Investigation of Annealing Process Effects on the Response and Stability of Sprayed Co₂SnO₄ Film under Ethanol Vapor

Round 1

Reviewer 1 Report

It is an interesting paper, I would suggest only to improve some captions i.e.

in fig.5 the caption should be for every a, b, c and d.

in experimental set-up eq.1  - pls. add some info how the partial pressure p_vap is measured - not only the reference.

The main question addressed by the research in the above paper is the annealing process effects on the response and  stability of sprayed Co2SnO4 film under ethanol vapor. This is an original topic in the field of gas sensors not investigated earlier.

The presented investigations compare the sensor structure before annealing and after the annealing processes with the estimating of the best suited temperature.

The author should explain in more details the dosing of ethanol vapours not only cite the reference. Especially, the measurement of the partial pressure of the ethanol vapour.

The conclusions are consistent with the performer invastigations.

The references are appropriate and actual.

The detailed captions for the fig.5 should be added.

 

 

Author Response

Answers for comments and Suggestions of Reviewer 1

 in fig.5 the caption should be for every a, b, c and d.

Dear reviewer, the Fig.5 caption was changed and the new caption became:

 Figure 5. CTO_as and CTO_ann ethanol sensing results: (a) Optimal working temperature determination of as-prepared film (CTO_As) under 500 ppm of ethanol, (b) Optimal working temperature determination of annealed film (CTO_Ann) under 500 ppm of ethanol, (c) Dynamic current variation of prepared layers at optimums temperatures, (d) layers responses evolutions versus ethanol concentration.

• in experimental set-up eq.1 - pls. add some info how the partial pressure p_vapis measured - not only the reference.

Dear reviewer, the following paragraph and reference were added to the text to give more information about the partial pressure P_vap  values, in our case we determine the partial pressure P_vap from database for given temperature T_vap  :

“For all ethanol concentrations, the heated bath circulator temperature T_vap was fixed at 30 ^oC in order to fix the vapor pressure P_vap which was deduced from database curve of ethanol pressure existing in the CRC Handbook of Chemistry and Physics [33]”

 

 

 

 

Reviewer 2 Report

1 Why the authors choose the Co2SnO4/Co3O4 as the sensitive material？this should be carefully illustrated in the introduction.

2 The Co3O4 plays a key role in the sensing properties of the composite, How is the sensing performance of the Co3O4 alone?

3 As a practical sensor, the selectivty should be carefully investigated.

4 How is the enviromental humidity affect the sensing properties? 

Author Response

Answers for comments and Suggestions of Reviewer 2

• Why the authors choose the Co2SnO4/Co3O4 as the sensitive material？this should be carefully illustrated in the introduction.

Dear reviewer, the Co2SnO4/Co3O4 was the results of annealing process and we have choose this composite as subject of our work, because  to the best of my knowledge, neither Co2SnO4 nor Co2SnO4/Co3O4 sensing properties under ethanol were studied. The following paragraph and references [20-26] were added to the introduction:

“Also, there are no work done on Co₂SnO₄/Co₃O₄ sensing properties under ethanol. Among the most used materials for ethanol detection we can mention the Co₃O₄ [20-22] and its composites like: ZnO/Co₃O₄, Fe₂O₃@Co₃O₄, Co₃O₄–TiO₂ and Co₃O₄/Ti₃C₂T_x [23-26].”

• The Co3O4 plays a key role in the sensing properties of the composite, How is the sensing performance of the Co3O4 alone?

Dear reviewer, there is a few studies concerning Co₃O₄ sensing performance under ethanol. The existent study exhibit good performances of this materials under ethanol, whereas sensing performance of Co₃O₄ in composite materials were widely investigated and they present good sensing performances and stability compared to pure Co₃O₄.

The following paragraph and references [20-26] were added to the introduction:

“Also, there are no work done on Co₂SnO₄/Co₃O₄ sensing properties under ethanol. Among the most used materials for ethanol detection we can mention the Co₃O₄ [20-22] and its composites like: ZnO/Co₃O₄, Fe₂O₃@Co₃O₄, Co₃O₄–TiO₂ and Co₃O₄/Ti₃C₂T_x [23-26].”

          3. As a practical sensor, the selectivity should be carefully investigated.

Dear reviewer, As perspective The Co₂SnO₄/Co₃O₄ selectivity, will be investigated in future works, the present work is a primary study of novel sensing properties of Co₂SnO₄layer under ethanol after annealing. The next step is the optimisation of layer thickness and sensor substrate and electrode architecture after that  a selectivity study could be conducted. The present results of CTO_Ann are promising for future carefully investigation of  CTO_Ann selectivity.

• How is the environmental humidity affect the sensing properties? 

