Observation of a crossover in kinetic aggregation of Palladium colloids

TL;DR

This study investigates how the aggregation behavior of palladium colloids on WO3/glass surfaces changes at a specific contact angle, revealing a kinetic crossover linked to surface wetting properties.

Contribution

It identifies a contact angle threshold that marks a transition in palladium colloid aggregation regimes and characterizes the fractal dimensions associated with each regime.

Findings

Two distinct aggregation regimes identified at CA = 46°

Fractal dimension shifts from 1.75 to 1.92 across the crossover

Fractal analysis consistent across different measurement methods

Abstract

We use field emission scanning electron microscope (FE-SEM) to investigate the growth of palladium colloids over the surface of thin films of WO3/glass. The film is prepared by Pulsed Laser Deposition (PLD) at different temperatures. A PdCl2 (aq) droplet is injected on the surface and in the presence of steam hydrogen the droplet is dried through a reduction reaction process. Two distinct aggregation regimes of palladium colloids are observed over the substrates. We argue that the change in aggregation dynamics emerges when the measured water drop Contact Angel (CA) for the WO3/glass thin films passes a certain threshold value, namely CA = 46 degrees, where a crossover in kinetic aggregation of palladium colloids occurs. Our results suggest that the mass fractal dimension of palladium aggregates follows a power-law behavior. The fractal dimension (Df) in the fast aggregation regime, where the measured CA values vary from 27 up to 46 degrees, according to different substrate deposition temperatures, is Df = 1.75 (0.02). This value of Df is in excellent agreement with kinetic aggregation of other colloidal systems in fast aggregation regime. Whereas for the slow aggregation regime, with CA = 58 degrees, the fractal dimension changes abruptly to Df=1.92 (0.03). We have also used a modified Box-Counting method to calculate fractal dimension of gray-level images and observe that the crossover at around CA = 46 degrees remains unchanged.