics at Ta3N5/electrolyte and Ta3N5/NiOx/electrolyte heterojunctions by operando ambient pressure X-ray photoelectron spectroscopy during photoelectrochemical water splitting. Full text in Research Archive
• Dahl, Øystein; Sunding, Martin Fleissner; Thøgersen, Annett; Killi, Veronica Anne-Line Kathrine; Svenum, Ingeborg-Helene & Grandcolas, Mathieu [Show all 10 contributors for this article] (2023). Operando observation of interfacial energetics at the Ta3N5/electrolyte heterojunction by ambient pressure X-ray photoelectron spectroscopy for photoelectrochemical 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). 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
• Kang, Xiaolan; Chaperman, Larissa; Galeckas, Augustinas; Ammar-Merah, Souad; Mammeri, Fayna & Norby, Truls [Show all 7 contributors for this article] (2022). Water Vapor Photoelectrolysis in an All Solid-State Photoelectrochemical Cell. 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
• 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. Full text in Research Archive
• Chatzitakis, Athanasios (2021). Solar Hydrogen Production. Full text in Research Archive
• Chaperman, Larissa; Mammeri, Fayna; Chatzitakis, Athanasios & Ammar, Souad (2021). Functionally graded, soft chemistry processed titania photoanode for water-splitting in liquid and gas phases. Full text in Research Archive
• Chatzitakis, Athanasios (2021). Her splittes vann til hydrogen og oksygen bare ved hjelp av sollys. Full text in Research Archive
• Touni, Aikaterini; Liu, Xin; Chatzitakis, Athanasios Eleftherios; Kang, Xiaolan; Spyridou, Olga & Sotiropoulos, Sotiris (2021). Platinized Black Titania Nanotube HER Cathodes prepared by the Galvanic Deposition Method at CaH2-reduced Titania Nanotubes. Full text in Research Archive
• Kuvshinov, Mikhail; Karazhanov, Smagul; Hallsteinsen, Ingrid & Chatzitakis, Athanasios (2021). Synthesis and characterization of Ni-Mo alloy for applications as cathodes in photoelectrochemical (PEC) cells. Full text in Research Archive
• Chatzitakis, Athanasios (2021). Fuel(s) from the Sun. Full text in Research Archive
• Karazhanov, Smagul; Zavorotynska, Olena & Chatzitakis, Athanasios (2021). Organizing online international school "Materials and processes for production of solar hydrogen" within the “Complex of online and onsite lectures on materials for hydrogen generation by solar water splitting” med acronym” (COOL LONGBOAT) project. Full text in Research Archive
• Liu, Xin; Risbakk, Sanne; Carvalho, Patricia Almeida; Yang, Mingyi; Backe, Paul Hoff & Bjorås, Magnar [Show all 8 contributors for this article] (2020). Hydrogenase-assisted catalysis on titania electrode oxides. Full text in Research Archive Show summary

  The widespread use of fuel cells and water splitting devices for energy generation and storage is limited by the dependence on noble metal catalysts. There is thus a tremendous need for the development of efficient electrocatalysts based on Earth-abundant elements. Nature inspired hydrogenases (HydA) are metallo-enzymes that catalyze the reversible reaction of H2 to protons and electrons. Hydrogenases containing Fe at the active sites, known as [FeFe]-HydA, show activities comparable to that of Pt. This work addresses a new class of electrodes for [FeFe]-HydA attachment and bio-assisted catalysis based on TiO2 nanotubes. The conducting oxide material provides suitable electronic conduction and hydrophilicity, while the nanostructure ensures tunability (tube length, crystal orientation and pore diameter) and high surface area for HydA attachment. In this work, electron microscopy is used to characterize the bio-electrodes. Based on the experimental findings, density functional theory (DFT) calculations are used to probe the catalytic reaction sites on the HydA and address the interaction between enzymes and TiO2. The novel bioelectrode will be employed in a system of artificial photosynthesis and generation of solar fuels by simultaneous water splitting and CO2 capture and utilization.

