Formation Mechanisms of Pt and Pt₃Gd Nanoparticles under Solvothermal Conditions: An in Situ Total X-ray Scattering Study

Abstract

Platinum-based nanoparticles play a crucial role as catalysts, and solvothermal synthesis methods are attractive due to the excellent control of nanoparticle characteristics such as size, crystallinity, and morphology, which strongly affect the chemical and physical properties. Insight into the reaction mechanism leading to nanoparticle formation under solvothermal conditions generally remains elusive. This is mainly due to the experimental difficulties that lie in obtaining atomistic information on the nanoscale during the progression of the synthesis. Using in situ total X-ray scattering and pair distribution function (PDF) analysis with a time resolution of 1 s, we unravel the formation mechanisms of Pt and Pt 3 Gd nanoparticles under solvothermal conditions. We demonstrate that an octahedral Pt 4+ platinic acid precursor complex is reduced in two steps. Both Pt and Pt 3 Gd nanocrystal formation proceeds via conversion of a square-planar Pt 2+ complex to an unsaturated nanocluster (Pt 0 ), which subsequently grows by a combination of aggregation and ripening. In contrast to Rietveld analysis of powder X-ray diffraction data, the PDF method is able to uniquely establish that the structure of the Pt−Gd alloy is Pt 3 Gd rather than PtGd.

In Situ PDF Study of the Nucleation and Growth of Intermetallic PtPb Nanocrystals

“…Previously we have shown that the formation of Pt nanoparticles under solvothermal conditions (ethanol solvent) from H 2 PtCl 6 ⋅6 H 2 O occurs via a two‐step reaction mechanism. The octahedral PtCl 6 2− complex (Pt is tetravalent) exists in the precursor solution and upon heating it first reduces to a square planar PtCl 4 2+ (Pt is divalent) before further reduction allows formation of metallic nanoparticles . The second reduction step occurs only after the first reduction from Pt 4+ to Pt 2+ is completed.…”

“…Here the formation of Pt nanoparticles was observed after 10 s of reaction in the case of H 2 PtCl 6 ⋅6 H 2 O at a temperature of 250 °C . For comparison we first investigated the formation of pure Pt nanoparticles from Pt(acac) 2 under the same conditions.…”

“…It has been suggested that a paradigm shift is needed towards focus on the atomic scale chemistry of nucleation . In the conventional nucleation approach, one global model is assumed to fit all nucleation phenomena, but the in situ PDF analysis has revealed that in reality a diverse array of nucleation mechanism take place depending on the chemical system . Another recent example of the strength of the PDF technique is the experimental verification of the restructuring of solvent molecules on nanoparticle surfaces in solution .…”

The chemistry of ZnWO4 nanoparticle formation

“…24 A major break-through in in situ studies of nucleation and growth came with the application of total X-ray scattering to follow the nucleation of nanoparticles under solvothermal conditions. 25,26 At this point a handful of such studies have been reported 17,25–30 and they reveal an amazing diversity in the nucleation and growth of metal oxides under solvothermal conditions. In the formation of CeO 2 nanoparticles 26 an initial dimerization was observed, whereas for SnO 2 and Fe 2 O 3 the nucleation takes places through clustering of specific pre-nucleation complexes.…”

Formation and Functioning of Bimetallic Nanocatalysts: The Power of X‐ray Probes

“…By tuning the incident Xray energy below the Pt edge (off-resonant), single-shot XES can be collected by ahigh-resolution dispersive spectrometer with sub-second time resolution. Notably,Saha et al [58] used in situ total X-ray scattering (1 sr esolution) to study Pt and Pt 3 Gd formation from their precursor states. Self-absorption does not occur in HEROS,o pposed to fluorescence XAS.Atwo-step Pt(acac) 2 decomposition mechanism is seen in H 2 :i )a decrease of the white line (WL) height owing to adecreasing density of unoccupied 5d states,f ollowed by ii)a peak shift showing reduction to metallic NPs (Figure 4e,f).…”