I. Abstract of the Paper’s Core Research

This study synthesizes Pd–Ag metal‑organic decomposition (MOD) inks suitable for both inkjet and aerosol‑jet printing processes. Two fabrication routes are compared: inkjet printing followed by 300°C nitrogen annealing, and aerosol‑jet printing combined with room‑temperature deposition and subsequent atmospheric‑pressure plasma post‑treatment. The microstructure and elemental distribution of the resulting thin films are characterized using XRD and SEHI super‑resolution surface spectroscopy. The key findings are as follows:

Printable Pd-Ag Bimetallic MOD Ink System

Palladium acetate and silver acetate–ammonia complex precursors, diluted with isopropyl alcohol to a viscosity of 5.58 cP and a Z‑number of 6.48, are compatible with ultrasonic atomization and droplet‑jetting, exhibiting no nanoparticle sedimentation and no risk of nozzle clogging.

Inkjet High-Temperature Forming Characteristics

Inkjet printing on a 60°C substrate, followed by annealing in nitrogen at 300°C for 1 hour, yields a Pd–Ag alloy with a stoichiometric ratio of 79:21; however, the resulting thin film exhibits micron‑scale lamellar pores and localized phase separation between Pd and Ag, leading to compositional nonuniformity over large areas.

Innovative Approach: Room-Temperature Aerosol Printing Combined with Low-Temperature Plasma Alloying

No substrate heating is required; pattern deposition is completed at room temperature. Combined with short‑duration atmospheric‑pressure Hairline plasma treatment, organic ligands decompose rapidly, enabling in situ formation of a uniform palladium–silver alloy. A PEDOT:PSS buffer layer is introduced to address film delamination, allowing the fabrication of custom microelectrodes ranging from 50 μm to 1 mm on both silicon and flexible PI substrates.

A New Method for Precise Multi-Scale Component Characterization

By employing secondary electron hyperspectral imaging (SEHI), we overcome the limitations of conventional EDX in probing depth, enabling nanoscale visualization of palladium-rich, silver-rich, and alloy‑mixed regions, and precisely pinpointing phase‑separation defects.

Differentiated validation of process scenarios

Inkjet printing is well-suited for large‑area, flat, rigid substrates; aerosol jet printing, with its long standoff distance and non‑contact advantages, is ideal for curved surfaces, flexible materials, and micro‑array applications; and plasma processing operates at low temperatures throughout the entire process, ensuring no damage to heat‑sensitive polymers.

II. Complete Process Flow: Aerosol Printing + Plasma Post‑Treatment

MOD ink formulation: Palladium acetate and silver acetate are complexed with concentrated ammonia solution, and the resulting colloid is stabilized by dilution with isopropyl alcohol.

Substrate pretreatment: A PEDOT:PSS buffer layer is spin-coated onto the silicon/flexible substrate to enhance the adhesion of the alloy film.

Optomec AJ equipment employs low-temperature aerosol printing, with the substrate maintained at ambient temperature throughout the process, using a 150 μm nozzle to fabricate square and interdigitated microstructures.

Under atmospheric pressure, localized scanning with an argon plasma beam rapidly decomposes organic ligands and in situ generates a palladium–silver alloy.

SEM, XRD, and SEHI are used for hyperspectral characterization of micro‑morphology, crystal structure, and the uniformity of elemental distribution.

III. Core Advantages of Three–Five Atmospheric Aerosol Processes (Comparison of Sputtering, Inkjet, and Thermal Evaporation)

All‑process deposition at room temperature, compatible with fully thermosensitive flexible substrates.

Inkjet printing requires high-temperature annealing at 300°C, whereas aerosol printing is performed at room temperature and followed by low-temperature plasma modification, ensuring that PET, PI, and conductive polymer substrates do not undergo thermal deformation.

Large spray‑distance, non‑contact forming; universally applicable to curved and irregularly shaped substrates.

The distance between the print head and the substrate is adjustable, enabling direct fabrication of textured, stepped, and curved devices, whereas sputtering is limited to planar wafer processing.

Maskless digital micro-patterning, resulting in substantial savings on precious metals.

CAD‑customized interdigitated electrodes and micro‑catalytic arrays, with site‑specific deposition of palladium–silver ink, reduce bulk‑film precious‑metal consumption by more than 70%.

