Peer review of: Unzipping of black phosphorus to form zigzag-phosphorene nanobelts (2020)

Reviewers' comments: 

Reviewer #1 (Remarks to the Author): 

The manuscript describes a method for producing nanostructured black phosphorus via its 
electrochemical intercalation with BF4- ions followed by oxygen-driven unzipping. The authors 
claim their method is useful since it can relatively-easily produce nanoribbons of phosphorene (the 
main focus of the paper) and tuned to produce phosphorene nanodots and sheets. While there are 
interesting aspects of this work, as it is currently written there are a number of serious issues that 
must be addressed (or the claims modified) for it to be suitable for publication: 

Major comments: 

1. Most importantly, the authors claim of ‘phosphorene nanoribbons’ (in the title and throughout) 
is not fully justified for two reasons 1) lack of clear demonstration of ‘nano’ribbons and 2) lack of 
clear demonstration of phosphorene. These are not pedantic comments: for the interesting 
properties and effects to emerge in phosphorene nanoribbons (as shown by the theory/simulation 
papers) their width must be a) narrow and b) must be uniform. Though it’s hard to tell from the 
images, not many presented ribbons here could be considered to be much below 100 nm. 
Moreover many of the ribbons certainly have very non-uniform widths see many examples in ED2f 
and ED3 – or look more like fragments or scrolls (also how did you determine the width for the 
histograms if it varied?). 

Also the ribbons, which have had more detailed experiments on are of considerably larger width 
e.g. Fig1a, Fig 1j, Fig 4. Also, notably, the AFM clearly shows the ribbons to be covered with 
fragments – what are these? Are they all like this? There are many apparent tip effects (or double 
stripes) in the AFM images, please discuss. 

Also ‘phosphorene’ is a single layer of bP and there does not seem to be any demonstration of a 
single layer (i.e. truly phosphorene) ribbon produced by this method. Did you find any? On a 
connected note why does a ribbon with layer 2.4 nm correspond to 3 layers? if layer spacing is 0.5 
nm? 

The reason these comments are so important is that the authors claim their method is beneficial 
over that in the recent Nature paper by Watts etc. due to it being less complicated/time-
consuming. This may be true but the method seems to produce ribbons of far less quality. Note in 
Watts the authors a) produce ribbons of far narrower widths (e.g. below 10 nm) b) do 
measurement that demonstrate the widths are uniform c) clearly demonstrate single layer ribbons 
d) their AFM images of ribbons have no fragments on them. In general their ribbons look much 
higher quality. 

Please note I’m not saying the current paper is not useful – but the authors should either hone 
their method to produce much narrower, more uniform-width, higher quality ribbons, including 
single layers (i.e. to justify their claims and title) or discuss the difference between their ribbons 
and those in Watts explicitly in the paper. Note that electrochemical intercalation to produce larger 
width nanoribbons has already been reported before 
[https://www.nature.com/articles/nnano.2015.194.pdf] – this work should be discussed - however 
in this paper the authors more accurately describe their ribbons as ‘nanobelts’ seeing as the large 
widths imply the confinement and edge effects expected from ribbons would be absent. 

2. If the electrochemical process produces intercalated bP, please include some data to show this 
happens e.g. XRD showing a layer increase, or maybe Raman spectra showing an associated 
change? This aspect of the mechanism is not clearly demonstrated. Please accordingly fix this 
sentence, also modifying wrt point 1 “After applying current, the BF4- was speeded to insert bulk 
BP and expand the interlayer distance to produce phosphorene, meanwhile, oxygen molecules 
formed at the anode continuously unzipped the phosphorene into nanoribbons.” 

3. It’s interesting you can produce dots but more characterisation than Raman spectra is required 
please. bP has a very strong Raman response so any fragment of residual bP would be expected to 
dominate the spectra. Intuitively bP nanodots would be expected to show a large increase of the 
Raman peaks width, therefore further evidence is required to demonstrate that the dots shown in 
TEM are indeed compromised of crystalline phosphorene fragments. 

4. When discussing the mechanism you should discuss this paper 
https://iopscience.iop.org/article/10.1088/2053-1591/1/4/045041 seeing as their discussion of the 
mechanism is similar to yours and from 2014. 

5. Given your proposed mechanism why do you not find any narrow ribbons? 

6. Please provide some measurements of the air stability of the nanostructures that are produced 
as this determines their usefulness. 

