Roland Marschall

The development of earth-abundant co-catalysts that enhance TiO2-driven hydrogen evolution remains crucial for sustainable photocatalytic H2 production. This work investigates Mo addition as a compositional strategy to tune the co-catalytic functionality of Fe3S4 for photocatalytic HER over TiO2. A series of x%Mo–Fe3S4 (x = 0, 1, 2, 5, and 7.5) materials was prepared and evaluated using both conventional solvothermal synthesis (18 h) and a microwave-assisted non-aqueous sol–gel route requiring 30 min. When combined with anatase TiO2 as a model photocatalyst at a fixed co-catalyst loading (5 wt %), Mo addition markedly improved the performance of the Fe3S4 co-catalyst. Structural, surface, and optical characterizations indicate that Mo addition modifies the sulfides, in parallel with the observed improvement in co-catalytic hydrogen evolution reaction (HER) performance. Under UV–Vis irradiation (300 W Xe lamp), the best solvothermal composite (5%Mo-Fe3S4@TiO2) reached ∼30 μmol H2 h–1 after 3 h irradiation, corresponding to 25 times the rate of bare TiO2. For the microwave-derived series, the 7.5%Mo-Fe3S4@TiO2 composite delivered ∼29 μmol H2 h–1 after 3 h, corresponding to 24 times higher than bare TiO2, and 5 times higher than undoped Fe3S4@TiO2. Overall, Mo-doped Fe3S4 emerges as an earth-abundant, non-noble metal co-catalyst platform that significantly improves photocatalytic HER with TiO2 while maintaining performance under a time-efficient synthesis protocol.

Layered Ruddlesden–Popper (RP) oxides provide a versatile platform for topochemical anion modification, enabling access to metastable mixed-anion phases with tuneable structural and electronic properties. In this work, we present a comprehensive investigation of how mixed B -site (indium and antimony) chemistry influences topochemical fluorination of Ba ₂ In _0.5 Sb _0.5 O ₄ using polyvinylidene fluoride (PVDF). Powder X-ray diffraction (PXRD) reveals anisotropic lattice expansion, dominated by elongation perpendicular to the perovskite layers, with minimal change in the in-plane lattice parameters. Fluorination proceeds through a composition-dependent multiphase regime in which several tetragonal ( I 4 /mmm ) RP components with progressively expanded c and slightly contracted a lattice parameters coexist, representing a distribution of fluorination states, followed by convergence to a single dominant fluorine-rich tetragonal phase at high fluorine content. Coupled X-ray/neutron Rietveld refinement establishes the end member as the anion-saturated oxyfluoride Ba ₂ In _0.5 Sb _0.5 O ₂ F ₄ , and locates the three fully occupied anion sites of the tetragonal I 4 /mmm framework. The preferred O/F distribution, with oxide at the equatorial site and fluoride at the apical and interstitial sites, is supported by bond-valence-sums (BVSs) analysis of the refined interatomic distances and the corresponding global instability indices (GIIs), together with charge balance, since O and F cannot be reliably distinguished from their neutron scattering lengths alone. X-ray absorption near-edge structure (XANES) measurements support the retention of the formal In ³⁺ and Sb ⁵⁺ oxidation states upon fluorination.

Layered Ruddlesden–Popper oxides provide a versatile platform for topochemical anion modification, enabling access to metastable mixed-anion phases with tuneable structural and electronic properties. In this work, we present a comprehensive investigation of how mixed B-site (indium and antimony) chemistry influences topochemical fluorination of Ba ₂ In _0.5 Sb _0.5 O ₄ using PVDF. Powder X-ray diffraction reveals anisotropic lattice expansion, dominated by elongation perpendicular to the perovskite layers, with minimal change in the in-plane lattice parameters. Fluorination proceeds through a composition-dependent multiphase regime in which several tetragonal ( I 4 /mmm ) RP components with progressively expanded c and slightly contracted a lattice parameters coexist, representing a distribution of fluorination states, followed by convergence to a single dominant fluorine-rich tetragonal phase at high fluorine content. Coupled X-ray/neutron Rietveld refinement establishes the end member as the anion-saturated oxyfluoride Ba ₂ In _0.5 Sb _0.5 O ₂ F ₄ , and locates the three fully occupied anion sites of the tetragonal I 4 /mmm framework. The preferred O/F distribution, with oxide at the equatorial site and fluoride at the apical and interstitial sites, is supported by bond-valence-sums (BVSs) analysis of the refined interatomic distances and the corresponding global instability indices (GIIs), together with charge balance, since O and F cannot be reliably distinguished from their neutron scattering lengths alone. XPS confirms that In remains in the +3 oxidation state, while the Sb spectral changes are consistent with modifications of the local O/F coordination environment rather than changes in the formal oxidation state. X-ray absorption near-edge structure measurements further support the retention of the formal In ³⁺ and Sb ⁵⁺ oxidation states upon fluorination.

Different synthesis parameters were studied influencing film thickness, aggregate formation, and pore ordering of ordered mesoporous silica thin film with vertically oriented pores.