Dear reviewer for the humidity effect the following sentence was added in the end of “3.2. Co₂SnO₄ and Co₂SnO₄/Co₃O₄ sensing results under ethanol” section:

“In addition in our previous work [37] we have demonstrate that the humidity has a neglected effect on the response stability of Co₂SnO₄. So we think that the Co₂SnO₄/Co₃O₄ composite which present a better stability will be not affected by the humidity, this prediction could be investigated in future research.”

 

Author Response File: Author Response.docx

Reviewer 3 Report

The author used the Co2SnO4 as promising sensitive active layer for ethanol vapor detection. My recommendation for this article is 'Major Revision'. The detailed comments are as follows.

 

1. It seems appropriate to delete the contents about the Li-ion battery in the Introduction part and add the reference of the ternary compound oxide sensor.

 

2. Explain the annealing conditions in detail in the Experimental procedure (gas atmosphere, heating rate, etc.) And why author choose 800 degrees as an annealing temperature? Also, why does the particle size decrease after annealing as shown in figure 1?

 

3. Write the caption of figure 5 in detail.

 

4. Please update the old references to the latest (in references 12 to 20).

 

5. Adding mapping data in figure 4 will make the distinction between Co3O4 and Co2SnO4 more obvious.

 

6. Is the p-type characteristic after annealing only due to the cobalt oxide? Does Co2SnO4 after heat treatment change to p-type? Do p-type Cobalt oxide and n-type Co2SnO4 exist together after heat treatment? It should be clearly explained in main manuscript.

Author Response

Answers for comments and Suggestions of Review 3

 

• It seems appropriate to delete the contents about the Li-ion battery in the Introduction part and add the reference of the ternary compound oxide sensor.

The contents about the Li-ion battery in the Introduction part were deleted and replaced by the following paragraph and references for ternary compound oxide sensor and also composites sensors were added in introduction:

“, pollution crisis is one of the worldwide major problem . In order to overcome this problem, several research groups attracted towards different kinds of nano materials because in nanoscale, the materials exhibit outstanding performance in gas sensors  applications for controlling the emission of Pollutants gases. However, a ternary metal oxide and composites like NiMn₂O₄, Fe₂O₃/ZnO and Ag-SnO₂/βC₃N₄ [1-3]  has developed as a promising material for gas sensing applications. Other materials such as metal stannates like Ba₂SnO₄, Ca₂SnO₄, Zn₂SnO₄, Cd₂SnO₄, Mg₂SnO₄, Mn₂SnO₄ and Co₂SnO₄ has been also developed by different research groups [4-10] have found interesting properties and showed good performance in optical and electrical properties and widely used in solar cells study, super capacitors etc.[11-18]. So far, there are very few studies on their gas sensing performances. In this context,”

• Explain the annealing conditions in detail in the Experimental procedure (gas atmosphere, heating rate, etc.) And why author choose 800 degrees as an annealing temperature? Also, why does the particle size decrease after annealing as shown in figure 1?

Dear reviewer, In order to give more details of the Experimental procedure and the raison of the choose 800 ^oC as annealing temperature, the following paragraph was added in the section “2.1. Co₂SnO₄ film deposition”:

“with a heating rate of 10 °C/min at standard atmospheric conditions, after that the sample allowed to let cool down to room temperature. the aim of this step was the investigation of the annealing effects on the Co₂SnO₄ layer sensitivity toward EtOH, which may be probably controlled by phase transition in as-prepared layer. The choose of temperature and calcination duration were in order to insure the synthetises of stable sensitive phase when the working temperatures under ethanol vapor will be varied.”

Dear reviewer, for explain the particle size decrease after annealing the new ref [34] and following paragraph were added to the section “3.2. Co₂SnO₄ and Co₂SnO₄/Co₃O₄ sensing results under ethanol”:

“The main reason for the observed grain size decreases is the porosity decreases of the grain with the increases of annealing temperature (i.e., the grain become more compact) as reported  in literature [34].”

 

• Write the caption of figure 5 in detail.

The Fig.5 caption was changed and the new caption became:

Figure 5. CTO_as and CTO_ann ethanol sensing results: (a) Optimal working temperature determination of as-prepared film (CTO_As) under 500 ppm of ethanol, (b) Optimal working temperature determination of annealed film (CTO_Ann) under 500 ppm of ethanol, (c) Dynamic current variation of prepared layers at optimums temperatures, (d) layers responses evolutions versus ethanol concentration.

 

• Please update the old references to the latest (in references 12 to 20).

Dear reviewer I am so sorry, the references couldn’t be updated because they present a selection of research works comparable to our work wherein they have used as sensitive layers derivatives of metal stannates like our layer and also the same concentration of 500 ppm of ethanol that we have used. Nevertheless, if you think it isn’t interesting, we can delete the table containing these references.

