Esko Kokkonen

Investigation of electron transport layer interfaces and ion migration dynamics in perovskite solar cells (PSCs) using near ambient-pressure X-ray photoelectron spectroscopy (AP-XPS).

Atomic layer deposition (ALD) of interphases is a promising strategy for stabilizing lithium metal negative electrode and mitigating dendrite formation in rechargeable batteries. In this work, we investigate the growth mechanism and electrochemical impact of titanium‐based ALD coatings on highly reactive lithium surfaces. Operando ambient‐pressure X‐ray photoelectron spectroscopy (APXPS) was employed to probe the initial stages of a Ti interphase growth under realistic ALD conditions using TiCl ₄ and H ₂ O precursors. The first H ₂ O pulse rapidly converts the Li surface into LiOH, while subsequent TiCl ₄ exposure reacts directly with lithium surface to form a stable LiCl interlayer. TiO _x nucleates on this LiCl‐modified surface, resulting in a composite TiO _x /LiCl interphase. This synergistic interphase improves electrolyte wettability, promotes uniform Li ⁺ flux, and suppresses dendritic growth. Electrochemical evaluation shows that TiO _x ‐LiCl‐coated Li anodes deliver substantially enhanced interfacial stability and cycling performance. Symmetric cells exhibit stable operation for over 600 h, while full Li‖LiNi _0.8 Mn _0.1 Co _0.1 O ₂ cells with a 10 ALD cycle TiO _x /LiCl coating achieve 100% Coulombic efficiency after 300 charge–discharge cycles at 200 mA g ⁻¹ , compared with ∼63% for bare lithium. These findings provide mechanistic insight and practical validation of ALD Ti‐based coatings as an effective protection strategy for durable lithium metal batteries.

Ambient pressure XPS studies of atomic layer deposition of AlN show that formation of AlO _x hinders formation of AlN.

The direct conversion of methane to methanol (DCMM) under continuous flow and atmospheric pressure offers notable environmental benefits and industrial promise, but remains a long-standing challenge due to the difficulty of activating CH₄ while avoiding overoxidation of methanol. Here, we demonstrate that pure ceria (CeO₂), without any metal promoters, enables gas-phase DCMM with up to 80% selectivity at 300–350 °C, upon optimization of the H₂O/O₂ ratio. At 550 °C, methanol and formaldehyde are formed at rates of 24 and 38 μmol g^–1 h^–1, respectively, both dropping below 1 μmol g^–1 h^–1 in the absence of O₂. Ex situ transmission electron microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy confirm that CeO₂ maintains structural integrity and resists carbon deposition during reaction. Combining kinetic studies, steady-state in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS), and density functional theory (DFT) reveals that hydroxyl groups (OH), generated from water dissociation, play a multifaceted role: they facilitate C–H bond activation, promote methoxy formation, and enhance methanol desorption. In situ ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) directly reveals the evolution of surface intermediates and shows that cofeeding O₂ and H₂O suppresses CH₃O and CH _x accumulation while boosting methanol yield, indicating a rapid intermediate turnover as key to sustained activity. AP-XPS O 1s spectra further highlight that O₂ promotes H₂O dissociation, regenerating reactive OH groups and maintaining performance at elevated temperature. These findings offer molecular-level insights into how water and oxygen cooperatively tune reactivity, enabling efficient methane-to-methanol conversion on a metal-free oxide catalyst.

Studies of the surface chemistry of the first few cycles of atomic layer deposition (ALD) using in situ and time-resolved operando techniques are attractive for realizing, understanding and obtaining true mechanistic information during the deposition. However, the latter techniques are yet to be applied to ALD of metal nitrides. Here we present a surface-chemistry investigation through a time-resolved ambient pressure X-ray photoelectron spectroscopy (APXPS) study of the initial growth of titanium nitride. The Ti 2p, O 1s, N 1s, C 1s and Si 2p core-level spectra recorded at different stages during the deposition show that chemisorption occurs immediately on the silicon dioxide surface due to the interaction of tetrakis(dimethylamido)titanium(IV) (TDMAT) with the surface. A delay in nucleation on the TDMAT-terminated surface was observed during the NH3 pulse. The intensity of the Ti 2p and N 1s core levels began to increase after four ALD cycles, showing that the surface was coated with Ti and N atoms and no Si signals were observed with time. The results show that ligand exchange reactions take place before transamination reactions. This was verified using the periodic changes in the intensity and peak positions of the above-mentioned spectra and complemented by residual gas analysis using mass spectrometry. These results can provide insights into the ALD surface growth of not only TiN but also other metal nitrides.

Ambient pressure X-ray photoelectron spectroscopy (APXPS) has emerged as an important technique for investigating surface and interface chemistry under realistic conditions, overcoming the limitations of conventional XPS restricted to ultrahigh vacuum. This review highlights the capabilities and scientific impact of APXPS at the MAX IV Laboratory, the world’s first fourth-generation synchrotron light source. With the APXPS beamlines SPECIES and HIPPIE, MAX IV offers state-of-the-art instrumentation for in situ and operando studies across a broad pressure range, enabling research in catalysis, corrosion, energy storage, and thin film growth. The high brilliance and small beam size of MAX IV’s synchrotron light are essential for pushing the time-resolution boundaries of APXPS, especially in the soft X-ray regime. We discuss representative studies at MAX IV, including investigations of single-atom catalysts, confined catalysis, time-resolved catalysis, atomic layer deposition, and electrochemical interfaces, showcasing the role of APXPS in advancing material and surface science.