## 1 Introduction  
The importance of palladium lies in its unique ability to adsorb hydrogen and catalyse hydrogenation and dehydrogenation reactions. Palladium plays a fundamental role in a wide range of applications: in the energy field for the production [1], storage [2] and use of hydrogen in catalytic fuel cells [3, 4, 5], and even in the chemical synthesis field with applications in optoelectronics [6] and biology [7, 8]. It is estimated that it is the element with the highest hydrogen adsorption capacity, managing to store volumes equal to 935 times its own [9]. These characteristics make this element extremely functional for the development and production of new energy systems that are not dependent on oil, such as fuel cells [2]. Nowadays, palladium is mainly used in the automotive sector in the production of catalytic converters. In 2020, China requested a 30% increase in the quantity of palladium inside cars and trucks [10]. The major problem related to the use of palladium is its high and constantly growing cost, due to the increasing need to transition from an economy based on oil to one based on hydrogen [11], and the very high demands from the automotive sector to achieve increasingly stringent environmental standards in vehicle emissions. To stem this problem and make an eco-sustainable system advantageous, even economically, it is essential to study techniques for producing thin films of palladium, to avoid the massive use of metal while preserving the surface properties that have made it so widespread. Palladium is also particularly popular in the electroplating industry for luxury jewellery and accessories in which it is used as intermediate layer or top layer with thicknesses ranging from 0.2 \(\upmu\)m to 1 \(\upmu\)m [12]. In this sector, palladium films obtained from an electroless deposition could open the doors to a more cost-effective industrial process with respect to the electrodeposition one. Electroless deposition techniques differ from electroplating due to the absence of an external current designed to promote a reduction in the species to be deposited on the substrate. In electroplating techniques, the growth of the deposit depends on the surface current density, which, for irregular geometries, is not uniformly distributed over the cathodic area, leading to non-uniform deposits. This problem does not occur with electroless depositions (given the non-use of external current) making these techniques excellent for use on substrates with exotic and irregular shapes [13]. The aim of this work is the development of a cost-effective technique or combination of techniques able to produce high-quality Pd thin films with tailorable thickness on copper substrates. To obtain a Pd layer with a very cost-effective procedure, electroless plating techniques were selected for the procedures. Copper was chosen as the base material because it is a common material used as a substrate, as well as intermediate layer, in electroplating and the surface finishing industry thanks to its semi-noble behaviour, high levelling power, and the brightness of copper films. Unfortunately, electroless deposition of palladium on copper is not trivial and can lead to some issues. These problems are due to the presence of concentrated ammonia in most electroless palladium baths; in fact, a pre-deposition step is mandatory for Cu substrates. As proof of the fact that ammonia acts as an etchant on copper surfaces, its effect has also been studied in the literature as a copper surface polishing agent [14]. In the case of this work, ammonia etchant behaviour is detrimental for the formation of a continuous and adherent Pd layer. In this work, two different types of electroless deposition techniques were used, displacement deposition and chemical deposition. The displacement deposition technique (PDP, palladium displacement plating, from now on) is a process in which the ions of a nobler metal, with respect to the metal substrate, are reduced on the substrate due to a galvanic exchange reaction, producing a deposited film [13].