processes Article Adsorption and Reaction Mechanisms of Direct Palladium Synthesis by ALD Using Pd(hfac) 2 and Ozone on Si (100) Surface Chunyu Cheng, Yiming Zou, Jiahui Li, Amanda Jiamin Ong , Ronn Goei , Jingfeng Huang , Shuzhou Li * and Alfred Iing Yoong Tok *   Citation: Cheng, C.; Zou, Y.; Li, J.; Ong, A.J.; Goei, R.; Huang, J.; Li, S.; Tok, A.I.Y. Adsorption and Reaction Mechanisms of Direct Palladium Synthesis by ALD Using Pd(hfac) 2 and Ozone on Si (100) Surface. Processes 2021, 9, 2246. https:// doi.org/10.3390/pr9122246 Academic Editor: Gianvito Vilé Received: 4 November 2021 Accepted: 26 November 2021 Published: 13 December 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore; chunyu.cheng@ntu.edu.sg (C.C.); yiming004@ntu.edu.sg (Y.Z.); jiahui.li@ntu.edu.sg (J.L.); jiamin.ong@ntu.edu.sg (A.J.O.); ronn_goei@ntu.edu.sg (R.G.); huang_jingfeng@rp.edu.sg (J.H.) * Correspondence: lisz@ntu.edu.sg (S.L.); miytok@ntu.edu.sg (A.I.Y.T.) Abstract: Palladium nanoparticles made by atomic layer deposition (ALD) normally involve formalde- hyde or H 2 as a reducing agent. Since formaldehyde is toxic and H 2 is explosive, it is advantageous to remove this reducing step during the fabrication of palladium metal by ALD. In this work we have successfully used Pd(hfac) 2 and ozone directly to prepare palladium nanoparticles, without the use of reducing or annealing agents. Density functional theory (DFT) was employed to explore the reaction mechanisms of palladium metal formation in this process. DFT results show that Pd(hfac) 2 dissociatively chemisorbed to form Pd(hfac)* and hfac* on the Si (100) surface. Subsequently, an O atom of the ozone could cleave the C–C bond of Pd(hfac)* to form Pd* with a low activation barrier of 0.46 eV. An O atom of the ozone could also be inserted into the hfac* to form Pd(hfac-O)* with a lower activation barrier of 0.29 eV. With more ozone, the C–C bond of Pd(hfac-O)* could be broken to produce Pd* with an activation barrier of 0.42 eV. The ozone could also chemisorb on the Pd atom of Pd(hfac-O)* to form O 3 -Pd(hfac-O)*, which could separate into O-Pd(hfac-O)* with a high activation barrier of 0.83 eV. Besides, the activation barrier was 0.64 eV for Pd* that was directly oxidized to PdOx by ozone. Based on activation barriers from DFT calculations, it was possible to prepare palladium without reducing steps when ALD conditions were carefully controlled, especially the ozone parameters, as shown by our experimental results. The mechanisms of this approach could be used to prepare other noble metals by ALD without reducing/annealing agents. Keywords: DFT study; palladium; ozone; atomic layer deposition 1. Introduction Atomic layer deposition (ALD) is widely used in many fields, owing to the fact that the large and complex 3D film surfaces can be used to grow nanoparticles uniformally and conformally over them. This can be done with precise control of the film thickness and nanoparticle size [1–3], where the films and nanoparticles chemisorb on the substrate surface with a strong binding. Many noble metals at the nanoscale have been synthesized successfully by ALD [4–8]. Among them, palladium has a wide range of applications at the nanoscale for catalysts [9,10], hydrogen storage [11–13], and sensors [14–16]. It is desirable in most of the applications to prepare uniform and conformal palladium particles on complex surfaces to improve palladium content per unit volume [17–19]. Elam et al. [20] have reported a method to nucleate and grow palladium directly on the surface of Al 2 O 3 using Pd(hfac) 2 (hfac = hexafluoroacetylacetone) and formalin (the aqueous solution of formaldehyde) by ALD. Weber et al. [21] have prepared palladium particles with high purity and low resistivity at 100~200 ◦ C using Pd(hfac) 2 and H 2 plasma followed by an O 2 plasma step, where the O 2 plasma step was necessary to remove hydrocarbon fragments on the surface that remained after the H 2 plasma step. Since formaldehyde is Processes 2021, 9, 2246. https://doi.org/10.3390/pr9122246 https://www.mdpi.com/journal/processes