Facile Solution Synthesis, Processing and Characterization of n- and p-Type Binary and Ternary Bi–Sb Tellurides

Round 1

Reviewer 1 Report

The author presents a rapid and reproducible method using microwave-assisted heating for the controlled growth of Bi2-xSbxTe3. But, a figure/table or data demonstrating the reproducibility of the method (standard deviation of measures...etc) is missing. I don't know if this data is included in the supplementary information.

Author Response

Point 1:  The author presents a rapid and reproducible method using microwave-assisted heating for the controlled growth of Bi2-xSbxTe3. But, a figure/table or data demonstrating the reproducibility of the method (standard deviation of measures...etc) is missing. I don't know if this data is included in the supplementary information.

Response 1: We have developed the presented synthetic protocols and fabricated samples with the same composition in various batches using a small-scale MW reactor. Structural and microstructural characterization of the compounds made in different batches (up to 8 batches are compared), under the same reaction conditions, have proven same microstructure and phase purity. Once the process reproducibility has been proven then we translated the developed protocol to a large-scale MW reactor, obtaining the same outcome for much large quantities in parallel processed reactors. The possibility of monitoring and controlling the temperature profile during the microwave assisted synthesis assures a high reproducibility of the synthetic process. Typical error range for the electronic measurements performed is also introduced into the experimental section.

Reviewer 2 Report

Comments to the Authors:

In this manuscript the authors used the microwave-assisted heating technique to produce high crystallinity and phase thermoelectric nanomaterials. In addition, electronic and thermal transport properties of the TE materials are investigated. From a general point of view, the results are interesting considering the recent efforts in the scientific community to rapid optimize the growth conditions in order to produce high performance TE materials in large scale. However, there are several issues that need to be addressed before the manuscript can be accepted for publication.

Please see my comments below:

Although the state of the art seems complete, the authors should mention other fast and reproducible synthetic methods. For instance, the synthesis of Bi_2−xSb_xTe compounds (0≤x≤2) by arc melting has been reported recently (e.g. Scientific Reports, 7, 6277, 2017 and Nanoscale Res. Lett. 11, 142, 2016). Note that in these works the authors also measured thermoelectric properties of the Bi₅Sb_1.5Te₃. Furthermore, recently has been reported one of the highest figure of merit of 1.71 in Bi_0.5Sb_1.5Te₃ obtained by the optimization of traditional melting-solidification method under a variable intensity magnetic field (Nano Energy 15, 709–718, 2015). The authors might want to include in the discussion as well. In the structural characterization the TEM measurements should be placed in the main text. A better discussion of the TEM measurements also would be useful in the manuscript. For instance, which is the crystal orientations that can be detected from the diffraction pattern of Fig. S5? Do the samples exhibit an inhomogeneous crystal texture over lengths scales of tens of nm, which is favorable for thermoelectricity? Does the increasing Sb content (x) has important effects on the crystallographic and vibrational properties? In order to better investigate the atomic crystal: Is it possible the authors to perform neutron powder diffraction (NPD) measurements from which the symmetry of crystal lattices, the dimensions of the unit cells of the crystal structures and the elemental composition can be revealed? A schematic of the layered crystal structure of the investigated materials would be useful for the readers. I suggest a combined figure 1 with XPS and representation of the crystal structures. Please improve also the visibility of figure 1, figure 3 and figure S6 (e.g. font size, axis etc.) The methodology used for Seebeck and electrical conductivity measurements in section 2.4 needs to be explained better. In section 3.3 is not clear how the authors measured the Seebeck coefficient and electrical conductivity of their samples (Figures 3a,b). A recent study showed a maximum Seebeck coefficient of about 209 μV/ K in Bi₅Sb_1.5Te₃ at 395 K (Scientific Reports, 7, 6277, 2017). This value is consistent with previous works. The data presented here show a decreased Seebeck by almost a factor of 2. In addition, the calculated thermal conductivity values of the Bi_0.5Sb_1.5Te₃ presented here are quite low in comparison with previous reported. How the authors explained this apparent discrepancy? Please comment on this. This discussion will help to better understand the high ZT values that the authors found.