• Xu, Kaiqi; Chatzitakis, Athanasios Eleftherios; Backe, Paul Hoff; Ruan, Qiushi; Tang, Junwang & Bjørås, Magnar [Show all 8 contributors for this article] (2020). The use of NMR to assess the activity of formate dehydrogenase biocatalysts for CO2 utilization. Full text in Research Archive
• Xu, Kaiqi; Grandcolas, Mathieu; Yang, Juan; Mei, Sen; Chatzitakis, Athanasios Eleftherios & Norby, Truls Eivind (2019). Photoelectrochemical water splitting on Ta3N5 nanotubes for H2 production. Full text in Research Archive
• Chatzitakis, Athanasios Eleftherios (2019). Artificial Photosynthesis: Mimicking nature for a sustainable energy future. Full text in Research Archive
• Chatzitakis, Athanasios Eleftherios (2019). Can we make fuel in the future from… simply air? Full text in Research Archive
• Sun, Xinwei; Chatzitakis, Athanasios Eleftherios & Norby, Truls Eivind (2019). Polymer-ceramic composite membranes for intermediate temperature PEM fuel cells. Full text in Research Archive
• Liu, Xin; Chatzitakis, Athanasios Eleftherios; Carvalho, Patricia Almeida; Risbakk, Sanne; Yang, Mingyi & Backe, Paul Hoff [Show all 7 contributors for this article] (2019). Enzyme-assisted Catalysis on Titania Electrodes. Full text in Research Archive
• Chatzitakis, Athanasios (2019). Sol + luftfuktighet = hydrogen = ren energi. Full text in Research Archive
• Chatzitakis, Athanasios Eleftherios (2019). Prospects of photoelectrochemical water electrolysis and the production of solar fuels. Full text in Research Archive
• Sun, Xinwei; Chatzitakis, Athanasios Eleftherios & Norby, Truls Eivind (2019). Composite membranes for high temperature PEM fuel cells and electrolysers. Full text in Research Archive Show summary

  Polymer electrolyte membrane(PEM) fuel cells and electrolysers offer efficient use and production of H2 for emission-free transport and sustainable energy systems. In order to reduce the cost,complexity,flooding,catalyst poisoning and degradation,operation above 80°C and at lower relative humidities(RH) is preferable. Acid doped polybenzimidazole(PBI),sulfonated polyetheretherketone(SPEEK) and their composites are being investigated for high temperature and low RH operation with sufficient conductivities. We take a critical view and a surface protonic approach to gain fundamental insight.

• Xu, Kaiqi; Chatzitakis, Athanasios Eleftherios; Norby, Truls Eivind & Bjørheim, Tor Svendsen (2019). Ta3N5 nanotubes loaded with Co(OH)x/Co-Pi-high photocurrent densities and improved photocorrosion resistance. Full text in Research Archive
• Xu, Kaiqi; Chatzitakis, Athanasios Eleftherios; Vøllestad, Einar; Ruan, Qiushi; Tang, Junwang & Norby, Truls Eivind (2018). Solid-state tandem photoelectrochemical cell for wet air electrolysis and hydrogen production. Full text in Research Archive
• Sun, Xinwei; Xu, Kaiqi; Chatzitakis, Athanasios Eleftherios & Norby, Truls Eivind (2018). Can hydroxyl radicals travel far? Gas phase transport and detection after photocatalytic generation at TiO2 nanorods. Full text in Research Archive
• Fontaine, Marie-Laure; Chatzitakis, Athanasios Eleftherios; Vøllestad, Einar; Thomassen, Magnus & Norby, Truls Eivind (2018). Principles of proton conduction in polymer-ceramic composite membranes for PEM applications. Full text in Research Archive
• Xu, Kaiqi; Chatzitakis, Athanasios Eleftherios; Bjørheim, Tor Svendsen & Norby, Truls Eivind (2018). Surface modification of Ta3N5 nanotubes as photocatalyst for photoelectrochemical water splitting. Full text in Research Archive
• Liu, Xin; Chatzitakis, Athanasios Eleftherios; Carvalho, Patricia Almeida; Backe, Paul Hoff; Yang, Mingyi & Bjørås, Magnar [Show all 7 contributors for this article] (2018). Enzyme-assisted Catalysis on Black Titania Electrodes. Full text in Research Archive
• Xu, Kaiqi; Chatzitakis, Athanasios Eleftherios; Norby, Truls Eivind; Grandcolas, Mathieu; Yang, Juan & Mei, Sen (2018). Ta3N5 / Co(OH)x composite as the photocatalyst for photoelectrochemical water splitting. Full text in Research Archive
• Grandcolas, Mathieu; Wabende, Brian; Yang, Juan; Mei, Sen; Xu, Kaiqi & Chatzitakis, Athanasios Eleftherios [Show all 7 contributors for this article] (2018). TiO2 Nanorods Decorated With Cobalt oxide as Photoanodes for a Bio-Hybrid PEC Cell. Full text in Research Archive
• Chatzitakis, Athanasios Eleftherios; Simonsen, Stian; Fontaine, Marie Laure; Thomassen, Magnus & Norby, Truls Eivind (2018). Conductivity of polymer-ceramic composite membranes at high T and p(H2O) using a novel PEEK sample holder. Full text in Research Archive
• Xu, Kaiqi; Chatzitakis, Athanasios Eleftherios; Vøllestad, Einar; Ruan, Qiushi; Tang, Junwang & Norby, Truls Eivind (2018). Solid-state photoelectrochemical cell for wet air electrolysis and hydrogen production. Full text in Research Archive
• Chatzitakis, Athanasios Eleftherios (2017). (Photo)electrochemical applications of TiO2 nanotubes. Full text in Research Archive
• Yang, Juan; Mei, Sen; Grandcolas, Mathieu; Simon, Christian; Xu, Kaiqi & Chatzitakis, Athanasios Eleftherios [Show all 10 contributors for this article] (2017). Synthesis of Nanosized Cobalt Oxides and Application in a Bio-hybrid Photoelectrochemical Cell for CO2 Utilization. Full text in Research Archive Show summary

