The thermoelectric properties of Au nanoparticle-incorporated Al-doped mesoporous ZnO thin films

Mesoporous Al-doped ZnO thin films incorporated with gold nanoparticles (Au NPs) were synthesized using a sol–gel and evaporation-induced self-assembly process. In this study, the complementary effects of Au NP incorporation and Al doping on the thermoelectric properties of mesoporous ZnO thin films were analysed. The incorporated Au NPs induced an increase in electrical conductivity but a detriment in the pore arrangement of the mesoporous ZnO thin film, which was accompanied by a decrease in porosity. However, the addition of the Al dopant minimized the pore structural collapse because of the inhibition of the grain growth in the ZnO skeletal structure, resulting in the enhancement of the pore arrangement and porosity. When the Au NPs and Al dopant were added at the same time, the degradation in the pore structure was minimized and the electrical conductivity was effectively increased, but the absolute value of the Seebeck coefficient was decreased. However, as a result, the thermoelectric power factor was increased by 2.4 times compared to that of the pristine mesoporous ZnO thin film. It was found that co-introducing the Au NPs and Al doping to the mesoporous ZnO structure was effective in preserving the pore structure and increasing the electric conductivity, thereby enhancing the thermoelectric property of the mesoporous ZnO thin film.

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Once again, thank you for submitting your manuscript to Royal Society Open Science and I look forward to receiving your revision. If you have any questions at all, please do not hesitate to get in touch. ********************************************** RSC Associate Editor: Comments to the Author: (There are no comments.) RSC Subject Editor: Comments to the Author: (There are no comments.) ********************************************** Reviewers' Comments to Author: Reviewer: 1 Comments to the Author(s) Comments on RSOS-181799 The present study reported a superior thermoelectric property of ZnO thin films by the assistance of Au nanoparticles and Al-doping. The topic is of general interest to the community, and the conclusions were supported by the experimental results. However, the manuscript still suffers from some problems before its publication. Detailed comments are listed below for further improvement: 1. The introduction section should be significantly enhanced. I can see the necessity of the study, but the novelty of the study was not well illustrated because a brief introduction of the state of art peer researches, especially mesoporous thermoelectric materials, was missing. 2. Though I can manage to follow the message in the introduction section, the logic in some places is rather confusing, e.g., line 46, page 1 "Hence, to control these factors…". Please check the entire section. 3. Please provide the fundamental data such as the porosity, or size of the SiO2/Si substrates. 4. I believe there was a mistake about the sentence of "mesoporous ZAO incorporated with Au NPs (hereafter, Au NPs-AZO) thin films)". 5. Detailed measurement process about the thermoelectric properties should be provided. 6. TEM images for both the pristine mesoporous ZnO and mesoporous ZnO incorporated with Au NPs should be provided for comparison. 7. I believe EDX mapping results of the samples may also provide more valuable information about the modification quality. 8. Please provide the Scherrer equation and size calculation process. Also only one Au NPs was indicated in Fig. 1, which cannot provide a solid support for the Scherrer calculation results. 9. Error bars should be provided in Fig. 4~6 to indicate the statistical significance, otherwise it is hard to determine the observed differences.

Reviewer: 2
Comments to the Author(s) 1. Figure 1 depicts the TEM image and EDS results of the Au NPs-AZO thin film, which reveals formation of Au nanoparticles only. How sample was prepared for TEM analysis? By ion milling or scratching-ff the material from Si/SiO2 substrate, dispersing it in some solvent and drop casting it on Cu grid? TEM images of pristine mesoporous ZnO and Au-NPs-ZnO should be provided for better understanding. In addition, the EDAX spectrum shows presence of Zn, Au, O, and Cu atomic species. There is no peak due to Al. Why? It is better to provide the atomic percentage of these elements in tabular form. are superior (or inferior) to the present ones? 3. What is effect of size of Au NPs on ZT? As the surface plasmon resonance depends on size and shape of the NPs, will it influence the ZT? 4. The manuscript contains a lot many typos and grammatical mistakes. The manuscript should be re-written carefully, avoiding vague statements. (For example, the statement "….the pore structural distortion due to the Au NP incorporation in mesoporous ZnO to enhance electrical conductivity was minimized by Al doping" seems unclear.)

Author's Response to Decision Letter for (RSOS-181799.R0)
See Appendix A.