Plasma‑based localized controlled alloying and zone‑specific functionalization design

Plasma beams can perform localized scanning, enabling alloy–pure metal patterned zoning on the same substrate—something that cannot be achieved through full‑surface heat treatment in a high‑temperature furnace.

MOD Particle-free ink ensures no clogging even with long-term printing.

Unlike nano-metal inks, molecular precursors exhibit no solid-phase agglomeration, resulting in superior long-term printing stability.

IV. Five Core Technological Innovations of This Study

Innovation 1: A Complete Process for Preparing Palladium–Silver Alloys via Aerosolization

By integrating MOD ink with aerosol jetting and atmospheric-pressure plasma, we overcome the limitations of conventional high-temperature annealing, enabling the direct fabrication of palladium–silver alloys on flexible substrates.

Innovation 2: A dual-process-compatible Pd-Ag organic precursor ink

A single ink system is compatible with both inkjet and aerosol direct-write devices, meeting the dual needs of large‑area mass production and micro‑scale customization in R&D.

Innovation: A New Paradigm for Nanoscale Component Characterization by SEHI

Outperforming conventional EDX shallow-depth probing, it enables nanoscale visualization of palladium–silver phase separation in thin films, providing a precise evaluation tool for optimizing alloy inks and post‑processing workflows.

Innovation: A 4 PEDOT:PSS buffer layer addresses the challenge of alloy film delamination.

Aerosol‑direct printing on silicon substrates tends to cause cracking and delamination, whereas a conductive polymer interlayer significantly enhances interfacial adhesion, yielding a continuous, crack‑free thin film.

Innovative Mechanism of Room-Temperature In-Situ Plasma Alloying in Five Chambers

Without requiring high‑temperature bulk treatment, plasma‑activated species rapidly strip away organic ligands, while metal atoms diffuse and coalesce in situ to form a functional alloy with an 79:21 composition.

V. Current Industry Challenges and Corresponding Solutions

Challenge 1: Conventional palladium–silver synthesis requires temperatures above 300°C, which the flexible polymer substrate cannot withstand.

✅ Ambient‑temperature aerosol printing combined with low‑temperature plasma modification, with the maximum processing temperature well below the polymer’s thermal tolerance threshold.

Challenge 2: Sputtering and electroplating achieve global uniformity of alloy composition but cannot be patterned locally.

✅ Maskless direct-write of aerosols enables on-demand printing of catalytic microarrays, with noble metals deposited only in the functional regions.

Challenge 3: Inkjet‑printed high‑temperature thin films are prone to micron‑scale phase separation, leading to degradation of catalytic and hydrogen‑separation performance.

✅ Plasma-assisted low-temperature atomic-scale diffusion ensures uniform thin-film composition, minimizing palladium‑rich or silver‑rich interphase defects.

Challenge 4: Nano-metal inks are prone to sedimentation and can cause nozzle clogging during prolonged printing.

✅MOD molecular‑type ink contains no solid particles, atomizes smoothly, and reduces equipment maintenance costs.

Challenge 5: Hard coating processes cannot form on curved surfaces or the surfaces of irregularly shaped reactors.

✅ Aerosol-based non-contact aerosol‑beam deposition enables the integrated fabrication of curved hydrogen‑separation membranes and microfluidic catalytic chips.

VI. Future Diversified Industrial Application Scenarios

1 Flexible Hydrogen-Selective Alloy Membrane Reactor

Low-temperature aerosol‑formed curved palladium–silver thin films replace rigid metal separation components, enabling lightweight hydrogen purification equipment for the chemical industry.

2 CO₂ Reduction Microelectrocatalytic Array

High-precision aerosol-printed palladium–silver microelectrodes for efficient carbon conversion in electrolytic cells, reducing the amount of precious-metal catalyst required.

3 Micro‑MEMS Hydrogen Sensing Chip

High-selectivity palladium–silver alloy microcircuits, high-precision trace hydrogen detection, and energy-storage and hydrogen‑energy safety monitoring devices.

4 Flexible Printed Electrochemical Devices

PI‑based low‑temperature palladium–silver conductive catalytic thin film, enabling rapid prototyping of wearable gas sensors and flexible electrochemical modules.

5 University-Based Research and Development Platform for Precious Metal Catalytic Materials

A desktop aerosol‑based apparatus enables the low‑cost synthesis of palladium–silver alloys with varying compositions, facilitating rapid screening of their catalytic and hydrogen‑separation properties.

