Peer review of: Imaging the kinetics of anisotropic dissolution of bimetallic core–shell nanocubes using graphene (2020)

Reviewers' comments: 

Reviewer #1 (Remarks to the Author): 

In this manuscript, the authors investigated the reaction kinetics during the dissolution of well-defined 
bimetallic core-shell nanocubes using real-time graphene-liquid-cell (GLC) TEM imaging and Monte Carlo 
simulations. They discovered the anisotropy reaction environment within GLCs and the slowdown in the 
Au shell etching by conducting control experiment and careful data analysis. Overall, this appears to an 
interesting work in which authors used advanced electron microscopy and simulations are used, and 
manuscript is well organized and written. This work can be accepted after addressing the following 
questions: 

1. How did the authors control the galvanic replacement between palladium and gold in such 
controllable way since the as-synthesized core-shell nanocubes show very uniform and distinct 
boundary? 

2. They author claimed the kinetics of the etching can be referred to the etching behavior, how did the 
author get this conclusion? Is there any derivation process should be included in this manuscript? 

3.How did the authors measure the volume of gold shell after the palladium shell was exposed, since the 
observed islands of gold were not uniform? 

4.In Figure 5, the author claimed that the slowdown effect came from the electron transfer from 
palladium to gold. However, the author also claimed that the electrons can be transferred from gold 
region to palladium region. 

5. For the etching of Cu@Au system, it should have the slowdown effect as well during the etching 
process. Did the authors observe the similar phenomenon? 

6. The authors claimed that the etching of Cu@Au system could be a method for measuring shell 
thickness and uniformity of particles. Can this in situ method be easily conducted with high 
reproducibility? 

7. It seems that the core is not really visible in the time-dependent TEM images, how the authors make 
sure it is indeed co-shell particle? To me, some of the 3D models given are not really comparable to 2D 
TEM images. For example, see 5a, 6(b,c), 6d. Is it possible look at the final particle obtained after etching 
by STEM? 

8. Can authors provide any statistics on the number of particles they looked at for the final conclusions? 

9. Is there any comparison between in situ TEM and in solution phase synthesis in a flask? Can authors 
translate this understanding to prepare a certain shape in large scale? I mean how this study lead to 
something useful for synthesis of shapes shown in 3D models in a flask? 

Reviewer #2 (Remarks to the Author): 

This is a joint experimental/simulation paper about the dissolution of core-shell nanocrystals - the 
shapes that occur and the mechanisms by which dissolution occurs. I do think this is interesting. 
However, I feel uncertain this work is sufficient for a high-impact journal for several reasons. 

1. I feel the manuscript is written in a misleading way in several places. First, when I read the abstract, I 
got the impression that most of the theoretical work was done by molecular dynamics. However, upon 
reading the paper I found that most of the theory was done with Monte Carlo simulations. At the 
bottom of p. 9 (lines 294-296) they say "Our observations revealed diverse dissolution reaction 
pathways that provide access to bimetallic multipods, segmental nanorods, nanoboxes and nanocages 
via controlled etching of core-shell nanocubes." However, when I searched through the paper, this 
sentence was the only place (other than the references) where "multipod", and "nanocage" were found. 
The paper should be cleaned up to more accurately reflect the work that was done. 

2. A significant conclusion from this work is that the graphene cell used in this work significantly 
influences the etching kinetics. While I feel it is valuable for others to know this, it also limits the 
applicability of the results to the anisotropic system studied here. It would not apply to general 
dissolution outside of the graphene cell. Because of this limited scope, I feel the work is not broadly 
significant and it should appear in a more specialized journal. 

3. In lines 454-456, the authors state that τ is a correction factor to restore detailed balance. I was trying 
to imagine why their simulations might not satisfy detailed balance and I guess this is because of the 
way they obtain ε - this should be clarified. It is also unclear to me how just one constant factor can 
address every instance of a transition not satisfying detailed balance. This definitely needs to be 
clarified. 

4. I feel their whole procedure for obtaining ε(η) is suspect. Why don't they enumerate the "before" and 
"after" local configurations as a crystal dissolves and get the change in energy that way? Better than this 
they should kinetic Monte Carlo based on DFT calculations. I feel that their model is very qualitative, 
parameters are fit to the experiments, and it could match any experimental observation they choose. 