RSOS-181799.R1 (Revision)
Review form: Reviewer 1 Is the manuscript scientifically sound in its present form? Yes

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The comments of the reviewer(s) who reviewed your manuscript are included at the end of this email.

Comments on RSOS-181799
The present study reported a superior thermoelectric property of ZnO thin films by the assistance of Au nanoparticles and Al-doping. The topic is of general interest to the community, and the conclusions were supported by the experimental results. However, the manuscript still suffers from some problems before its publication. Detailed comments are listed below for further improvement: Comment #1. The introduction section should be significantly enhanced. I can see the necessity of the study, but the novelty of the study was not well illustrated because a brief introduction of the state of art peer researches, especially mesoporous thermoelectric materials, was missing.
▶ The authors revised the Introduction part according to the comment of the reviewer.

Before revision (Results and discussion):
However, these factors have an interdependent relationship, for example in that the Seebeck coefficient value decreases and thermal conductivity increases as the electrical conductivity increases [1][2][3]. Hence, to control these factors independently, a mesoporous structure was adopted as a thermoelectric material in this work because it has low thermal conductivity.
A mesoporous structure has pores within the range 2 to 50 nm [4], and since one was first reported in 1992 [5], the analysis of these structures has markedly increased because of their low density, low thermal conductivity, high specific surface area, etc. [6,7]. Moreover, these properties can be changed depending on the pore size and pore arrangement of the mesoporous structure [8].

After revision (Results and discussion):
However, there is a limit to increasing the ZT value because the Seebeck coefficient, electrical conductivity, and thermal conductivity have an organic correlation. Seebeck coefficient and electrical conductivity in inversely relationship, and electrical conductivity and thermal conductivity are in a proportional relationship [1][2][3]. Therefore, in this study, the mesoporous structure was introduced to maximize the drop in the thermal conductivity while minimizing the decrease in the electrical conductivity, and to increase the thermoelectric properties. The mesoporous structure is a structure in which pores in the range of 2-50 nm are distributed inside the material [4], and it was first reported by Mobil in 1992 [5]. The pores structure acts as a phonon scattering center due to smaller size of pores and shorter distance between pores than phonon mean free path, and drastically decrease the thermal conductivity term by phonon (κph). In addition, the open-pore structure has very large specific surface area and can be applied to various devices [6,7]. Their properties could be controlled by pore structure such as pore size, porosity, and pore distribution in mesoporous structure [8]. Electrical conductivity is a term related to electron transport, which is influenced by carrier concentration and mobility. However, thermal conductivity is divided into term by electron (κel) and phonon (κph).
When a mesoporous structure is introduced, κph could be selectively decreased by an introduction of mesoporous structure, and the thermoelectric properties could be enhanced. Comment #2. Though I can manage to follow the message in the introduction section, the logic in some places is rather confusing, e.g., line 46, page 1 "Hence, to control these factors…". Please check the entire section.
▶ As mentioned in comment # 1, authors modified the Introduction part. Comment #3. Please provide the fundamental data such as the porosity, or size of the SiO2/Si substrates.
▶ According to the comments of the reviewer, authors referred to the size of the SiO 2 / Si substrate in the "Experimental procedure" part and added the porosity estimation basis in the "Results and Discussion" part. The porosity values had been already given in the previously submitted manuscript.

After revision (Experimental Procedure):
In this paper, SiO 2 / Si substrate was prepared by setting the size to 2 cm x 2 cm.

After revision (Results and discussion):
Porosity could be calculated from the difference between the refractive index of the original material and the refractive index measured after the structure was synthesized. In this paper, the refractive index of ZnO is estimated to be 2.

Comment #4. I believe there was a mistake about the sentence of "mesoporous ZAO incorporated with Au NPs (hereafter, Au NPs-AZO) thin films)".
▶ According to the comment of the reviewer, typo was revised.

Comment #5. Detailed measurement process about the thermoelectric properties should be provided.
▶ According to the comments of the reviewer, authors added detailed measurement process about thermoelectric properties in "Experimental procedure" part.

After revision (Experimental Procedure):
The instrument allows measurement of film samples. The Seebeck coefficient is defined as the ratio of the voltage difference to the temperature difference between two sides of the materials (S=ΔV/∆T). The temperature range could be set by using software system [19]. In this paper, the temperature difference from 323 to 478 K at intervals of 50 K under a helium gas flow atmosphere.

Comment #6. TEM images for both the pristine mesoporous ZnO and mesoporous ZnO incorporated with Au NPs should be provided for comparison.
▶ According to the comments of the reviewer, authors added TEM image and EDS data.