To produce green hydrogen through water electrolysis, highly efficient catalysts that use inexpensive, earth‐abundant materials are required. In search of novel and inexpensive electrocatalysts, we investigate the adjustment of the composition of transition metal‐based catalysts for the alkaline oxygen evolution reaction (OER) and evaluate the effects of both redox‐active and ‐inactive elements. Based on the known synthesis of AFe ₂ O ₄ ‐type spinels (A = transition metal cation), we adjusted the composition of the A and B positions (B = Fe position), achieving a significant improvement in catalytic activity. Additionally, we are investigating the high‐entropy effect, which is expected to enhance the activity and stability of electrocatalysts. Introducing cobalt and manganese and combining these elements with nickel and zinc formed a NiO/(NiZn)(MnCoFe) ₂ O ₄ composite that reduced the overpotential for OER to 360 mV. However, increasing the number of A‐cations in the spinel beyond six to reach a high‐entropy spinel oxide had no positive effect on the overpotential. In terms of composition, the amount of nickel, and thus NiO, could be reduced to 5%, maintaining low overpotentials. The same applies to the proportion of cobalt, which does not scale linearly with activity.

Copper oxide-based materials are promising candidates as photocathodes for photoelectrochemical (PEC) hydrogen production due to their suitable band gap energy for visible light absorption and appropriate band edge positions for driving the hydrogen evolution reaction. However, the application of CuO suffers from inherent drawbacks, such as poor photostability upon illumination, caused by the position of the Cu²⁺ reduction potential within the energy band gap of CuO. To protect CuO from photocorrosion, Pt was sputter-coated on nanoporous sol–gel-based CuO thin films prepared by dip-coating using Pluronic® F-127 as a structure-directing agent and, for the first time, citric acid as a complexing agent. Electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy collectively confirmed that Pt can be homogeneously deposited with controlled thicknesses on the nanoporous CuO surface, possessing pore dimensions between 10 nm and 100 nm. The impact of the Pt coating thickness on the PEC performance and stability of CuO was investigated in a neutral aqueous electrolyte. PEC analysis showed that Pt can act as a protective layer, which was indeed found to slow down the photodegradation process of CuO substantially (57% versus 2.9% of the initial photocurrent retained after 30 min of PEC operation). However, Pt coating did not increase gas evolution under irradiation and applied potential, when compared to pristine CuO. The optimum Pt layer thickness in terms of photostability was found to be approximately 20 nm. The empirically derived optoelectronic properties of CuO thin film photocathodes (indirect optical E_gap = 1.5 eV) were supported by density functional theory calculations, providing fundamental insight into the electronic band structure. The outcomes of this study demonstrate a reproducible and up-scalable synthetic approach to increase the photostability of CuO-based materials under operating conditions in PEC cells.

Photoelectrochemical (PEC) water splitting is a promising strategy for providing clean, sustainable fuel. However, its efficiency is limited by the high overpotential and sluggish kinetics of the oxygen evolution reaction. To improve oxidation kinetics and produce a higher-value product, it is possible to oxidize organic molecules instead. One such molecule is 5-hydroxymethylfurfural (HMF), which can be converted into the high-value platform chemical 2,5-furandicarboxylic acid (FDCA). In this study, we used porous tungsten oxide (WO3) photoanodes to oxidize HMF photoelectrochemically. Our work focuses on electrolyte selection for this conversion reaction, and we therefore conducted a detailed reproducible study. Furthermore, we carried out a mechanistic investigation and studied the capability of oxidizing the reaction intermediates to support our electrolyte choice. Using the most suitable electrolytes in a self-designed flow-cell setup enabled us to produce FDCA (0.02 mM) for the first time using WO3 photoanodes under AM 1.5 G illumination. Due to the identification of the side products maleic acid and formic acid, this work also provides knowledge for further optimization of PEC HMF oxidation.

We present a comparative study of n = 1 and n = 2 Ruddlesden–Popper (RP) titanates Sr₂TiO₄ and Sr₃Ti₂O₇, as well as their oxyfluorides Sr₂TiO₃F₂ and Sr₃Ti₂O₅F₄, synthesized via low-temperature topochemical fluorination to study structure–property relationships in photocatalytic hydrogen (H₂) evolution. In Sr₂TiO₄, fluorination lowers the symmetry from I4/mmm to P4/nmm, placing Ti in a locally non-centrosymmetric site and producing ordered, asymmetric TiO₅F units. By contrast, Sr₃Ti₂O₇ retains its tetragonal I4/mmm symmetry upon fluorination with Ti remaining in a locally non-centrosymmetric environment. Photocatalytic H₂ evolution shows a ∼6.3-fold enhancement for Sr₂TiO₃F₂ compared to Sr₂TiO₄, which might be attributed to inversion symmetry breaking and the resulting internal local dipole fields that promote charge separation. In contrast, Sr₃Ti₂O₇ exhibits much higher intrinsic activity than Sr₂TiO₄ (∼30-fold), likely governed by dimensionality, while its fluorinated phase Sr₃Ti₂O₅F₄ shows suppressed performance due to the absence of local polarity, which limits beneficial fluorination effects and diminishes charge-separation efficiency, as supported by transient absorption spectroscopy (TAS) results. TAS reveals distinct charge-carrier dynamics, with Sr₃Ti₂O₇ showing the highest photogenerated electron population and Sr₂TiO₃F₂ exhibiting enhanced long-lived electrons, consistent with improved charge separation. Density functional theory (DFT) calculations show that fluorination deepens the O 2p valence band, slightly raises the Ti 3d conduction edge, and reduces c-axis dispersion, consistent with the UV-Vis and Butler–Ginley analyses. The resulting increase in electron effective mass suppresses charge mobility along the c-axis, lowering conduction dimensionality. These findings establish anion sublattice engineering as a selective route to tune local symmetry, charge-carrier dynamics, and photocatalytic performance in RP-type titanates.