Reviewer #3 (Remarks to the Author): 

This paper by Lei Chen et al reports findings on the chemical etching kinetics of bimetallic core-shell 
nanoparticles uncovered using in-situ liquid cell TEM. In addition to previously reported behavior from 
similar studies on monometallic particles, the authors experimentally observe a marked slowing in 
dissolution rate for Pd@Au nanocubes at the point both metals are exposed, both for the particle 
overall and for the less-reactive Au metal. They further demonstrate the significance of metal reactivity 
on dissolution kinetics by rapidly etching the Cu core of Cu@Au nanocubes. The authors also find that 
sides of the particle likely in contact or in close proximity to the enveloping graphene sheets of the liquid 
cell are etched much more slowly due to limited exposure to the oxidative environment. Lastly, they 
support the majority of their experimental findings with agreeing Monte Carlo simulation results. 

I believe the paper is appropriate for publication, with a few potential revisions. These findings are 
significant in building on the rapidly expanding field of in-situ liquid cell TEM as well as multimetallic 
nanoparticle chemistry by combining the two. Namely, they extend previous liquid cell experimental 
frameworks to a more complex material system involving multiple reactive metal species. While the 
findings from their model nanocube system are interesting in their own right, I believe the main 
contribution is the demonstration that our current liquid cell TEM techniques can capture the chemistry 
of more catalytically-relevant (for instance) materials with sufficient temporal and spatial resolution. 

In my view of the recorded in-situ videos and subsequent analysis, with results corroborated by 
simulations, I see no glaring issue with the authors’ conclusions and agree with their major findings. That 
being said, I suggest the following critical feedback be taken into consideration for the final manuscript: 

1. Throughout the “Intrinsic and extrinsic factors responsible for anisotropic dissolution” section, when 
discussing the orientation of the Pd@Au nanocube relative to the confining graphene windows, the 
language is unclear regarding what described directions refer to what face of the original cube. In the 
first paragraph, “top and bottom” are both defined with respect to Fig. 3a after rotation as well as with 
respect to the initial cube orientation orthogonal to the electron beam. In subsequent paragraphs, 
which “top and bottom” facets are being referred to is not explicitly clear, especially to readers less 
familiar with the sealing behavior of graphene liquid cells. I suggest defining an e-beam axis and defining 
orientations relative to that. 

2. I think the absence of anisotropic etching in many other GLC-confined experiments is worth exploring 
further past the putative statements given—it is strange that monometallic etching in both this and 
previous papers had not observed that behavior, and I don’t believe it is fully explained by the smaller 
particle size (other than chance with small sample size) since GLC pockets are subject to large variations 
in thickness and in the reference cited are likely to “wrap” even smaller particles than the ones studied. 
Rather, the later remark that there were simply a range of liquid thicknesses present and some 
happened to wrap on particles (more likely the larger ones) could be verified by measurements of the 
liquid cell thickness for both cases where the anisotropic etching is and isn’t observed. Such a 
measurement before and after etching could also verify whether or not the pocket thickness is defined 
by the particle size in certain size regimes. 

3. It was not immediately obvious to me the purpose of investigating the etching of cubes with thinner 
Au shells (as opposed to simply gathering more statistics with the same shell thickness). What was the 
expected effect of a larger Pd:Au ratio? 

4. Please include experimental details of the AFM measurements in the methods. 

5. I would include a STEM-EDX map of the Cu@Au nanocages similar to that of Fig. 1 to verify the 
elemental distribution (placed in the SI). 

6. The Cu@Au nanocube experiments seems overall disconnected from the larger picture. The abstract 
implies they experience similar behavior to Pd@Au nanocubes, while the discussion/conclusion section 
disregards them entirely. In the earlier kinetics analysis, the lack of a linear V vs. t relationship already 
ruled out rate limiting from the oxidative species—was this section just to verify this? Were there similar 
observations regarding galvanic corrosion? This section could be motivated better to be less of an 
afterthought, especially where it lies near the conclusion of the paper. 

7. In Fig. 4, it may help to mark the expected nanocube locations in 4d (or S6), since the bright spots in 
the AFM don’t match up exactly with the cubes are located in the correlated SEM image. (Also, is this 
due to particle movement? Even though in a month the cubes are barely seen to move.)