Structural Disorder of CuO, ZnO, and CuO/ZnO Nanowires and Their Effect on Thermal Conductivity

Round 1

Reviewer 1 Report

Upon reviewing your manuscript intitled: Structural disorder of CuO, ZnO, and CuO/ZnO nanowires and their effect on the thermal conductivity. I find your work interesting, but I do not believe it can be published in its current form. Therefore, some revisions must be done so that it may be published in this journal. I have the following comments and recommendations.

-The introduction of relevant background and research progress was not comprehensive enough.

- The lattice parameters can be slightly altered depending on the synthesis method, pH, temperature, etc. How and why were the parameters shown in Table 1 selected?

- In the case of lattice deformation, dislocation density and Zn-O and Cu-O distance. Are these parameters not considered in the study?

- What is the feasibility of a similar study, but from an experimental point of view? Are there any experimental results that I can corroborate this study by simulation?

- Why do the authors consider this study important? from the point of view of applications of these materials, how can this study help?

 

 

Author Response

We appreciate the referees' comments and criticisms. We included a detailed answer to each of the points raised by the referees. Language and writing style allows making the manuscript easier for the readers. It highlighted in green the major corrections and included them in the revised version of the manuscript. With all these changes, our paper is now suitable for publication in Crystals.

 

Best regards,

 

Upon reviewing your manuscript intitled: Structural disorder of CuO, ZnO, and CuO/ZnO nanowires and their effect on the thermal conductivity. I find your work interesting, but I do not believe it can be published in its current form. Therefore, some revisions must be done so that it may be published in this journal. I have the following comments and recommendations.

 

1. The introduction of relevant background and research progress was not comprehensive enough.

 

Author's reply: Thank you for the comment. We agree with the reviewer that the relevant background and research progress was not comprehensive enough in the introduction section. To clarify this point, we completed the introduction section, line 44:

 

“In nanostructures, the effects of structural deviations on physicochemical properties remain a discrepancy due to experimental difficulties.”

 

Line 50:

“CuO is a semiconductor with a narrow indirect bandgap (1.2 eV in bulk), monoclinic structure (C2/c symmetry), and four formulas per unit cell [4]. In contrast, ZnO is a semiconductor with a wide direct bandgap (3.37 eV in bulk), hexagonal structure (P63mc symmetry), and two formulas per unit cell [5]. Cupric oxide and zinc oxide conductivities are associated with native defects [4,6]. The usual structural defects observed in cupric oxide and zinc oxide nanowires are twin planes, stacking faults, and inversion domain walls [7,8]. In general, twin planes result from crystals joining that grew on adjacent facets of performed grains [9]. Stacking faults appear by the changes in the stacking sequence of close-packed planes [8]. The other structural defects become increasingly important as the surface contribution to the total energy increases due to the size decreases [8].”

 

Line 61:

“Experimentally, nanowires growing is associated with the differences in surface chemistry between two types of faces: polar faces and non-polar faces. In the first one, the surface dipoles are thermodynamically less stable than in the second one [10]. In the simulation case, the atoms usually suffer from rearrangement to minimize their surface energy and tend to grow stably. Structural defects show markable importance, for ex-ample:

• Cupric oxide nanowires growing by thermal oxidation exhibits compres-sive stress. It is the driving force in the growth mechanism [9].
• Structural defects play a fundamental role in improving the CO2 electro-reduction to ethylene [11].

Thermal conductivity has been widely investigated for zinc oxide nanowires, in theoretical studies [3], in simulations [12], and experimentally [13]. The focus in this case was:

1. Determine the effect of confinement in the radial direction on thermal conductivity [3].
2. Determine the surface effects on the thermal conductivity [14].
3. Compare the thermal conductivity among different nanostructures as gal-lium nitride [12].
4. Determine the size dependence of the nanowires with thermal conductiv-ity [15].
5. Determine the effects of doping the nanowires and their relationship with thermal conductivity.
6. The thermal conductivity of zinc oxide nanowires embedded on polymeric matrices [13,16].

