Inkjet Printing of a Highly Loaded Palladium Ink for Integrated, LowCost pH Sensors

© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 wileyonlinelibrary.com applications, metal nanoparticles, oxides, polymers, and many other materials were inkjet-printed.[1] However, the inkjet printing of an essential material for electrochemical applications, palladium (Pd), is less commonly reported. Pd is of significant importance for electrochemical applications due to its high electrocatalytic activity and affinity for hydrogen.[9] Pd and Pd-based materials have been used as electrocatalysts in power sources,[9,10] absorbers in hydrogen purification and storage devices,[9,11] and sensing materials in gas/bio/chemical sensors.[9,12,13] Previously, we demonstrated that the reversible redox reactions between palladium/palladium oxide (Pd/PdO) and hydronium ions (H3O) could be used for highly sensitive pH monitoring.[13] Besides, Pd can be used for electrical interconnections in electronic devices. For instance, Pd provided good ohmic contacts in carbon nanotube-based field-effect transistors because of its high work function.[14–16] Also, Pd was used for inter-chip interconnections due to its high electrical conductivity and resistance to chemicals.[17] Thus, Pd and Pd-based materials have the potential to play a dual role as active sensing materials and electrical interconnections in future electrochemical sensing/monitoring systems. One important challenge of applying Pd and Pd-based materials in low-cost electrochemical systems is related to its deposition techniques. Usually, Pd and Pd-based materials are deposited via hydrothermal growth, electrochemical deposition, thermal oxidation, and physical vapor deposition.[9,18,19] These processing techniques require a large amount of chemicals, high-pressure chambers, high temperatures, and vacuum equipment. Also, additional patterning steps are needed to integrate the deposited materials into functional systems. These processing and patterning steps result in costly manufacturing. Therefore, the less complicated and more cost-efficient inkjet printing technique for Pd deposition should be developed. To the best of our knowledge, inkjet printing of Pd has been used to deposit and pattern seed layers for electroless deposition,[20–22] and also as the contact electrodes for thin-film resistors.[15] The Pd inks used in these studies had low metal content (normally <5 wt%), which resulted in very thin, discontinuous, and high-resistance Pd films. Such Pd films were undesirable for electrochemical sensing because the inefficient Inkjet Printing of a Highly Loaded Palladium Ink for Integrated, Low-Cost pH Sensors