Small adsorbed molecules play a key role in the morphology of inorganic nanoparticles. The presence of in situ 11 generated hydrogen during the synthesis of platinum nanoparticles is found to drive the growth of cubic nanocrystals, but little is 12 known about the processes occurring at the molecular level. In this paper, we use standard ab initio calculations to show that 13 hydrogen preferentially adsorbs on (100) Pt facets compared to (111) stabilizing the cubic morphology. Moreover, we provide 14 experimental and theoretical evidence that moderate partial pressure of hydrogen is needed to obtain nanocubes. In the absence 15 of hydrogen, or for low partial pressures, small nanoparticles with undefined shape are formed; however, longer exposure to 16 hydrogen pressure around 1 atm leads to the formation of cubes. Finally, this theoretical result allows presenting an experimental 17 protocol to be used to obtain platinum nanocubes with different degree of truncation. 18 ■ INTRODUCTION 19 Many physical and chemical properties depend on the shape of 20 the particles forming the material, 1 hence the challenge of 21 controlling the crystal morphology in a wide range of scientific 22 and technological applications. The chemical route is largely used 23 to control the shape of inorganic nanocrystals, although there is 24 still no theoretical drawback of the main forces driving it. Despite 25 many studies found in the literature, the theoretical and 26 computer simulation of nanoparticles' synthesis and growth are 27 still in the initial stage of development, and most of these works 28 do not take into account the complexity of the chemical 29 synthesis. It is still paramount to explain the mechanisms at the 30 origin of the uniformity of shape. Indeed, the field is still wide 31 open to future research aiming at identifying the conditions to 32 control the nanomorphology. 33 In the chemical synthesis of the nanocrystals, the chemical 34 bath possesses a complex composition containing various 35 reactants, solvents, surfactants, (counter)ions, and impurities. 36 Moreover, during the reaction, byproducts can be formed, which 37 could play a role in the nuclei formation and the following 38 nanoparticles' growth. 2 For instance, the role of dissolved gas H 2 39 on the shape control of platinum nanocrystals has recently been 40 demonstrated experimentally. 3 Some authors also report the role 41 of halide, ligands, or adsorbed CO on the nanomorphology. 4−6 42 Besides, the role of the initial shape of the nanocrystals and the 43 growth kinetics have been emphasized. 7,8 Beyond the exper-44 imental recipe, a theoretical approach is needed to rationalize and 45 predict the stability of the nano-objects. In the present paper, the 46 goal is to emphasize the role of a byproduct generated during the 47 chemical synthesis in the final nanoparticle morphology. We 48 focus here on the specific case of platinum. We use ab initio 49 calculations to explain how the presence of in situ generated H 2 50 drives the formation of cubic nanoparticles. We also explain the 51 impact of the experimental conditions, namely, the partial