Ligand-Assisted Growth of Nanowires from Solution

Round 1

Reviewer 1 Report

Great work.

Its experimental evidence/application would be a great addition to the fundamental understanding for the scientific community working in this area.

Author Response

We are thankful to the Referee for favourable review.

Reviewer 2 Report

In this manuscript the authors have used statistical mechanics to analyse the conditions under which ligand-assisted growth of shape-anisotropic crystalline nanomaterials from solution can take place. The study presents the conditions for ligand-assisted growth of nanoplatelets and nanowires from isotropic or anisotropic seed nanocrystals of cuboid shape. In contrast to nanoplatelets ligand-assisted growth of nanowires requires certain anisotropy in the ligand-facet interaction energy.

 

Although the title is appealing, the article is not beyond “intuitive” even according to the authors’ own words. The model starts from a cuboid seed morphology, which is not a general requirement based on the available literature on anisotropic materials. The guideline should help for the synthesis of anisotropic metallic and ceramic nanostructures through LAG. I find the work a bit disconnected from the reality to guide the experimental attempts.

 

For example, metallic nanostructures, such as Au nanorods, can be grown from solution by using a specific surfactant (CTAB) where the ligand-facet interaction energy is anisotropic. Though the seed crytals do not have to be of cuboid type. Among the ceramic materials ZnO is the mostly studied one in the nanoorod architecture, which is easy to control due to the anisotropy in the surface energy of the crystal facets in z-direction.

 

Could the authors make connections to real life scenarios from available literature to exemplify the value of their findings. Even a bit further to lead the readers, could they provide some guidelines on how to obtain surface/facet energies of certain crystals, as well as their interaction energy with some well know ligands. Without a strong connection to experimental work I do not recommend the work for publication in its current form.

Author Response

We thank the Referee for useful comments.

 

Although the title is appealing, the article is not beyond “intuitive” even according to the authors’ own words. The model starts from a cuboid seed morphology, which is not a general requirement based on the available literature on anisotropic materials. The guideline should help for the synthesis of anisotropic metallic and ceramic nanostructures through LAG. I find the work a bit disconnected from the reality to guide the experimental attempts.

For example, metallic nanostructures, such as Au nanorods, can be grown from solution by using a specific surfactant (CTAB) where the ligand-facet interaction energy is anisotropic. Though the seed crytals do not have to be of cuboid type. Among the ceramic materials ZnO is the mostly studied one in the nanoorod architecture, which is easy to control due to the anisotropy in the surface energy of the crystal facets in z-direction.

 

In case of cuboid crystals it is easy to relate the growth process on a particular crystal facet with the extension of a certain crystal edge. In fact, the cuboid morphology characterized by three different ligand-facet interaction energies is quite generic. If all the energies are different it describes the crystals with orthorhombic symmetry. In case of equal ligand-facet interaction energies we have the case of cubic crystal symmetry, and using the LAG regime it is possible to generate a platelet if the seed shape is a platelet of appropriate thickness (see discussion of Fig. 1). If only two ligand-facet interaction energies are equal then we reproduce tetragonal crystal symmetry. The approach can easily be extended for hexagonal crystal symmetry, and even to cylindrical shape of the seed crystals. To qualitatively illustrate this recall that the key parameters in our approach are the ligand-facet and ligand-ligand interaction energies, which along with the ligand concentration in the solution determine the ligand critical nucleus size. The latter is specific for each crystal facet. In the case of uniaxial seed shape (tetragonal, hexagonal, cylindrical) the growth of nanowires along z-axis takes place when the side crystal facets are covered with the compact layer of ligands. This happens when the ligand critical nucleus size is small enough compared to the shortest edge of the side facet. At the same time the ligand critical nucleus size for the facet perpendicular to z-axis has to be large enough to prevent formation of the compact ligand layer on this facet. In most cases these two conditions are fulfilled when the ligand-facet interaction energies for the side facets (or side surface of a cylinder) are significantly higher than that for the facet (surface) perpendicular to z-axis (see Fig. 2), which is in line with the examples mentioned by the Referee. We have added a paragraph (marked in red) in Results and Discussion of the revised manuscript to emphasize the usefulness of the cuboid seed shape.

 

 

Could the authors make connections to real life scenarios from available literature to exemplify the value of their findings. Even a bit further to lead the readers, could they provide some guidelines on how to obtain surface/facet energies of certain crystals, as well as their interaction energy with some well know ligands. Without a strong connection to experimental work I do not recommend the work for publication in its current form.

 

It was not a purpose of our study to give a quantitative description of a real crystal ligand-assisted growth process from solution. This would be too ambitious, as it requires much more extensive calculations with a very limited possibility of drawing any systematic conclusions. A sensation of how expansive can be such calculations one can obtain from the example of calculating ligand-protein binding energies – see, for example, J. Chem. Inf. Model. 2017, 57, 2911-2937. Similarly limited are the possibilities of experimental determination the molecule-surface interaction energies – see www.pnas.org/cgi/doi/10.1073/pnas.1505874112. Therefore we do not think that any strong, e.g. quantitative connection to experiment is feasible at this stage.

 

With our study we aimed to improve our understanding of the parameters controlling the growth of highly anisotropic shapes of crystals obtained by ligand assisted growth from solutions. For the growth of nanowires we found, for example, that besides such ‘intuitive’ parameters as anisotropy of ligand-facet binding energies there is an additional parameter such as the ligand critical nucleus size relative to the crystal facet size. This parameter simply means that for even very favourable anisotropy of ligand-facet binding energies there are certain shapes of seed crystals which would not allow a nanowire growth but rather support a platelet (z<1 in Fig. 2), or cuboid (z>alpha) shapes of the growing crystal. 

Reviewer 3 Report

Dear author,

Congratulations for such a thorough study, analysing the ligand-assisted growth of nanomaterials from a mathematical/quantitative perspective. I appreciate all the effort you and your colleague have devoted to explaining this question to the reader, not only from a mathematical and physical perspective, but also carefully describing the interaction of ligands with facets and their influence over the nanomaterial growth very clearly, and even using graphical information to provide a better visual perception of the question, by drawing the boundary functions over certain circumstances, which is from my point of view very helpful.

However, let me share with you some minor suggestions that might improve your manuscript:

1. The introduction provides sufficient background and include many relevant references. However, references to classical use of ligands in nanomaterials synthesis, capping agents (not only as stabilizers but also intendedly exploited into preferential growth area) and preferential growth might also be included to gain perspective. Maybe a slight description within 1 or 2 lines, putting LAG into the right temporal context, and good references (first references or reviews) is enough, and provides the reader with a temporal perspective of this issue.
2. Some typos should be revised. For instance, the word “which” in conclusions (line 189) might be “whose”. Another issue comes from the expression “shape-anisotropic crystalline nanomaterials, or nanocrystal” (line 10, 30…). In my opinion, it seems more natural to say “anisotropic-shape material”. However, some other authors use “shape-anisotropic” expression within colloids and nanomaterials field, so this might be fine.

Taking into account these suggestions, my overall recommendation is accepting this manuscript after minor revision.

Comments for author File: Comments.docx

Author Response

We are grateful to the Referee for favourable review and useful comments. Following the Referee suggestion we have added a couple of lines to the Introduction (marked in red) and a reference to the review (Ref [29] in the revised manuscript) on the role of ligands in chemical synthesis.