Peer review of: Atomic-scale phase separation induced clustering of solute atoms (2020)

1Reviewers' comments: 

Reviewer #1 (Remarks to the Author): 

Comments: 

In this manuscript, the authors reported an atomic-scale self-filtration process that results in the 
dealloying of a solid solution. The use of dynamic, atomic-scale electron microscopy observations and 
theoretical modeling to delineate the atomic-scale mechanisms is impressive. The texts and figures 
were well organized and logical, so I'd recommend to accept this manuscript after major revision. 

1.During the high-temperature reaction, the Au clusters are amorphous-crystallization oscillations to 
adjust the crystal orientation due to the lattice mismatch of Cu, so whether the underlying Cu is also 
affected at the same time, resulting in a certain amorphization or a certain structural disorder, the 
final result is a two-phase dynamic adjustment? 

2.If getting an ideal nanoporous structure, the prolonged dealloying duration is always required. Is the 
situation (dissolution, nucleation, crystallization, expansion, stacking, etc) within the initial surface 
dealloying period of Cu-Au similar to the stage of long-term dealloying? i.e., after 30 min or 60 min. 

3.Is the initial crystalinity transformation during the dealloying to cluster universal or not? Because the 
dealloying-preferred alloy system is diverse and the dealloying phenomenon is always dependent of 
several factors such as alloy kind, composition ratio, phase content (single or multiple), corrosive 
potential gap between involved constituents, etc. For example, the dealloying-preferred alloy system 
generally contains continuous solid solution (Ag-Au) and fixed intermetallic compounds (CuAu, Cu3Au, 
Au3Cu, etc), which belongs to totally different counterpart and different re-crystalization rule during 
the selective leaching. 

4.Why the surface step in Fig. S4 b isn’t marked? 

5.Minor mistakes: 

1) Line 179, “the angle is marked by black and green lines in the inset diffractogram in Fig. 3e”, the 
lines are missing. 

2) The captions of Supplemental video 4 and 5 are incorrect, 4 is amorphous-to-crystalline while 5 is 
crystalline-to-amorphous transition. 

3) Please check carefully about the format of references. 



Reviewer #2 (Remarks to the Author): 

In “Atomic-scale self-dealloying and clustering of solute atoms” the authors report on the surface 
reorganization of Cu-10at.%Au nanofoils annealed at 600°C in hydrogen. The kinetics of this process 
is studied using in-situ TEM and is corroborated by a combination of first-principle, kinetic Monte Carlo 
and molecular dynamics simulations. 

Thermodynamics of small systems are a fascinating field as it often breaks with classical 
thermodynamics. Known phenomena at the nanoscale include mixing of macroscopically immiscible 
systems or interfacial sharpening at the interface of two fully miscible phases (Science 306, 1913 
(2004), Appl. Phys. Let. 99, 181902 (2011)). 

Here, the authors use Cu-Auto study the possibility of phase separation in a miscible system. 

Segregation, surface alloys and order-disorder transition in Cu-Au are not new and have been 
discussed extensively in the literature, see e.g.: 

Phys. Rev. Lett. 51,1, 43-46 (1983) 
Phys. Rev. B 45, 3703 (1992) 
Phys. Rev. B 57, 6427 (1998) 
Nanoscale, 2017, 9, 9267 
Modelling Simul. Mater. Sci. Eng. 13 657 (2005) 

Quite interestingly the authors have already published a very similar work on the same Cu-10%Au 
system, see: 

Segregation induced order-disorder transition in Cu(Au) surface alloys (Acta Mater.154, p. 220-227 
(2018)). 

The authors should try to put their new work into context to what is known and stress clearly the 
difference between this and their previous work. 

The manuscript needs significant improvement with respect to its clarity and description of the 
different processes at play. The authors should provide the reader a clear and comprehensive picture 
about the chemistry and kinetics in this system and compare with literature (e.g. Science 306, 1913 
(2004))). I recommend major revisions including additional experiments before the manuscript is 
accepted. 

Below I highlight some items I recommend revising: 

1.) Chemical Analysis 
The authors claim that the nanoparticles that form on the surface are made of gold. This claim clearly 
lacks experimental support. To distinguish between the two materials Au and Cu-10at.% the authors 
rely on the measurement of the local lattice spacing, which is not sufficient. 
To substantiate their claim, the authors should perform detailed EDX mappings and line scans, aiming 
to visualize the chemical differences between the surface and the Cu-10at.%Au phase, see e.g. 
Nanoscale,2017, 9, 9267. Furthermore, the authors should provide an EDX spectrum of the as-
deposited sample to confirm the initial composition. 

2.) Which effect has the presence of the void on the observed cluster formation/”phase separation” 
and its kinetics? It is know that edges act as sources for capillary instabilities in thin films leading to 
enhanced surface diffusion in the vicinity of the edge, see e.g. (E. Jiran and C. V. Thompson, Capillary 
instabilities in thin films, J. Electron. Mater., 1990, 19, 1153–1160). The authors should carefully 
discuss which processes are responsible for the cluster formation. 

3.) Hydrogen 

Here, the samples have been annealed in hydrogen at elevated temperatures. It is know that the 
presence of hydrogen and solution in metals facilitates dislocation formation and increases their 
mobility (see e.g. Hydrogen in Metals in Physical Metallurgy (Elsevier)). This can induce grain rotation 
during recrystallization. The authors should discuss the role of hydrogen on the kinetics and observed 
recrystallization phenomena. Similar to their paper in Acta Mater., the authors should also include the 
presence of hydrogen in their first-principle simulations. 

4.) Sample preparation 

The procedure of TEM sample preparation and imaging is not clear. The authors should add a 
schematic to the supplementary illustrating the difference steps of sample preparation including the 
transfer on the heating chip. 

Minor issues: 
1. Please state the manufacturer of the heating chip that is used. 
2. The scale bar in all diffractograms is missing. 
3. Please report the acceleration voltage. 
4. Please omit the word “self-filtration” and use established wording 


Reviewer #3 (Remarks to the Author): 

Taking the results and simulations at face value, this article certainly contains new, surprising (in 
places) and provocative information. As such, it satisfies the requirements to be published in a high-
impact journal. Essentially, the authors find that the surface of a homogeneous metallic alloy can 
undergo a kind of vertical phase separation, wherein 3D clusters, and eventually crystallites, of Au 
emerge from the homogeneous bulk Cu-10%Au alloy. The maximum height is a surprising number of 
atom layers (>20), and goes well beyond ordinary surface segregation, or anything that is seen in 
STM studies that I am familiar with. All that is required to trigger this "phase transformation" is simple 
heating at 600 deg C in a low pressure of hydrogen. 
There are a few topics that I would like to have seen discussed in more detail - 
Is this phenomenon unique to thin alloy films of about the given thickness dimension (50 nm thick), or 
would it happen even on a bulk crystal surface? 
Can the absence of a beam effect be quantified beyond "likely" (line 228)? 
Is it possible that hydrogen plays some role? 
How low is the oxygen content of the film (referring to bulk, not surface oxide)? Such oxygen could 
react with incoming hydrogen. 
Although the exposure temperature is well above any order-disorder transition, is it possible that the 
type of pairwise atomic interaction implied by that ordering tendency plays some role in the 
phenomenon? 
I am not really in agreement with calling this "dealloying", as that is bound to cause confusion, and 
the physics of this are quite different from what is normally understood by dealloying.