Han Gardeniers’s research while affiliated with University of Twente and other places

Lanthanide-doped ZrO2 ceramics are promising materials for optics due to their high refractive index and tunable luminescent properties. In this study, we investigated the impact of Yb3+ and Er3+ dopant concentrations on the emission behavior of lanthanide-doped 3D ZrO2 microarchitectures fabricated using two-photon lithography. Thermal treatments have been carried out at 600 C and 750 C to promote the stabilization of the ZrO2 tetragonal phase (t-ZrO2) and at 1000 C to induce phase transition in ZrO2 to the monoclinic (m-ZrO2) phase in the 3D microarchitectures. Scanning transmission electron microscopy confirmed the crystallinity changes across the thermal treatments. Photoluminescence (PL) and cathodoluminescence (CL) measurements confirm emission bands of Yb3+ and Er3+ single dopants and Yb3+:Er3+ co-dopants. Variations in Yb3+ content reveal that the PL emission of Er3+ increases (e.g., 4 S 3/2 → 4 I 15/2), which is attributed to the interplay between the dopant concentrations, defect structures and the ZrO2 host. The results highlight the importance of ZrO2 microarchitectures' crystallinity and co-doping relationship, which enable the promotion of Er3+ emissions. We expect our research will find applications in 3D optical systems.

Feynman's statement, “There is plenty of room at the bottom”, underscores vast potential at the atomic scale, envisioning microscopic machines. Today, this vision extends into 3D space, where thousands of atoms and molecules are volumetrically patterned to create light‐driven technologies. To fully harness their potential, 3D designs must incorporate high‐refractive‐index elements with exceptional mechanical and chemical resilience. The frontier, however, lies in creating spatially patterned micro‐optical architectures in glass and ceramic materials of dissimilar compositions. This multi‐material capability enables novel ways of shaping light, leveraging the interaction between diverse interfaced chemical compositions to push optical boundaries. Specifically, it encompasses both multi‐material integration within the same architectures and the use of different materials for distinct architectural features in an optical system. Integrating fluid handling systems with two‐photon lithography (TPL) provides a promising approach for rapidly prototyping such complex components. This review examines single and multi‐material TPL processes, discussing photoresin customization, essential physico‐chemical conditions, and the need for cross‐scale characterization to assess optical quality. It reflects on challenges in characterizing multi‐scale architectures and outlines advancements in TPL for both single and spatially patterned multi‐material structures. The roadmap provides a bridge between research and industry, emphasizing collaboration and contributions to advancing micro‐optics.

The small dimensions of microfluidic channels allow for fast diffusive or passive mixing, which is beneficial for time-sensitive applications such as chemical reactions, biological assays, and the transport of to-be-detected species to sensors. In microfluidics, the need for fast mixing within milliseconds arises primarily because these devices are often used in fields where rapid and efficient mixing significantly impacts the performance and outcome of the processes. Active mixing with acoustics in microfluidic devices involves using acoustic waves to enhance the mixing of fluids within microchannels. Using sharp corners and wall patterns in acoustofluidic devices significantly enhances the mixing by acoustic streaming around these features. The streaming patterns around the sharp edges are particularly effective for the mixing because they can produce strong lateral flows that rapidly homogenize liquids. This work presents extensive characterizations of the effect of sharp-edged structures on acoustic mixing in bulk acoustic wave (BAW) mode in a silicon microdevice. The effect of side wall patterns in different angles and shapes, their positions, the type of piezoelectric transducer, and its amplitude and frequency have been studied. Following the patterning of the channel walls, a mixing time of 25 times faster was reached, compared to channels with smooth side walls exhibiting conventional BAW behavior. The average locally determined acoustic streaming velocity inside the channel becomes 14 times faster if sharp corners of 10° are added to the wall.