• Adding mapping data in figure 4 will make the distinction between Co3O4 and Co2SnO4 more obvious.

Dear reviewer, unfortunately no mapping data was realised for elaborated samples

• Is the p-type characteristic after annealing only due to the cobalt oxide? Does Co2SnO4 after heat treatment change to p-type? Do p-type Cobalt oxide and n-type Co2SnO4 exist together after heat treatment? It should be clearly explained in main manuscript.

Dear reviewer, to explain this point the following paragraph was added to the section “3.2. Co2SnO4 and Co2SnO4/Co3O4 sensing results under ethanol”:

 

 “The previous results of AC investigation allow us to affirm that the response enhancement of CTO_ann sensor is mainly controlled by the oxidation reactions with oxygen adsorbed in grain boundaries regions formed by Co₃O₄/Co₂SnO₄ p–n heterojunctions. In the others sensor regions we have probably a coexistence of Co₃O₄/Co₃O₄ p–p homojunctions and Co₂SnO₄/Co₂SnO₄  n–n homojunctions, so theirs electrical (response) behaviours goes against with each other.”

 

 

Author Response File: Author Response.docx

Reviewer 4 Report

1-The chemical composition of the deposited layer Co2SnO4 should be analyzed (e.g. Raman spectrum)

2-Please explain the effect of morphology on sensor sensitivity.

3-XRD pattern in Figure 3 should be better explained.

4-Formulas (3) and (4) are duplicated.

5-What is causing a switch from n to p-type conductivity?

6-Page 6 line 286 and page 9 line 384: The sentence should be ended with a full stop.

Author Response

Answers for comments and Suggestions of Review 4

 

1-The chemical composition of the deposited layer Co2SnO4 should be analyzed (e.g. Raman spectrum)

Dear reviewer, unfortunately no chemical composition were realised for elaborated samples.

2-Please explain the effect of morphology on sensor sensitivity.

Dear reviewer, the effect of morphology on sensor sensitivity was discussed  in the text, the following paragraph was added to section “3.1. Characterization of elaborated films”:

 

“Figure 4(a) and 4(b) shows the high magnification SEM images of the samples obtained before and after the annealing treatment at 800 ^oC.

We remarks that the high magnification SEM image shows a randomly combination of compact grains agglomerations with average sizes ranging from about 50 nm to 150 nm in size. After annealing the film shows a large number of individual grains with average sizes ranging from about 20 nm to 50 nm in size, leading to an increases of porosity and offer a large active surface area available for oxygen molecules adsorption.

High surface area is the main factors in a sensor sensitive layer to achieve higher sensitivity.

The polycrystalline nature of annealed film, offer more surface area available for the atoms fractions and oxygen molecules adsorption at the surfaces of grains and interfaces between the grains ( grain boundary ) suggests that the microstructure of the films is suitable for ethanol vapor sensing applications. The variations in sizes of particles are seen in SEM images investigation were comparable to the grains sizes estimated by XRD measurements.”

3-XRD pattern in Figure 3 should be better explained.

For best explains of XRD pattern in Figure 3 the following paragraph was added in the section “3.1. Characterization of elaborated films”:

 

“The crystallites sizes of prepared samples were calculated using the following formula of scherrer (eq. (3)) :

 

                                                                                                                    (3)

 

where λ is the wavelength of used x-rays diffraction, β is the full width at half maximum (FWHM) of (hkl) diffraction plane in radians. and h is the corresponding incident angle.

The average crystallites sizes were found to be about 80 nm for CTO_as pure Co₂SnO₄ (311) phase and 50 nm and 30 nm for Co₂SnO₄ (311) and Co₃O₄ (111) directions respectively, in polycrystalline CTO_ann layer, no other phase formation of impurities was observed in the diffractogram.”

4-Formulas (3) and (4) are duplicated.

Dear reviewer, Formulas (3) and (4) are not duplicated, for Formula (3) we have a division by I₀, whereas in Formula (4) we have a division by I_gas.

5-What is causing a switch from n to p-type conductivity?

Dear reviewer, the switch from n to p-type conductivity was caused by the apparition of p-type Co₃O₄ phase in the Co₂SnO₄ layer. This point was already mentioned in the text.

6-Page 6 line 286 and page 9 line 384: The sentence should be ended with a full stop.

Dear reviewer, the full stop were added in Page 6 line 286 and page 9 line 384

 

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

After revision, the manuscript is recommended for publication.

Reviewer 3 Report

The author responded well to the reviewer's questions and requests, although there were some shortcomings. Considering the parts modified and added by the author, the level of the current manuscript could be accepted.