www.nature.com/scientificreports OPEN The infuence of dielectric permittivity of water on the shape of PtNPs synthesized in high‑pressure high‑temperature microwave reactor Marek Wojnicki1*, Magdalena Luty‑Błocho1, Przemysław Kwolek2, Marta Gajewska3, Robert P. Socha4, Zbigniew Pędzich5, Edit Csapó6,7 & Volker Hessel8 In this paper, a novel method for the synthesis of Pt nanoparticles (PtNPs) using a microwave autoclave reactor is proposed. For benchmarking, the obtained results are compared with the traditional, batch method. A novel process window is proposed, which is the application of high‑ temperature and high‑pressure. The main fnding is that this only brings advantage, when the ionic strength of the system is enough low. It is explained, that at high pressure and high temperature, water behaves like only a slightly polar solvent, approaching a subcritical state. This reduces the electrostatic stabilization of the particles. Moreover, a change in the Pt particle shape is observed under high pressure and temperature conditions, suggesting that additional physical–chemical processes are involved. Nanoparticles attract attention, mainly due to the fact, that they exhibit diferent properties than bulk materials. Tese properties are not linearly correlated to the particles size and do not follow traditional physics, therefore, it is difcult to predict the fnal properties of nanoparticles without further tests. One of the reasons why nanoparticles are so interesting is the fact, that the active surface area increases with the decrease of the particle size. In other words, it is possible to synthesize catalysts with a high activity using a relatively little amount of active metal 1. Tis is especially important when rare and precious metals are used 2. Te latter is the case of platinum. Tis metal exhibits excellent catalytic properties 3–5. It is resistance to all mineral and organic acids is outstanding 6 with the exemption of exposure to boiling sulfuric acid (temperature above 320 °C) and aqua regia. Terefore, this metal is frequently used as a catalyst in the chemistry/fertilizer industrial processes such as e.g. ammonia and nitric acid synthesis7,8. Te latest research shows that Pt nanoparticles can also be used in fuel cells9, as well as efective and stable catalyst for selective hydrogenation of 5-hydroxymethylfurfural10. Platinum nanoparticles (PtNPs) can be obtained via a chemical or physical route. In practice, however, the frst one is mostly applied due to the lower costs of equipment and easy implementation. PtNPs are usually syn- thesized by chemical reduction of Pt(IV)11 or Pt(II)12 salts with sodium borohydrides13, iron(II)14, ethanol15,16, N-heterocyclic thiones17, plants aqueous extractsor 18, other reductants 19, in the presence of sodium citrate 20 or other stabilizing agent20. Te common way is carrying out the synthesis in batch reactor. Besides and as a more modern approach, several papers reported efcient PtNPs synthesis in microreactors 21. For example, the appli- cation of a microreactor equipped with a back pressure regulator enables to heat the aqueous reaction mixture 1Faculty of Non-Ferrous Metals, AGH University of Science and Technology, Mickiewicza Ave. 30, 30-059 Kraków, Poland. 2Department of Materials Science, Faculty of Mechanical Engineering and Aeronautics, Rzeszow University of Technology, Aleja Powstańców Warszawy 12, 35-959 Rzeszow, Poland. 3Academic Centre for Materials and Nanotechnology, AGH University of Science and Technology, al. A. Mickiewicza 30, 30-059 Kraków, Poland. 4Institute of Catalysis and Surface Chemistry Polish Academy of Sciences, Niezapominajek 8, 30-239 Kraków, Poland. 5Faculty of Materials Science and Ceramics, AGH University of Science and Technology, al. A. Mickiewicza 30, 30-059 Kraków, Poland. 6MTA-SZTE Biomimetic Systems Research Group, University of Szeged, Dóm tér 8, 6720 Szeged, Hungary. 7Department of Physical Chemistry and Materials Science, University of Szeged, Rerrich B. tér 1, 6720 Szeged, Hungary. 8School of Chemical Engineering and Advanced Materials, The University of Adelaide, Adelaide, Australia. *email: [email protected] Scientifc Reports | (2021) 11:4851 | https://doi.org/10.1038/s41598-021-84388-2 1 Vol.:(0123456789) www.nature.com/scientificreports/ above 105 °C. Te use of solvents of even higher boiling point allows the application of even higher temperature. On this path, Chen et al.22 applied an ethylene glycol–water solution (volume ratio 3:1), for PtNPs synthesis at ~ 140 °C. Te synthesis time was c.a. 1 h. In many applications (e.g. for catalysis), PtNPs with a small size are obtained via chemical reduction using strong reductant like the aforementioned sodium borohydride. Te reduction process is rapid, yet it has limits in the scope of application. Unfortunately NaBH4 is not applicable when the nanoparticles are produced for e.g. medical use, due to the nature of this strong reductant and its oxidized product. More friendly reductants like ascorbic acid, glucose, citric acid, etc. usually lead to production