Most of the papers reported in the literature about cupric oxide nanowires focus on enhanced the thermal conductivity in nanofluids [4]. This property has been taken advantage of for applications in wastewater treatments [17].”

 

Line 98:

ZnO/CuO core-shell nanowires heterojunctions have been grown and vertically aligned with a good p-n junction with promising performance as a photodetector [27].

 

1. The lattice parameters can be slightly altered depending on the synthesis method, pH, temperature, etc. How and why were the parameters shown in Table 1 selected?

 

Author's reply: Thank you for the observations. We agree with the reviewer that the synthesis method influences the lattice parameters (pH, temperature, etc.). The initial parameters shown in Table 1, were selected as the ideal crystalline structure reported in the literature. Once the simulation is performed, the crystal structure was deformed modifying the distance between anions and cations in the lattice.

 

1. In the case of lattice deformation, dislocation density and Zn-O and Cu-O distance. Are these parameters not considered in the study?

 

Author's reply: Thank you for the comment. As the reviewer pointed out, we do not consider lattice deformations and dislocation density. However, it was considered Zn-Zn, Cu-Cu, Zn-O, Cu-O, and O-O distances. Moreover, to improve the manuscript quality, we included a discussion of lattice deformation in line 153:

 

“Structural defects present in cupric oxide are associated with the formation of the grains in the nanowire and suggest a polycrystalline structure and agree with experimental reports [36].”

 

Line 220:

“Experimentally wurtzite nanostructures have demonstrated a favorable growth direction along the c-axis [37]. Calculations of surface energies based on bond density reveal that the  planes in the wurtzite structure are the energetically favorable surface in the 1D nanostructure system [38]. The wurtzite space group is described as several alternating planes composed of tetrahedrally coordinated cations and anions, stacking alternatively along the c-axis [39]. The oppositely charged ions produce positively charged  and negatively charged  polar surfaces, resulting in a dipole moment and sometimes in spontaneous polarization along the c-axis as well as a divergence in surface energy. Stacking faults can be easily formed in the I1 type of basal plane  [8], as in this case.”

 

1. What is the feasibility of a similar study, but from an experimental point of view? Are there any experimental results that I can corroborate this study by simulation?

 

Author's reply: Thank you for the comment. We agree with the reviewer that important to find experimental results that corroborate our simulation study. For that, we included in the manuscript some works that reported similar results to those included here, line 55:

 

“The usual structural defects observed in cupric oxide and zinc oxide nanowires are twin planes, stacking faults, and inversion domain walls [7,8]. In general, twin planes result from crystals joining that grew on adjacent facets of performed grains [9]. Stacking faults appear by the changes in the stacking sequence of close-packed planes [8]. The other structural defects become increasingly important as the surface contribution to the total energy increases due to the size decreases [8].”

 

Line 61:

“Experimentally, nanowires growing is associated with the differences in surface chemistry between two types of faces: polar faces and non-polar faces. In the first one, the surface dipoles are thermodynamically less stable than in the second one [10]. In the simulation case, the atoms usually suffer from rearrangement to minimize their surface energy and tend to grow stably. Structural defects show markable importance, for example:

• Cupric oxide nanowires growing by thermal oxidation exhibits compressive stress. It is the driving force in the growth mechanism [9].
• Structural defects play a fundamental role in improving the CO₂ electroreduction to ethylene [11].”

 

Line 174:

“Structural defects present in cupric oxide are associated with the formation of the grains in the nanowire and suggest a polycrystalline structure and agree with experimental reports [36].”

 

Line 225:

“The oppositely charged ions produce positively charged  and negatively charged  polar surfaces, resulting in a dipole moment and sometimes in spontaneous polarization along the c-axis as well as a divergence in surface energy. Stacking faults can be easily formed in the I1 type of basal plane  [8], as in this case.”

