Fabrication of ZnO Ceramics with Defects by Spark Plasma Sintering Method and Investigations of Their Photoelectrochemical Properties

Round 1

Reviewer 1 Report

This is an interesting manuscript reporting the preparation of zinc oxide with a high concentration of the point defects and studying the relevant effects on the bulk properties. Although the novelty of the manuscript is not very clear, it could be recommended for publication after addressing the following issues:

1. This study does not involve nanomaterials and, thus, probably does not fit the scope of the journal.
2. The authors attribute the weakening and broadening of the peak (100) to the formation of some defects. This should be explained in more detail and supported by the references. Moreover, the changes in the diffraction intensity are also visible for the peak (002) (and probably even for (101). However, the latter is not explained. In fact, SPS could promote some preferential orientation of the growing crystallites, and this might result in the different intensities.
3. The authors involve the grain size in the discussion. The information on how it was calculated should be provided.
4. The statement “But the absorption edge of the two ceramics (ZnO(C)-1A and 2A) don’t return to the original absorption edge at 384 nm (Figure 2 and Figure S2), which clarifies that there are other defects (such as VZn, Zni) in the ZnO(C)-1 and ZnO(C)-2” is not clear. In fact, the defect equilibrium even in ZnO is quite complex and involves many defect reactions, which should be considered altogether. As an example, ZnO containing Zn interstitials can be viewed as oxygen-deficient ZnO. Thus, treatment in air atmosphere could lead to the disappearance of Zn(i) in the same way as it happens with oxygen vacancies. In turn, introducing the oxygen excess may promote the formation of zinc vacancies. The authors should clarify their discussion, preferably providing the corresponding defect reactions.
5. It is not clear why the charge carrier mobility increases with the concentration of the defects. In fact, an opposite dependence could be expected.
6. The values of the charge carrier mobility and density presented in Table S2 appear to be negative. The precision of the provided values, including resistivity, should also be revised.
7. The text should be double-checked for misprints (e.g., “victimized” – page 4).

Author Response

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Reviewer 2 Report

It is a valid contribution suitable Nanomaterials Journal. However, the writing and the structuring of the manuscript require extensive improvements.  

Introduction:

It might be worth mentioning other processing techniques like flash cold sintering and UHS for processing of ZnO.

The experimental procedure is rather unclear. How the parameters were selected? “Then the sintering temperature  was raised to the target temperature (700, 900 or 1100 ℃) at 80 ℃ / min and kept at this temperature for 30 minutes”. 

Experimental:

“Then we marked the heat-treated ceramics with ZnO(C)-1A, ZnO(C)-2A, 100 and ZnO(C)-3A, respectively.” Labelling is needless complicated.

 Results and discussion:

The  carbothermal reduction promoted by the contact with the graphite tooling is not discussed. Photographs of the samples should be shown. There is no discussion on electrochemical reduction occurring when SPS is carried out under a DC field  https://doi.org/10.3390/ma14112826

“The ZnO ceramics are prepared under high temperature and high pressure as well as in a vacuum atmosphere, therefore, oxygen defects first appear in ceramics”. Use numbers instead of high or low, I guess pressure should be in the order of GPa not MPa.

Data for reference ZnO sample prepared using conventional sintering should be added in all the Figures.

Typos and Language issues : “victimized by vacuum pump”, “the ceramic samples don’t move obviously”; “as vacuum, hypoxia,” Erroneous use of  Kröger-Vink Notation

Author Response

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Round 2

Reviewer 1 Report

Although the authors have satisfactorily addressed most of the questions, some of them still remain:

1. While explaining the weakening of the diffraction peak in [002] direction, the authors state: “The (002) plane is composed of oxygen atomic plane and zinc atomic plane. At high temperature, oxygen and zinc atoms will volatilize, which will also weaken the XRD diffraction peak in this direction.” In fact, according to such an explanation, the intensities of all diffraction peaks should be changed (which does not appear to be the case). Secondly, if the volatilization occurs, how significant are these losses compared to the weight of the sample?
2. The information about measurements of the grain size should be added to the Experimental section.

Author Response

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Reviewer 2 Report

The paper has been improved.  For future work, I might suggest the authors to consider electrochemical effects occurring during flash cold sintering https://doi.org/10.1016/j.jeurceramsoc.2020.06.051

Author Response

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