Minor comments 

1. I don’t agree with the statement in the first line of the abstract or paper. While phosphorene is 
interesting there are many 2D materials that precede its discovery and are post-graphene (notably 
TMDs) and arguably more useful. 

2. Nanoribbons are not 1D they are 2D strips. 

3. In some places though the discussion is nice, it is overlong and distracts from the results e.g. 
I’m don’t think such a long description of the black phosphorus crystal structure is needed. 



Reviewer #2 (Remarks to the Author): 

The authors reported a top-down electrochemical strategy to produce one-dimensional zigzag 
phosphorene nanoribbons from the 3D bulk black phosphorus. The methods are reasonable and 
supported by theoretical DFT calculations. The results are interesting and the manuscript is well 
written. I recommend publishing after a minor revision after appropriately address the following. 
1. The authors reported DFT calculations results in Fig. 3 and extended Fig. 6&7. However, in the 
DFT methodology session, essential information is missing. For example, the simulation cell size 
used in the calculations. The cell size should be big enough to avoid interaction between O2 (or 
H2O) molecules between periodic images resulted from periodic boundary condition. 

2. The author mentioned the vacuum space is more than 10 Å. This needs to be more specific, 
especially for a slab model with adsorption molecules. 10 Å is at the very small end of vacuum 
range to be considered in a slab model with adsorption species, given that the layer-spacing in BP 
is 5.3 Å. The authors are recommend to discuss this and verify that the vacuum space in their 
calculations are sufficient. 

3. Not many literature work has been reported for 1D phosphorene nanoribbon synthesis. Ref #2 
as an early work reporting a top-down process in producing discrete phosphorene nanoribbons. 
The authors are encouraged to compare their methods with Ref. #2, such as advantages of each 
method. 




Reviewer #3 (Remarks to the Author): 

The paper deals with the fabrication of zigzag-phosphorene nanoribbons via electrochemical 
oxidation. Due to importance of 2D materials and processed, size-modulated material in particular, 
this article is of great interest to a broad readership in natural science, materials science and 
physics. Especially anisotropic phosphorene nanoribbons are expected to be of interest due to 
predicted interesting and record-braking properties. Whether these predictions are on any value 
can only be judged if such ribbons are available. In this article an elegant way is described to 
achieve such ribbons on a large scale. The explained mechanism is clever and straight forward. 
The characterization of the ribbons is performed using state-of-the-art techniques, like AFM, 
Raman Spectroscopy, TEM including electron diffraction which prove the existence, type and 
crystallinity of the products. Equation 1 is trivial and can be removed to my point of view. A very 
impressive and fundamental piece of this work are the quantum chemical calculations on al 
intermediates during oxidation and its interpretation. Especially the relation of the current density 
of the experiment and the final products are of immense value. This can directly be used for 
effective scale up and commercialization. Indeed, very impressive. The authors also showed first 
applications of their ribbons in FET devices which will gain interests in different communities. All 
experiments are done with high accuracy and data are interpreted with care including suitable 
statistics. To my point of view researchers can reproduce the given experiments based on the 
statements and descriptions given in the manuscript. 

Upon reading this excellent manuscript only a few questions arose. Is the preparation mechanism 
of bulk black phosphorus critical or product-directing for the formation of phosphorene ribbons? 
There are several synthesis procedures like high pressure, complex annealing and tempering and a 
mineralizator based way (Sn/SnI4 as precursor) to synthesize it. Due to the variations and the 
synthesis history during formation the original procedure might be of interest. I recommend to 
discuss the different procedures briefly in the beginning and to check the way how the black 
phosphorus used in this manuscript was synthesized. I believe that a different stress-strain and 
impurity level can directly influence the oxidation behavior in the final product. 
One major point to be readjusted is the name of the products. Due to the fact that at the edges of 
each ribbon P-O bonds of different kinds (P=O; P-OH) are formed there is a certain oxygen content 
present in each ribbon. The thinner and smaller the ribbons are the higher the ratio oxygen vs. 
phosphorus will be. I suggest to call the resulting species therefore phosphorene oxide 
nanoribbons instead of phosphorene nanoribbons in the entire manuscript. 
In sum, I recommend to publish this article in Nature Commun. in its present form, maybe 
addressing the questions I asked in the last part of my comments.