Overall, the discussion of the results should be further developed. In particular, the experimental measurements should be carefully explained and compared with recent studied. The authors should include ALL the recent works where the thermoelectric performance of Bi_2−xSb_xTe compounds have been investigated and commented accordingly. The result is interesting and contain substantial information to a broader readership. The manuscript might comply with the standards of Applied Science after revision of the above mentioned issues.

Comments for author File: Comments.pdf

Author Response

Point 1) Although the state of the art seems complete, the authors should mention other fast and reproducible synthetic methods. For instance, the synthesis of Bi2−xSbxTe compounds (0≤x≤2) by arc melting has been reported recently (e.g. Scientific Reports, 7, 6277, 2017 and Nanoscale Res. Lett. 11, 142, 2016). Note that in these works the authors also measured thermoelectric properties of the Bi0.5Sb1.5Te3. Furthermore, recently has been reported one of the highest figure of merit of 1.71 in Bi0.5Sb1.5Te3 obtained by the optimization of traditional melting-solidification method under a variable intensity magnetic field (Nano Energy 15, 709–718, 2015). The authors might want to include in the discussion as well.

Response 1: Thanks for the suggestions. We have now included the arc-melting process in the state-of-the-art section in the manuscript and the related references were added.

Point 2) In the structural characterization the TEM measurements should be placed in the main text. A better discussion of the TEM measurements also would be useful in the manuscript. For instance, which is the crystal orientations that can be detected from the diffraction pattern of Fig. S5? Do the samples exhibit an inhomogeneous crystal texture over lengths scales of tens of nm, which is favorable for thermoelectricity? Does the increasing Sb content (x) has important effects on the crystallographic and vibrational properties?

Response 2:Due to the polycrystalline nature of the material, we did not prioritize the TEM discussion. From the XRD analysis we observe the same intensity profile of diffraction planes for all four materials systems reported.

The d spacing measured from the TEM image is 0.22 nm, corresponding to (110) plane of the rhombohedral Bi2Te3lattice. This info is added to the caption of Figure S5. 

The suggested detailed investigations using HR TEM for as-made and thinned SPS sintered samples are planned as future work and will be undertaken. Increasing Sb content has not been observed to have an impact on crystallographic properties as can be seen from the XRD patterns of sintered samples presented in Figure 1.

Point 3) In order to better investigate the atomic crystal: Is it possible the authors to perform neutron powder diffraction (NPD) measurements from which the symmetry of crystal lattices, the dimensions of the unit cells of the crystal structures and the elemental composition can be revealed?

Response 3:  We thank the Reviewer for this suggestion. We do not have a direct access to the beam-lines for performing neutron diffraction studies. However, we do understand the value of the suggestion and will make necessary contact for performing neutron diffraction studies in the future. This will help us not only in revealing detailed structural information but may also be useful in identifying phonon modes, providing further insight into the thermal conductivity. 

Point 4) A schematic of the layered crystal structure of the investigated materials would be useful for the readers. I suggest a combined figure 1 with XPS and representation of the crystal structures. Please improve also the visibility of figure 1, figure 3 and figure S6 (e.g. font size, axis etc.)

Response 4: Figure1, Figure 3 and Figure S6 have been replaced with higher resolution ones.

The crystal structure schematic is inserted into Figure S4(a).

Point 5) The methodology used for Seebeck and electrical conductivity measurements in section 2.4 needs to be explained better. In section 3.3 is not clear how the authors measured the Seebeck coefficient and electrical conductivity of their samples (Figures 3a,b).

 Response 5: The methodology used for Seebeck coefficient and electrical conductivity measurements in Section 2.4 are detailed in the revised manuscript, which reads as follows:

The Seebeck coefficient S and the electrical conductivity σ, were measured simultaneously on the pellets obtained after the SPS process using a commercial instrument ZEM-ULVAC M8 model. This sytems measures the S and σ based on the four-probe point method. Typical error in both electrical conductivity and Seebeck coefficient measurements is estimated as 4% in total.