Image and Text Guide

Figure 1. Optical images (×80) showing the optimal line widths of inkjet-printed Pd, Ag, and Pd–Ag MOD inks on glass.

Unified process: scanning speed of 13 mm/s, droplet pitch of 0.05 mm, and glass substrate at 60 °C. The linewidth of pure Pd is 394 ± 16.8 μm, that of pure Ag is 553 ± 9.9 μm, while the Pd–Ag alloy designed with a molar ratio of 79:21 exhibits a linewidth of only 290 ± 6.0 μm—after alloying, ink spreading is significantly suppressed, and the line edges are much sharper; distinct “ring effects”—annular accumulations—appear on both sides of the Ag lines, whereas the Pd–Ag lines show the cleanest edges.

Figure 2. Macroscopic photographs of inkjet-printed Pd, Ag, and Pd–Ag square films after 1 pass and 5 passes (glass substrate, annealed in N₂ at 300 °C for 1 h).

Left column, Pd: At 1 pass, the film is gray‑black with visible parallel scan streaks; at 5 passes, the color deepens but edge scattering becomes severe. Middle column, Ag: At 1 pass, the film appears as a pale yellow, transparent layer; at 5 passes, it turns gray‑white. Right column, Pd–Ag: At 1 pass, the film is dark brown‑black with an uneven, textured surface; at 5 passes, the surface evolves into a rough, porous, island‑like morphology. All film edges are relatively sharp, with the most pronounced edge scattering observed for the 5‑pass Pd sample—likely attributable to the secondary atomization of ammonium hydroxide (boiling point 38 °C) on the substrate at 60 °C.

Figure 3. LV-SEM morphology of a single-pass thin film on a silicon substrate (scale bar: 500 nm)

A) Pd: Nanoparticles assemble into a two-dimensional (2D) network with a characteristic size of ~100 nm, leaving the underlying silicon substrate uncovered. B) Ag: In addition to spherical nanoparticle arrays, large-scale three-dimensional agglomerates also form; as the particles move farther from the aggregates, their diameters decrease. C) Pd–Ag: The nanoparticles are more densely packed and exhibit a narrower size distribution, yet dark regions on the silicon substrate remain uncovered. This indicates that alloying narrows the particle size distribution, but single-layer coverage remains incomplete.

Figure 4. LV-SEM of inkjet-printed droplet remnants

A–C) 5-pass Ag / 5-pass Ag / 5-pass Pd: After annealing, the Ag droplet remnants leave behind a porous 3D scaffold, whereas the Pd remnants form a dense, sheet-like film. D–F) 1-pass Pd–Ag: At low magnification, the droplet remnants exhibit highly uneven sizes (D); in E), an individual droplet remnant has a diameter of approximately 200 μm; and in F), at high magnification, the sheet‑like structure is observed to wrinkle, fold, and locally rupture—this sheet contraction and cracking are precisely what give rise to the micrometer‑scale surface topography.

Figure 5. Microstructure of the film layer on a glass substrate, magnified ×270 (scale bar: 50 μm)

A/B) Pd 1/5 pass: a sheet-like structure is observed after 1 pass, with partial collapse occurring by 5 passes; C/D) Ag 1/5 pass: a dark film forms after 1 pass, and by 5 passes, porous particle aggregates have grown; E/F) Pd–Ag 1/5 pass: the film surface is relatively smooth after 1 pass, with a few residual droplets scattered about, while by 5 passes, a continuous network of porous folds extends across the entire area. The sheet-like morphology on the glass substrate is identical to that on the silicon substrate, demonstrating that the morphology is determined by the ink and the annealing process, and is independent of the substrate.

Figure 6. XRD pattern + Table 3: Verification of Pd-Ag alloying after annealing at 300 °C

Black denotes pure Ag, red denotes pure Pd, and blue denotes Pd–Ag. The three principal diffraction peaks of the alloy film (111, 200, and 220) are shifted relative to those of pure Ag and pure Pd, falling between the two; linear interpolation according to Bragg’s law (Table 3) yields molar fractions of Pd in the alloy of 79.53, 79.37, and 79.88 at%, which closely match the ink formulation of 79:21. The peaks remain broad and are accompanied by minor peaks corresponding to the pure phases, indicating that locally unalloyed Pd/Ag regions persist within the film—consistent with the droplet‑remnant morphology observed by SEM.