Fig. 1 shows the TEM and EDS results of the pristine ZnO and Au NPs-AZO samples. The
pores were not clearly distinguishable because of planar overlapping in the two dimensional TEM image but in both images, several tens nanometer sized pores were well observed, especially with pristine ZnO sample ( Fig. 1(a)). In the TEM image of Au NPs-AZO sample ( Fig.   1(b)), the presence of the Au NPs at a size of almost 20 nm was clearly confirmed. The comparison data of EDS results of both samples also showed the existence of the Au NPs in the mesoporous Au NPs-AZO sample.

Fig. 1. TEM images and EDS results of (a) pristine ZnO and (b) Au NPs-AZO samples.
Comment #7. I believe EDX mapping results of the samples may also provide more valuable information about the modification quality.
▶ Authors agreed with the reviewer's opinion. However, at this time, the EDS analysis by using TEM was only possible with the point technique, there is a limit to the overall composition analysis.

The object of TEM and EDS analyses was limited on the confirmation of Au PNs in mesoporous
ZnO matrix, and darker Au NP image and EDS result including Au NPs can be sufficient for this purpose. Authors request a generous understanding of reviewer. Fig. 1, which cannot provide a solid support for the Scherrer calculation results.

NPs was indicated in
▶ According to the comments of the reviewer, the authors added Scherrer calculation as follows.

After revision (Results and discussion):
Using the Scherrer equation [23], the size of the Au NPs was calculated with the full-width at half-maximum value of the Au (111) peak. The Scherrer equation is shown below.

= cos θ
Here, β is the width of the observed diffraction line at its half intensity maximum, K is the shape factor set to 0.9, and λ is the wavelength of Cu Kα. The size of Au NPs in AZO and ZnO was calculated to be 20.7 nm and 21.2 nm, respectively which matched well with the TEM observations in Fig. 1. Comment #9. Error bars should be provided in Fig. 4~6 to indicate the statistical significance, otherwise it is hard to determine the observed differences.
▶ According to the comments of the reviewer, authors added the error bar in Figs. 4~6.
After revision (Fig. 4, Fig. 5, Fig. 6 Concerning with the TEM and EDS data of mesoporous ZnO for comparison with ZnO containing Au NPs (Au NPs-AZO), the additional TEM and EDS analyses of mesoporous ZnO were carried out and the results were given as Fig. 1(a). Please refer to the responses of the Comments #6 and #7 of Reviewer 1.
The composition of Al, Au and Zn in starting precursors were given in Experimental procedure part as "the molar ratios of the Al and Au precursors to Zn precursor were fixed at 0.02 (i.e. the atomic ratio of both Al to Zn and Au to Zn was 2 at%)". This is not the final composition ratio in the mesoporous Au NPs-AZO thin film but all the precursors were preserved in the film formation process without a loss and then the starting composition can be used for the overall composition.
Concerning with the emission peak of Al, the emission peak from Al (around 1.5 KeV) should have been observed in the EDS data of Fig. 1(b). However as shown in the TEM and EDX data, Au NPs was focused during the observation and the emission intensity of Au would have been greatly exaggerated. Then as shown in Fig. 1(b incorporating Au NPs in mesoporous ZnO thin film was found with XRD, GISXAS, Seebeck coefficient, and electrical conductivity measurements data in this study as superior thermoelectric properties of Au NPs-AZO than those of Au NPs-ZnO.
Comment #3. What is effect of size of Au NPs on ZT? As the surface plasmon resonance depends on size and shape of the NPs, will it influence the ZT?
▶ In nanocomposite, the size of the nanoparticle is a factor that could significantly change the ZT.
Generally, as the size of the nanoparticle decreases, the grain boundary increases, and this boundary could scatter the electron and phonon, affecting the electrical conductivity / thermal conductivity. In this study, free electrons are generated by surface plasmon resonance to improve the electrical conductivity.
However, as reviewer commented, as the surface plasmon resonance depends on size and shape of the NPs, it influences the ZT. That is because the electrical conductivity and thermal conductivity decrease as the size of the nanoparticle decreases [Ref#1]. In the previous paper, electrical conductivity changes with the addition of Au nanoparticles was analyzed [17]. ▶ The sentence was corrected and the entire manuscript was English corrected by English edition service. Authors hope no more typos and grammatical mistakes in the revised manuscript.