 

Line 269:

“The bulk thermal conductivity at room temperature for cupric oxide and zinc oxide reported in the literature are 76.5 W/m.K [4] and 46 W/m.K [42], respectively. Size reduction of CuO and ZnO at the nanometric scale produces different changes in thermal conductivity. In the first case, a particle size reduction increases the thermal conductivity, compared with the bulk state, because of the long mean-free path of phonon vibration [43]. This observation agrees with the results obtained by our simulations for CuO nanowires at room temperature (135 W/m.K). In contrast, in the second one, a particle size reduction decreases the thermal conductivity, compared with the bulk state [12]. The ZnO nanowire's thermal conductivity decreases with the diameter of the nanowires [15]. This observation agrees with the results obtained by our simulations for ZnO nanowires at room temperature and those reported in the literature for zinc oxide (Table 2). The thermal conductivity of metal oxide particles decreases as the temperature increases because of the phonon-phonon scattering [44]. However, the grain boundaries will scatter phonons, and thus the size of the crystalline domain acts as a limiting length for phonons [12]. The defects scattering and the phonon-phonon scattering affects the thermal conductivity, first increasing to a maximum and then decreasing (Figure 5) [45]. The temperature of maximum thermal conductivity is sensitive to the nanowire diameter (Table 2).”

 

Table 2. Summary of the thermal conductivity of ZnO nanowires reported in the literature at room temperature with their diameter (d), length (L) and temperature maximum (T_max).

d (nm)	L (µm)	Thermal conductivity (W/m.K)	Tmax	Reference
25 0.1 0.9 40 150 160 215 250 250 480	0.4 0.0009 - 35 - 5 69 69 69 40	6 10 3 12 12 12 17 15 22 32	250 - 350 - 180 125 - 100 - -	This work [46] [45] [13] [47] [3] [13] [16] [13] [13]

 

 

 

1. Why do the authors consider this study important? from the point of view of applications of these materials, how can this study help?

 

Author's reply: Thank you for the observation. We agree with the reviewer that the importance of this study contributes to the application's point of view. In this case, we demonstrated that structural defects, appear in nanowires grown on the free substrate. It is not related to the lattice mismatch. These defects produce a polycrystalline structure of the nanowires with a formation of grains to reduce the system energy. These grains act as scattering points of the phonons allowing for tunning the thermal conductivity with the temperature. As mentioned in the introduction these characteristics are fundamental for thermoelectric applications, line 48:

 

“Among all one-dimensional nanostructures, cupric oxide (CuO) and zinc oxide (ZnO) are interesting due to their semiconductor properties (type p and n, respectively) and high demand for thermoelectric applications [3].”

 

 

 

Line 85:

“Most of the papers reported in the literature about cupric oxide nanowires focus on enhanced thermal conductivity in nanofluids [4]. This property has been taken advantage of for applications in wastewater treatments [17].”

 

Line 98:

“ZnO/CuO core-shell nanowires heterojunctions have been grown and vertically aligned with a good p-n junction with promising performance as a photodetector [27].”

Reviewer 2 Report

The authors studied the disorder effect on thermal conductivities in ZnO and CuO nanowires by molecular dynamics. Nanowires are attracting great interest in various scientific fields. In particular, in thermoelectrics and electronics, understanding the thermal transport is very important. This hot topic will intrigue a lot of researchers. However, there are some concerns about introduction and discussions. If the authors revised them appropriately, this study would meet the criteria for the publication in Crystals.

 

Comment list

Comment 1: There are a lot of research about thermal conductivity of ZnO and CuO nanowires. To show the motivation of simulating thermal transport, the authors should cite more research: e.g. the thermal conductivity of embedded ZnO nanowire structure was reported in thermoelectric research in 2018. The thermal conductivity of Ga-doped ZnO nanowire was reported in 2012.