 Point 6) A recent study showed a maximum Seebeck coefficient of about 209 μV/ K in Bi0.5Sb1.5Te3 at 395 K (Scientific Reports, 7, 6277, 2017 –). This value is consistent with previous works. The data presented here show a decreased Seebeck by almost a factor of 2. In addition, the calculated thermal conductivity values of the Bi0.5Sb1.5Te3 presented here are quite low in comparison with previous reported. How the authors explained this apparent discrepancy? Please comment on this. This discussion will help to better understand the high ZT values that the authors found.

 Response 6: Electronic transport properties are strongly correlated, they are also affected strongly by charge carrier density, defects and scattering events (J. Goldsmid, Introduction to Thermoelectricty). Therefore, most likely it is possible to observe different transport values in the same composition due to the microstructure of composition, which is mostly determined by the synthesis method. Regarding the comparison of our result with the indicated paper by the Reviewer, our sample shows a higher electrical conductivity and a lower Seebeck coefficient in comparison with the literature. In correlated systems an increased electronic conduction will lead to a decrease in the Seebeck coefficient, as observed for our samples. As the Reviewer indicated, our Bi_0.5Sb_1.5Te₃ composition have higher electrical conductivity and lower Seebeck coefficient in comparison to F.S Sanchez’ paper (Scientific Reports, 7, 6277, 2017). This difference mainly originates from the microstructure difference between the two samples. F.S Sanchez et al, synthesized these compositions with Arc Melting process and solidification under magnetic field. This method provides larger grains with texture, micro-nano size of grains or layers cause strong scattering of charge carriers and heat carrier phonons. Therefore, the texture decreases electrical and thermal conductivity of samples while it enhances the Seebeck coefficient. In comparison to this sample, our sample has a rather different microstructure. As it can be seen in SEM and TEM micrographs, samples prepared with solution chemical process have small nano size, nearly identical crystalline structure. This microstructure provides with higher electrical conductivity and lower thermal conductivity due to energy difference between charge carriers and heat carrier phonons (L.D. Hicks and M.S. Dresselhaus  Phys. Rev. B 47, 12727). The thermal conductivity can be decoupled from the electronic transport by engineering the interfaces; by introducing point defects or via nanostructuring leading to increase in the density of grain boundaries thus increased scattering of phonons (mainly reducing the lattice component of the thermal conductivity). The obtained nanostructure in our work is the origin of reduced thermal conductivity.

--> Part of this discussion in a brief form is inserted in Section 3.3 of the revised manuscript

Reviewer 3 Report

The authors have demonstrated a quick yet simple solution synthesis route for Bi2-xSbxTe3 nanopaltelets with a promising TE performance in terms of ZT values above 1. The manuscript is well written and organized with a clear scientific understanding. I believe this work can be published after the minor corrections

What about the stability of the material(s)? What are the error bars of TE measurements (thermal conductivity, Seebeck coefficient etc)? Add them in the plots or tables. In line 278 the authors have mentioned the n-type and p-type nature of the material changes depending on x. Mention the possible reason(s). The scale bars in XPS plots of the SI are not visible.

Author Response

Point 1: What about the stability of the material(s)? What are the error bars of TE measurements (thermal conductivity, Seebeck coefficient etc)? Add them in the plots or tables. In line 278 the authors have mentioned the ntype and p-type nature of the material changes depending on x. Mention the possible reason(s). The scale bars in XPS plots of the SI are not visible.

Response 1: Typical errors originating from the measurements are given in the details of the characterization section; the following lines are added: Typical error in both electrical conductivity and Seebeck coefficient measurements is estimated as 4% in total.

  Bi₂Te₃ has typically n- type and Sb₂Te₃ has p-type electronic conduction, mainly due to their electronic DOS. The transition from n- to p-type electronic conduction is observed for compositions where Sb content, x, in Bi_2-xSb_xTe₃ is higher than 0.5. In order to demonstrate the feasibility of the synthetic process we have chosen two n- type and two p-type compositions from this family if compounds.

  XPS spectra in FigS6 is replaced with better resolution ones.

Round 2

Reviewer 2 Report

The authors replied sufficiently to my comments and the manuscript has been improved. I suggest the manuscript to be published in present form.