Adding iron to a copper-based propellant MOF simultaneously solves its water stability problem and boosts laser propulsion efficiency, breaking a trade-off once thought inherent.
(Nanowerk Spotlight) Microsatellites weighing less than a kilogram can now photograph Earth, relay communications, and monitor weather. Steering them in orbit requires propulsion, and the options available today all involve managing fluids in some form. Cold gas thrusters release pressurized gas through nozzles. Electric thrusters ionize gas or vaporize solid material using electric arcs. Chemical systems burn liquid propellants. Each approach needs pressurized tanks, valves, and feed lines, hardware that competes for the limited volume inside spacecraft sometimes no larger than a coffee cup.
Pulsed laser micropropulsion (PLMP) takes a different approach, one that eliminates fluid handling entirely. A small onboard laser fires nanosecond pulses at a solid propellant target, vaporizing a thin surface layer into a jet of plasma that pushes the spacecraft forward. The propellant is an inert solid slab, so the system needs no pressurized tanks, no valves, and no feed lines. This makes it simpler, more compact, and less prone to the leaks and mechanical failures that plague fluid-based systems at small scales. The technology has not yet flown in space, but recent laboratory results have brought it closer to practical use.
The bottleneck is the propellant. Early candidates, metals and polymers sometimes blended with nanoparticles, could absorb laser energy but failed to vaporize cleanly. Embedded particles did not fully decompose during the nanosecond pulse, leaving large residues and wasting energy.
A class of crystalline materials called metal-organic frameworks, or MOFs, offered a way forward. MOFs are built from metal atoms linked by organic molecules into porous, repeating lattices. This atomic-level regularity means laser energy is absorbed and distributed uniformly, producing clean, efficient ablation.
One copper-based MOF called HKUST-1 proved particularly effective, setting performance records in laboratory propulsion tests. But it has a critical flaw: moisture attacks the copper-oxygen bonds that hold its lattice together, and within 6 hours the crystal structure collapses. For a material that must survive humid launch conditions and potentially water-rich environments in space, this rules it out.
The researchers replaced a fraction of the copper atoms in HKUST-1 with iron during a one-step synthesis, producing a bimetallic variant called FeCu-MOF. The chemistry behind this choice is straightforward. Iron(III) binds more tightly to oxygen than copper(II) does: iron-oxygen bonds have a dissociation energy of roughly 397 kJ/mol, more than double the approximately 156 kJ/mol of copper-oxygen bonds in standard HKUST-1. Copper(II) ions in the original framework also suffer from a geometric distortion called Jahn-Teller instability, which loosens their grip on the organic linkers and accelerates hydrolysis. Iron(III), by contrast, adopts a stable coordination geometry that resists water attack.
(A) Illustration of the PLMP mechanism. (B) Illustration of the advantages of doped bimetallic MOF propellants in the PLMP absorption and ablation process. (C) Schematic for illustrating the stability of doped bimetallic MOF in water. (D) Illustration of the ablation process of the pulsed laser ablation in three different propellants. (Image: Reproduced from DOI:10.1002/adma.72795, CC BY) (click on image to enlarge)
The team synthesized three variants with iron-to-copper ratios of 1:20, 1:10, and 1:5. X-ray diffraction confirmed that all retained the parent crystal structure without lattice distortion, and elemental mapping showed iron and copper distributed uniformly throughout each crystal. Surface area measurements verified that iron substitution does not block the framework’s pores. The bimetallic MOFs maintained their crystal shape after 120 hours of water immersion, compared with just 6 hours for pure HKUST-1.
The same atomic substitution that strengthens the framework also makes it a better propellant. When a laser pulse strikes a target, the fraction of photon energy converted to heat determines how efficiently the material ablates. In HKUST-1, broadband light absorption reaches 71%. The medium-doped variant, FeCu-MOF-M, achieved 91%. This gain arises because iron and copper atoms sitting at neighboring sites within the lattice exchange electron density through their d-orbitals, a process called metal-to-metal charge transfer. This broadens the range of wavelengths the material can absorb and channels the captured energy into heat rather than re-emitting it as light.
Quantum-mechanical modeling supported this interpretation, showing that iron incorporation shifts electronic energy levels in ways that favor charge transfer and non-radiative energy dissipation. Physically mixing iron nanoparticles into HKUST-1 powder did not produce the same effect. Such mixtures reached only 84% absorption because the nanoparticles clumped together, scattering light and creating uneven thermal zones. The performance advantage requires iron and copper atoms to occupy neighboring sites within the same crystal lattice.
In propulsion testing, the optimized variant achieved an ablation efficiency of 59.32%, meaning nearly 60% of the laser energy was converted into kinetic energy of the exhaust. This surpasses HKUST-1 by 15.7% and exceeds physically mixed iron-MOF composites by 125%. Electron microscopy showed why: ablation residues from FeCu-MOF averaged just 1.7 nm in diameter, compared with 4.3 nm for HKUST-1. Smaller residues mean more complete vaporization, with a greater share of ablated mass exiting as high-velocity exhaust rather than coalescing into larger, slower particles.
After 12 hours of water immersion, FeCu-MOF retained more than 95% of its propulsion performance. HKUST-1 subjected to just 6 hours of soaking suffered a 40% drop in ablation efficiency.
The iron-to-copper ratio proved to be a precise tuning lever. Performance peaked at the 1:10 ratio, where electronic coupling between iron and copper maximized photon harvesting. Higher iron content introduced clustering and local structural disorder that reduced returns. This stoichiometric sensitivity underscores that the gains stem from atomic-level integration, not merely from the presence of iron in the material.
The FeCu-MOF platform combines hydrolysis resistance, enhanced light absorption, and stoichiometric tunability in a material produced through a single synthesis step. More broadly, the work validates isomorphic metal substitution, swapping one metal for another within an existing crystal structure, as a general strategy for stabilizing moisture-sensitive frameworks without sacrificing performance. For pulsed laser micropropulsion, it brings the field closer to a propellant that can withstand real operating conditions while delivering record efficiency.
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