 

 

Comment 2: I guess that thermal conductivity depends on the diameter of nanowire. In this study, the author performed the calculation using the model with the diameter of 25 nm. It would be better to show how temperature dependence of thermal conductivity varies when the diameter is varied.

 

Comment 3: The authors mentioned that the calculated thermal conductivity agreed with the value obtained experimentally. However, in the present version, the nanowire structural parameters such as diameter, etc. in the preceding study are missing. Please describe them. I am concerned that the simulation model is closed to the nanowire structure in the experimental study. 

Author Response

We appreciate the referees' comments and criticisms. We included a detailed answer to each of the points raised by the referees. Language and writing style allows making the manuscript easier for the readers. It highlighted in green the major corrections and included them in the revised version of the manuscript. With all these changes, our paper is now suitable for publication in Crystals.

 Best regards,

 

The authors studied the disorder effect on thermal conductivities in ZnO and CuO nanowires by molecular dynamics. Nanowires are attracting great interest in various scientific fields. In particular, in thermoelectrics and electronics, understanding the thermal transport is very important. This hot topic will intrigue a lot of researchers. However, there are some concerns about introduction and discussions. If the authors revised them appropriately, this study would meet the criteria for the publication in Crystals.

 

1. Comment 1: There are a lot of research about thermal conductivity of ZnO and CuO nanowires. To show the motivation of simulating thermal transport, the authors should cite more research: e.g. the thermal conductivity of embedded ZnO nanowire structure was reported in thermoelectric research in 2018. The thermal conductivity of Ga-doped ZnO nanowire was reported in 2012.

 

 Author's reply: Author's reply: Thank you for the observations. We agree with the reviewer that the necessity to cite more research for showing the motivation for simulating thermal transport. To clarify this point, we include this in the introduction section, line 71:

 “            Thermal conductivity has been widely investigated for zinc oxide nanowires, in theoretical studies [3], in simulations [12], and experimentally [13]. The focus in this case was:

1. Determine the effect of confinement in the radial direction on thermal conductivity [3].
2. Determine the surface effects on the thermal conductivity [14].
3. Compare the thermal conductivity among different nanostructures as gallium nitride [12].
4. Determine the size dependence of the nanowires with thermal conductivity [15].
5. Determine the effects of doping the nanowires and their relationship with thermal conductivity.
6. The thermal conductivity of zinc oxide nanowires embedded on polymeric matrices [13,16].

Most of the papers reported in the literature about cupric oxide nanowires focus on enhanced thermal conductivity in nanofluids [4]. This property has been taken advantage of for applications in wastewater treatments [17].“

 

2. Comment 2: I guess that thermal conductivity depends on the diameter of nanowire. In this study, the author performed the calculation using the model with the diameter of 25 nm. It would be better to show how temperature dependence of thermal conductivity varies when the diameter is varied.

 

Author's reply: Thank you for the observations. We agree with the reviewer that the thermal conductivity depends on the diameter of the nanowire. The revised version of the manuscript includes a study performed in ZnO, line 276:

 

“The ZnO nanowire's thermal conductivity decreases with the diameter of the nanowires [15].”

 

The dependence of thermal conductivity as a function of the diameter of CuO and ZnO nanowires is out of the scope of this paper. However, now we are working on these simulations and expect to publish them soon.

 

3. Comment 3: The authors mentioned that the calculated thermal conductivity agreed with the value obtained experimentally. However, in the present version, the nanowire structural parameters such as diameter, etc. in the preceding study are missing. Please describe them. I am concerned that the simulation model is closed to the nanowire structure in the experimental study.