  CO2 capture and utilization (CCU) can alleviate the disastrous effects of greenhouse gas (GHG) emissions form the use of fossil fuels. The use of CO2 emission as feedstock for direct conversion into valuable chemicals has attracted increasing research interest worldwide. A number of approaches, such as chemical, electrochemical, photochemical and biological routes have used CO2 as a sustainable carbon resource for the production of chemicals. In the present CO2BioPEC project, a novel hybrid photoelectrochemical concept, based on solar energy and formate dehydrogenase biocatalyst is applied, as illustrated in Figure 1 (left). This multidisciplinary project aims at demonstrating a bio-hybrid photoelectrochemical cell, in which solar energy is efficiently captured and utilized to convert CO2 into energy-rich compounds, using formate dehydrogenase enzymes as biocatalysts. Cobalt oxide is a low-cost material with a band gap that can absorb visible light. It is also a promising co-catalyst that has been widely used for oxygen evolution route. In the present work, nanosized cobalt oxide was synthesized and used as a co-catalyst on the photoanode. The results showed that the formation of cobalt oxides (Co3O4, CoOOH and Co(OH)2) is strongly dependent on the temperature and reaction time. The photochemical efficiency will be further investigated by impregnating nanosized cobalt oxides onto Ta3N4 /TaON nanotubes.

• Chatzitakis, Athanasios Eleftherios; Xu, Kaiqi & Norby, Truls Eivind (2017). TiO2 nanotubes as photoanode electrodes in solid-state photoelectrochemical cells. Full text in Research Archive
• Xu, Kaiqi; Chatzitakis, Athanasios Eleftherios; Norby, Truls Eivind; Mei, Sen; Juan, Yang & Grandcolas, Mathieu [Show all 7 contributors for this article] (2017). Solid-state photoelectrochemical Cell with Tantalum Nitride Nanotubes as Photoanode. Full text in Research Archive
• Xu, Kaiqi; Chatzitakis, Athanasios Eleftherios; Bjørheim, Tor Svendsen & Norby, Truls Eivind (2017). Fuels from the sun, sea and air-by solid-state photoelectrochemical cell. Full text in Research Archive
• Xu, Kaiqi; Chatzitakis, Athanasios Eleftherios; Bjørheim, Tor Svendsen & Norby, Truls Eivind (2017). Tantalum oxynitride for photoelectrochemical water splitting. Full text in Research Archive
• Fleischer, Christian; Chatzitakis, Athanasios Eleftherios; Grandcolas, Mathieu; Mei, Sen & Norby, Truls Eivind (2017). Solid-state photoelectrochemical water splitting: Application and properties of TiO2 nanotubes. Full text in Research Archive
• Grandcolas, Mathieu; Sahoo, Prangya Parimita; Foss, Carl Erik Lie; Xing, Wen; Polfus, Jonathan M. & Sunde, Svein [Show all 8 contributors for this article] (2016). Novel photoelectrocatalytic concepts for conversion of nitrogen to fuels and chemicals. Full text in Research Archive
• Both, Kevin Gregor; Chatzitakis, Athanasios; Bergum, Kristin; Hansen, Per-Anders Stensby; Prytz, Øystein & Norby, Truls (2023). Plasmonically Enhanced Photocatalysis: Synthesis, Physical Properties, and Applications. Universitetet i Oslo. 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.