A 3D-printing approach is used to fabricate green bodies/precursor microarchitectures that, upon annealing, allow the fabrication of hierarchical 3D hollow microarchitectures (3DHMs). The 3DHMs are composed mainly of TiO2 and inorganic stabilizers that enable the production of inorganic cellular units upon thermal annealing at 650 °C. Morphological inspection reveals that the 3D architecture beams comprise TiO2 nanoparticles (NPs). The inner and outer diameters of the hollow beams are ∼80 μm and ∼150 μm, retained throughout the 3D hollow network. A proof-of-concept photo-Fenton reaction is assessed. The 3DHMs are impregnated with α-Fe2O3 NPs to evaluate solar photo-Fenton degradation of organic compounds, such as MB used as control and acetaminophen, an organic pollutant. The optical, structural, and chemical environment characteristics, alongside scavenger analysis, generate insights into the proposed solar photo-Fenton degradation reaction over TiO2 3DHMs loaded with α-Fe2O3. Our work demonstrates newly hollow printed microarchitecture with interconnected networks, which can help direct catalytic reactions.

Two‐photon lithography (TPL) is a powerful technique for creating 3D microarchitectures. Applied to high‐refractive‐index materials like ZrO2, it promises advanced optics. This is the case of ZrO2 host matrixes in combination with luminescent dopants. However, due to the nonideal crystallinity attained to the TPL pre‐ceramic replica from a custom‐made photoresin, the emission of lanthanide (Ln) dopants in ZrO2 microarchitectures can be suboptimal. However, crystallinity exacerbated by annealing can promote Ln‐emission, thereby enabling the integration of ceramic micro‐optic into a low‐temperature process. This work presents a photoresin containing a metal‐organic monomer tailored for TPL, enabling the fabrication of Ln‐doped tetragonal ZrO2 (t‐ZrO2) microarchitectures. The emission properties of Ln‐doped microarchitectures with trivalent Ln ions (Ln³⁺), i.e., Yb³⁺ (2.5 mol%), Er³⁺ (0.35 mol%), and Tm³⁺ (0.35 mol%) are studied. The results demonstrate that Ln emission is absent when annealing the microarchitectures at 600 °C. Annealing at 750 °C activates Ln³⁺ emissions, including ²F5/2–²F7/2 (infrared), ⁴S3/2–⁴I15/2 (green), and ³H4–³F6 (near‐infrared) transitions corresponding to Yb, Er, and Tm species. Transmission electron microscopy (TEM) confirms that t‐ZrO2 crystallinity becomes more prominent at 750 °C, demonstrating the promotion of Ln emissions upon thermal treatment and underscoring the role of crystalline in TPL micro‐optical ceramics.

The small dimensions of microfluidic channels allow for fast diffusive or passive mixing, which is beneficial for time-sensitive applications such as chemical reactions, biological assays, and the transport of to-be-detected species to sensors. In microfluidics, the need for fast mixing within milliseconds arises primarily because these devices are often used in fields where rapid and efficient mixing significantly impacts the performance and outcome of the processes. Active mixing with acoustics in microfluidic devices involves using acoustic waves to enhance the mixing of fluids within microchannels. Using sharp corners and wall patterns in acoustofluidic devices significantly enhances mixing by acoustic streaming around these features. The streaming patterns around sharp edges are particularly effective for mixing because they can produce strong lateral flows that rapidly homogenize liquids. This work presents extensive characterizations of the effect of sharp-edged structures on acoustic mixing in bulk acoustic wave (BAW) mode in a silicon microdevice. The effect of side wall patterns in different angles and shapes, their position, the type of piezoelectric transducer, and its amplitude and frequency have been studied. Following the patterning of the channel walls, a mixing time of 25 times faster was reached, compared to channels with smooth sidewalls exhibiting conventional BAW behavior. The average locally determined acoustic streaming velocity inside the channel becomes 14 times faster if sharp corners of 10° are added to the wall.

