For the high concentration reaction, the peak area plot is more complex and can be divided into four stages based on the slope: Stage I: ~60 100 min.; Stage II: 100 180 min.; Stage III: 180 300 min.; Stage IV: 300 500 min. The peak area increases rapidly and linearly in Stage I, indicating a ten fold increase in the growth rate of [Pt_NC] compared to the low concentration reaction. Remarkably, during Stage II, the peak area remains almost constant, indicating almost no growth in [Pt_NC]. In Stage III, the peak area resumes a rapid increase; the gradient is ~60% of that observed in Stage I. From the beginning of Stage IV to the end of experiment, the slope of the area plot decreases slightly, with a gradient of about 1/3 of that in the previous stage (Stage III), indicative of a slower increase in [Pt_NC].

The X-ray correlation length (L) for the low concentration reaction increases from 3 nm at 75 minutes to 8.6 nm at 230 minutes. It then remains almost constant at ~8.4 ± 0.3 nm for the remainder of the reaction. For the high concentration reaction, L increases during Stage I from ~5 nm to ~8.2 nm at 100 minutes. During Stage II, L remains almost constant at 8.0 ± 0.2 nm. Interestingly during Stage III, L decreases from 8.1 nm to 7.6 nm, differing markedly from the low concentration reaction. After 300 minutes, L continues to decrease gradually and is calculated to be ~7.4 nm at the end of Stage IV.

Transmission electron microscopy
TEM images of platinum nanoparticles obtained from the low concentration reaction show faceted nanoparticles of irregular shapes, with particle sizes of 6 ± 2 nm. The size range is consistent with the X-ray correlation length shown in Figure 1C. TEM images of the platinum nanoparticles collected after 400 minutes display similar morphologies to those observed at 80 mins but are of bigger size at 11 ± 3 nm, again consistent with XRD.

Figures 2A, C, E, and G show the TEM images of platinum nanoparticles collected from the high concentration reaction at 75, 120, 240, and 500 minutes, corresponding to Stages I to IV, respectively. As shown in Figure 2A, the platinum nanocrystals formed in the middle of Stage I adopt a nearly cubic morphology with eight arms/pods, described as quasi-octapods. Figure 2C shows the platinum nanocrystals collected in the middle of Stage II are larger and have thinner cross-sections towards the center of the {100} faces, indicative of etching. For this reason we describe these nanocrystals as etched-octapods. Figure 2E and G shows the platinum nanocrystals formed in Stage III and are described as porous platinum or porous nanocrystals.

Growth mechanism: Low concentration.
The peak area plot in Figure 1B shows the [Pt_NC] growth rate is much slower for the low concentration reaction, compared to the high concentration reaction. TEM indicates the formation of faceted and non-branched morphologies from an early growth stage. Slow nanocrystal growth and formation of faceted, non-branched morphologies are known to be characteristic of growth in a thermodynamically controlled regime. Nanocrystal growth in the low concentration reaction therefore occurs under thermodynamic control via a slow, nearly layer-by-layer monomer addition onto the crystallite faces to yield stable, faceted morphologies.

Growth mechanism: High concentration reaction.
Stage I: Formation of quasi-octapods. The XRD data show that the increase of [Pt_NC] during Stage I is rapid: there is fast nanocrystal growth. A fast growth rate has been associated with kinetically controlled growth, which often leads to a complex growth pattern and nanocrystals having branched quasi-octapods morphologies.

Stage II: Quasi-octapods → etched-octapods. There is little change in the total amount of platinum atoms in the nanocrystals ([Pt_NC]) in Stage II. TEM images show growth on the quasi-octapods takes place over the arms along the <111> directions, while etching has occurred on the {100} facets, leading to the formation of etched-octapods. By correlating the observations in XRD and TEM, the increase in [Pt_NC] due to nanocrystal growth is balanced by the loss of re-dissolved platinum atoms from the nanocrystals as a consequence of etching; hence the [Pt_NC] peak area remains constant over this growth stage.

Stage III: Etched-octapods → porous nanocrystals. The relatively rapid increase in [Pt_NC] in Stage III is similar to that observed in Stage I, again indicating kinetic control. However, the slight decrease in L suggests an unconventional type of growth. The cross-sections towards the center of the (100) faces of the porous nanocrystals at 240 minutes are noticeably thinner (Figure 2F) as compared with those of the etched-octapods (Figure 2D). A significant change in the particle morphology is gradually taking place, which is characterized by etching of much of the central core and the formation of more branches. These changes lead to an increase of porosity in the nanocrystals. As the nanocrystals become increasingly porous, the average size of the single crystalline domains, and hence L, decreases. Figure 3 illustrates the proposed growth mechanism from Stage I to III.

Figure 3. Schematic illustration of the growth mechanism for platinum nanocrystals in the high concentration reaction, through the first three growth stages (I - III). Platinum monomers first nucleate into cuboctahedral nuclei (A), and then grow into single-crystalline quasi-octapods (B). Growth of the arms of the quasi-octapods, coupled with selective etching on the edges and centers of facets leads to formation of etched-octapods (C). These processes continue and transform the nanocrystals to porous nanocrystals (D).