• Kang, Xiaolan; Norby, Truls & Chatzitakis, Athanasios (2021). Novel nanostructured materials for energy applications. Universitetet i Oslo, Det matematisk-naturvitenskapelige fakultet. Full text in Research Archive Show summary

  Nanomaterials and nanotechnology have been flourishing greatly over the last decades and their applications are spreading almost over all the branches of science and technology. In this work, we zoomed into the synthesis and characterization of nanostructured materials. The central focus was to manipulate the compositions, morphology and structures of nanomaterials by combining various strategies like defect engineering and exsolution. Several interesting features and properties of nanostructured materials were investigated and highlighted, including the optical, plasmonic, electronic and surface properties. This process deepened our fundamental understanding of surface protonic conduction of nanocrystalline oxides and increased our control over nanostructuring of materials in order to achieve targeted functionalities. The as-prepared nanomaterials have been applied in energy- and environment-related applications, showing improved and controllable efficiency in pollutants degradation and water (vapor) splitting in photocatalytic, (photo) electrocatalytic, as well as in a solid-state photoelectrocatalytic systems.

• Andreassen, Magnus Haga; Norby, Truls Eivind & Chatzitakis, Athanasios Eleftherios (2020). Synthesis, characterization, and optimization of Cu2O photocathodes for solid-state photoelectrochemical cells. Universitetet i Oslo. Full text in Research Archive
• Simonsen, Stian Christopher; Norby, Truls Eivind & Chatzitakis, Athanasios Eleftherios (2020). Nafion®-Ceramic Composite Electrolytes for Proton Exchange Membrane Fuel Cells at Elevated Temperatures. Universitetet i Oslo. Full text in Research Archive
• Xu, Kaiqi; Norby, Truls Eivind; Chatzitakis, Athanasios Eleftherios & Bjørheim, Tor Svendsen (2020). Artificial photosynthesis: Advanced nanomaterials and use of biocatalysts for novel photoelectrochemical cells. Universitetet i Oslo. Full text in Research Archive Show summary

  The capture and storage of solar energy in chemical form by a photoelectrochemical (PEC) cell is an elegant and simple solution to the increasing energy demands and needs for chemicals and fuels from non-fossil sources. We first developed a facile fabrication method of a monolithic solid-state PEC (SSPEC) cell, which utilizes a solid-state polymer electrolyte and minimizes the distance between the two electrodes. The recent progress in understanding of surface proton conduction in porous ceramic-based electrolytes led to the idea of using the SSPEC cell for hydrogen production through water vapor splitting. The traditionally Pt-based cathode was replaced by an earth-abundant photocathode, g-C3N4, which increased the intrinsic photo-induced electrical field by a factor of three. Such a monolithic SSPEC cell working fully in the gas phase was demonstrated first time, and can play an important role for clean energy production in rural or other areas where grid infrastructure and clean water sources are limited or absent. CO2 utilization is considered a key player in a carbon-free energy economy, emphasising the importance of CO2 capture and conversion. We have successfully demonstrated how a bio-catalyst (enzyme) can be coupled with traditional chemical engineering for this purpose. To approach this, a stable, highly photo-active material - Ta3N5 nanotubes - has been synthesized and tested in a photoelectrochemical cell. It reaches close to its theoretical performance in photo-assisted water splitting. We then coupled the Ta3N5 nanotubes as the photoabsorber together with an optimised enzyme - formate dehydrogenase (FDH) - in a photoelectrochemical cell. The aim is to mimic natural photosynthesis to reduce CO2 into a valuable chemical form – formic acid. A solar-driven reduction of CO2 and water to formic acid at close to 100% faradaic efficiency has been reached.

• Andersen, Karoline Sjøen; Norby, Truls; Sartori, Sabrina & Chatzitakis, Athanasios (2020). Hydrophobic coatings for offshore PV modules: Impact of precipitation from seawater on the protective glass. Universitetet i Oslo. Full text in Research Archive
• Andersen, Karoline Sjøen; Norby, Truls Eivind & Chatzitakis, Athanasios Eleftherios (2020). Hydrophobic coatings for offshore PV modules: Impact of precipitation from seawater on the protective glass. Universitetet i Oslo. Full text in Research Archive

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