larger particles and the process of nucleation and growth is much longer than in the case of sodium borohydride. Terefore, it would be very desirable to develop a method, where small and stable PtNPs are obtained in a short time using a reductant, and preferably a “mild” one. In this paper, we applied trisodium citrate (TSC) to stabilize and synthesize PtNPs, via a faster reduction compared to citric acid. Application of the latter is less popular mainly because this is a weak carboxylic acid, and its deprotonation requires some time. Moreover, sodium citrate is known and well described in the literature mainly for AuNPs using the so-called Turkevich method 23–25. Te size of obtained gold particles varies from 5 to 25 nm depending on the adopted procedure. Unfortunately, this method is time-consuming. It is required to boil the reacting solution for circa1h to obtain colloid with deep red color. Even more, using other reductants for the fabrication of PtNPs may take an even longer processing time. Tus, the Turkevich method was applied for AuNPs synthesis in microreactor, where efcient heating of a small volume of the solution allowed for quick Au(III) ions reduction and obtaining the red colloidal gold within few minutes26. Te use of a microreactor for the synthesis of precious metals ofers many advantages, yet also implies complications, which need to overcome. Te latter are is related to the nucleation, sedimentation and adsorption on the microchannel walls27. Such an autocatalytic particle growth is disadvantageous for at least two reasons. Te frst one is related to the efect of the metal deposit, which begins to act as a heterogenic catalyst for the chemical reaction, changing its kinetics28,29. Te other one, in turn, is related to the physical blocking of the microreactor channels by deposited metal. Tis is quite troublesome because in order to unblock the channel it is necessary to use concentrated acids or their mixtures, e.g. aqua regia in the case of gold or platinum. Tese adverse efects can be eliminated by appropriate modifcation of the surface of the microreactor channels 30,31. Terefore, we decided to “speed up” the synthesis of PtNPs not using the chemical and chemical engineering (microreactor) opportunities, but rather by changing the physical state of the processing solution. We propose to apply a microwave heated high-pressure and high-temperature reactor. Te use of microwave radiation in inorganic chemical synthesis began in the 1970s 32. Among its several advantages, a signifcant acceleration of the chemical reaction compared to "classic" techniques (the use of sand, oil baths, heating jackets, etc.) should be considered as the most important one. Besides, because microwave radiation usually does not heat the reaction vessel but only the solvent and reagents, it is easier to achieve an even temperature distribution in the reactor, avoiding the large temperature gradients observed when using "classical" techniques32,33. Te efect of acceleration of a chemical reaction can be qualitatively explained using the Arrhenius equation, and thus the major advantage of microwaves is to facilitate reaching unhabitual high temperatures for organic chemistry in a way that these are well defned. Te use of the microwave radiation increases the frequency factor and thus the reaction rate constant 32. Two efects would be responsible for this phenomenon: enthalpy, associated with the provision of energy supplied by microwaves in the form of the vibrational energy of reacting particles, and entropy, associated with the arrange- ment of molecules34. Tere are two mechanisms responsible for heating the reaction mixture by microwave radiation. Te frst one relates to the presence of ions in the reaction mixture. Tey move under the infuence of Coulomb interactions with an alternating electric feld. Teir movements are accompanied by collisions with other ions and kinetic energy is converted into thermal energy35. Generating heat in this way is very efcient 32.Te other mechanism is associated with the rotation of electric dipoles (friction and collisions, e.g. between water molecules) under the infuence of an alternating electric feld. Tis efect is observed when the rotational motion of water molecules is incompatible with the radiation phase, which is given for the microwave frequency range. Te more polar the solvent, the more efcient the heating. It can be assumed that during the synthesis of nanoparticles, the heating of the reaction mixture is carried out by both mechanisms, with a dipole mechanism dominating at low tem- perature, and ionic one at high. Tat is because the dielectric constant (relative permittivity) of water depends on the temperature and varies from 78 for 298 K to 20 at 573 K32. Te even heating of the reaction mixture depends on the volume and geometry of the reaction vessel. Micro- wave radiation is refected from its walls and standing waves are generated, therefore some areas will be heated more strongly, others less so. Tis efect can be particularly evident when heating small samples32.