 

Author's reply: Thank you for the comments. We agree with the reviewer about the necessity to include details of the structural parameters of the systems for which the thermal conductivity agree with experimental results. To clarify this point, it included these details in the revised version of the manuscript, line 269:

 

“The bulk thermal conductivity at room temperature for cupric oxide and zinc oxide reported in the literature are 76.5 W/m.K [4] and 46 W/m.K [42], respectively. Size re-duction of CuO and ZnO at the nanometric scale produces different changes in thermal conductivity. In the first case, a particle size reduction increases the thermal conductiv-ity, compared with the bulk state, because of the long mean-free path of phonon vibra-tion [43]. This observation agrees with the results obtained by our simulations for CuO nanowires at room temperature (135 W/m.K). In contrast, in the second one, a particle size reduction decreases the thermal conductivity, compared with the bulk state [12]. The ZnO nanowire's thermal conductivity decreases with the diameter of the nanowires [15]. This observation agrees with the results obtained by our simulations for ZnO nanowires at room temperature and those reported in the literature for zinc oxide (Table 2). The thermal conductivity of metal oxide particles decreases as the temperature increases because of the phonon-phonon scattering [44]. However, the grain boundaries will scatter phonons, and thus the size of the crystalline domain acts as a limiting length for phonons [12]. The defects scattering and the phonon-phonon scattering affects the thermal conductivity, first increasing to a maximum and then decreasing (Figure 5) [45]. The temperature of maximum thermal conductivity is sensitive to the nanowire diam-eter (Table 2).”

 

 

 

 

 

 

 

Table 2. Summary of the thermal conductivity of ZnO nanowires reported in the literature at room temperature with their diameter (d), length (L) and temperature maximum (T_max).

d (nm)	L (µm)	Thermal conductivity (W/m.K)	Tmax	Reference
25 0.1 0.9 40 150 160 215 250 250 480	0.4 0.0009 - 35 - 5 69 69 69 40	6 10 3 12 12 12 17 15 22 32	250 - 350 - 180 125 - 100 - -	This work [46] [45] [13] [47] [3] [13] [16] [13] [13]

 

About the relation of the nanowire structure in our simulation with the experimental studies, we clarify the point in the revised version of the manuscript, line 54:

 

“Cupric oxide and zinc oxide conductivities are associated with native defects [4,6]. The usual structural defects observed in cupric oxide and zinc oxide nanowires are twin planes, stacking faults, and inversion domain walls [7,8]. In general, twin planes result from crystals joining that grew on adjacent facets of performed grains [9]. Stacking faults appear by the changes in the stacking sequence of close-packed planes [8]. The other structural defects become increasingly important as the surface contribution to the total energy increases due to the size decreases [8].

Experimentally, nanowires growing is associated with the differences in surface chemistry between two types of faces: polar faces and non-polar faces. In the first one, the surface dipoles are thermodynamically less stable than in the second one [10]. In the simulation case, the atoms usually suffer from rearrangement to minimize their surface energy and tend to grow stably. Structural defects show markable importance, for example:

• Cupric oxide nanowires growing by thermal oxidation exhibits compressive stress. It is the driving force in the growth mechanism [9].
• Structural defects play a fundamental role in improving the CO2 electroreduction to ethylene [11].”

 

Line 174:

“Structural defects present in cupric oxide are associated with the formation of the grains in the nanowire and suggest a polycrystalline structure and agree with experimental reports [36].”

 

Line 220:

“Experimentally wurtzite nanostructures have demonstrated a favorable growth direction along the c-axis [37]. Calculations of surface energies based on bond density reveal that the {011 ̅0} planes in the wurtzite structure are the energetically favorable surface in the 1D nanostructure system [38]. The wurtzite space group is described as several alternating planes composed of tetrahedrally coordinated cations and anions, stacking alternatively along the c-axis [39]. The oppositely charged ions produce positively charged (001) and negatively charged (001 ̅) polar surfaces, resulting in a dipole moment and sometimes in spontaneous polarization along the c-axis as well as a divergence in surface energy. Stacking faults can be easily formed in the I1 type of basal plane [0001] [8], as in this case.”

Round 2

Reviewer 1 Report

This version may be considered for publication

Reviewer 2 Report

Everything was cleared. This study is worth publishing.