Magnetically guided untethered devices are used in a variety of medical applications. These devices are typically powered by onboard battery units. Instead, a hydrogen fuel cell(FC) is a promising alternative power source for such small-scale devices as it relies on a sustainable fuel. They function as electric power sources by utilizing the redox reaction of hydrogen and oxygen, using a proton exchange membrane(PEM). Understanding the impact of reducing FC active area is crucial for deploying FCs in untethered devices and gaining insights into the challenges of downscaling the devices. This research investigates the performance of PEM FCs (PEMFCs)when their active area is reduced, and when the FC is supplied with reactants at different flow rates. PEMFCs with three active areas of electrodes, 3.5×3.5, 2.7×2.7, and 1.6×1.6 cm2were designed, fabricated, and characterised. Maximum fuel cell output powers of 0.3, 0.09, and 0.03 W (maximum power densities of 0.025, 0.012, and 0.012 W/cm2) were achieved, respectively. Mathematical modelling of the PEMFC simulated the FC response, providing insights into the activation kinetics of the fuel cell. In the context of small-scale magnetic actuation, the smallest PEMFC with an active area of 1.6×1.6 cm2 was used to power an inductor coil (rated 130 mA, 150 mH, 8 Ω).The resistive behavior of the coil was captured at a power of0.0277 W (0.0108 W/cm2).

Electrolyzers operate over a range of temperatures; hence, it is crucial to design electrocatalysts that do not compromise the product distribution unless temperature can promote selectivity. This work reports a synthetic approach based on electrospinning to produce NiO:SnO2 nanofibers (NFs) for selectively reducing CO2 to formate above room temperature. The NFs comprise compact but disjoined NiO and SnO2 nanocrystals identified with STEM. The results are attributed to the segregation of NiO and SnO2 confirmed with XRD. The NFs are evaluated for the CO2 reduction reaction (CO2RR) over various temperatures (25, 30, 35, and 40 °C). The highest Faradaic efficiencies to formate (FEHCOO-) are reached by NiO:SnO2 NFs containing 50% of NiO and 50% SnO2 (NiOSnO50NF), and 25% of NiO and 75% SnO2 (NiOSnO75NF), at an electroreduction temperature of 40 °C. At 40 °C, product distribution is assessed with in-situ differential electrochemical mass spectrometry (DEMS), identifying methane besides other products, like formate, hydrogen, and carbon monoxide, in the flow electrochemical cell. XPS and EELS unveiled the FEHCOO- variations due to a synergistic effect between Ni and Sn. DFT-based calculations reveal the superior thermodynamic stability and activity of Ni-containing SnO2 systems towards CO2RR over the pure oxide systems. Furthermore, computational surface Pourbaix diagrams showed that the presence of Ni as a surface dopant increases the reduction of the SnO2 surface and enables the production of formate. Our results highlight the synergy between NiO and SnO2, which can promote the electroreduction of CO2 at temperatures above room temperature.

Porous separators are a key component in alkaline water electrolyzers and are significant sources of overpotential. In this paper, porous silicon separators were fabricated by etching precise arrays of cylindrical pores into silicon substrates through lithography. Chemical stability of the silicon-based separators is ensured through the deposition of a silicon nitride layer. Platinum or nickel were vapor-deposited directly on the faces of the separator to complete a zero-gap configuration. Separator porosity (ε) was varied by changing the pore diameter and the pore spacing. These well-controlled porous silicon zero gap electrodes (PSi-ZGEs) were used to study the trade-off between separator resistance and gas-crossover at different porosities. It was found that separator resistances comparable to commercially used Zirfon UTP 500 at much lower ε. Gas crossover remained within the explosive limits for ε ≤ 0.15 %. In the broad perspective, the current work can pave the path for the development of ionomer-free separators for alkaline water electrolysis which rely on the separator geometry to limit gas-crossover. The PSi-ZGEs achieved stable performance at 100 mA/cm2 for 24 hours without significant surface damage in the alkaline electrolyte