The Effect of pH Solution in the Sol–Gel Process on the Structure and Properties of Thin SnO₂ Films

Round 1

Reviewer 1 Report

Specific comments from referee to Editor

The effect of pH solution in the sol-gel process on the structure and properties of thin SnO2 films

 

Comments:

The work entitled “The effect of pH solution in the sol-gel process on the structure and properties of thin SnO2 films,” describes an interesting results of the influence of pH on the structural properties of SnO2 films. However, it is not clear the experimental procedure, is losing some information and is necessary a better discussion the results

 In this conditions the paper is accepted after major correction for publications

 

Abstract

OK

 

Introduction

OK

 

Experimental  

Materials and Method

1. In general this part should be rewritten.
2. It is unclear how the film was prepares,
3. How was the film with more layer prepared?
4. How was the heat treatment performed?
5. Were adhesion tests performed on the film on the susbtrate?
6. On page 3, line 67 , Could you pxplain about the micro-weihing method? And add the reference?
7. On page 3, line 76, who means Q – 2Q configuration?
8. The thickness of the film was measurement after heat treatment?

 

Results and discussion

1. About discussion of figure 1, It is unclear which film is being discussed
2. In Figure 1, you are talking about which temperature?
3. On page 7, about figura 4, how these multilayers were made and heat treated. It would be interesting to put the values of all thickness of the films.
4. About figure 11, whats about the thickness of these films

 

 

Author Response

Point 1: In general the part «Materials and methods» should be rewritten.

Response 1: Thank you for pointing this out.

1)We have divided this section into two parts: 2.1 - Materials and Synthesis, 2.2 - Characterization methods.

2) One section of deposition the film on the substrate was added.

3) In addition, the text was shortened.

Point 2: It is unclear how the film was prepares.

How was the film with more layer prepared?

Response 2: Thank you for this excellent observation.

The film-forming system was obtained by the sol-gel method:

A crystalline tin tetrachloride hydrate (SnCl₄^.5H₂O) (pure), rectified alcohol were used for the preparation of film-forming systems. The acidity of the systems was regulated by the addition of ammonia aqueous solution (high-purity). Precleaned microscope slides with dimensions 76 mm x 26 mm x 1 mm were used as substrates for the film’s deposition.

The crystalline hydrate of the tin tetrachloride was crushed to a powdery state. A sample weighing 3.9072 g was placed in a 100 ml flask and dissolved in 50 ml of ethanol. Concentrated aqueous ammonia solution (0.2 ml; 0.4 ml; 0.8 ml; 1.2 ml; 1.6 ml; 2.0 ml; 2.4 ml) was diluted with ethanol to 25 ml and dropwise, at constant stirring, was added into the initial solution. The resulting mixture was brought to 100 ml by adding ethanol to obtain a system with a tin ion concentration of 0.11 mol/l.

In this case, the following reactions with ethanol occur in the SnCl₄/ethanol system:

 

         (1)                                       

 

The stability of alkoxy compounds of the general formula SnCl_(4-x)(C₂H₅O)_х decreases as the number of chlorine atoms in the molecule decreases. Therefore, in solutions, they can only be in certain proportions [19].

Reactions with water also take place in the SnCl₄/ethanol system:

                     (2)                                              

 

Interaction with water leads to the appearance of reactive OH-groups and the subsequent processes of polycondensation and sol formation. Hydrochloric acid released during reactions (1, 2) suppresses hydrolysis and shifts the equilibrium of system (2) to the left. Synthesis of tin hydroxide are unlikely under conditions of water shortage. Complete hydrolysis of tin tetrachloride occurs on the substrate surface under the influence of moisture from the air.

The pH level was increased by adding an aqueous solution of ammonia, which has a slightly alkaline reaction due to the following process:

                            (3)                                                

 

Neutralization of HCl by adding an aqueous solution of ammonia occurs according to the following reaction:

                 (4)                                 

                  

 

This catalyzes the hydrolysis of tin tetrachloride and the formation of tin hydroxide.

The overall reaction is as follows:

 

         (5)                                           

 

The film-forming systems were kept for a day in a dark place at 20˚C to establish a dynamic equilibrium between the substance molecules and the formation of stable chemical bonds. These conditions are necessary for the formation of a highly dispersed colloidal system (sol).

 

The film deposition method

For deposition of thin films of SnO₂, we used the dip-coating method: The SnCl₄/ethanol system was deposited on the substrate (at an angle of 45⁰) by dipping into the solution and followed by drying at room temperature for 30 minutes. In this case, reactions (2, 5) and the formation of tin hydroxide from alkoxy compounds occurred:

 

   (6)                                 

In order to crystallize the structure, annealing was performed in a muffle furnace for 15 minutes at a temperature of 400 ° C in an air atmosphere. The solvent (and other volatile compounds) evaporates and decomposition reaction of tin hydroxide into SnO₂ and water proceeds on the surface of the sample:

 

            (7)                                                       

To obtain a multilayer system, this process was repeated 4 times. Then the final annealing was carried out in a muffle furnace for 1 hour at a temperature of 400 ° C in an air atmosphere. The final thickness of the 4-layer film was about 166 ± 7 nm. The time interval between the deposition of individual layers was 1 h. The result was 8 samples obtained from film-forming systems with different pH.

Point 3: How was the heat treatment performed?

Response 3: Thanks to the reviewer for this comment.

The samples were synthesized by the sol-gel method and deposited on the substrate by the dip-coating method. The current process includes annealing, after deposition of each layer (15 min, 400 °C in an air atmosphere), and after deposition of 4 layers (1 h,  400 °C in an air atmosphere) to crystallize the resulting structures. These procedures were carried out for all samples.

Additional annealing of the films was carried out at temperatures of 100, 250, 350 and 400 °C in XRD studies. In this case, with an increase in the annealing temperature, a significant increase in the signal amplitude is observed.

Point 4: Were adhesion tests performed on the film on the substrate?

Response 4: Thank you so much for catching these errors.

Previously, it was found out SnO₂ films obtained from SnCl4/EtOH film-forming systems with a tin ion concentration of less than 0.2 mol/l comprise stable adhesion to a glass substrate. The least resistance includes films with a concentration of tin ions of 0.11 mol/l.

In order to obtain films with good adhesion to the substrate as well as maximum conductivity, a concentration of tin ions of 0.11 mol/l was consumed in this work.

Point 5: On page 3, line 67 , Could you explain about the micro-weighing method? And add the reference?

Response 5: Thank you for this important observation.

The film thickness was determined by the micro-weighing method. The following formula was used to calculate the film thickness:

d=(m _sample – m _substrate)/(ρ _{tin oxide} * S _substrate)

where d is the film thickness, m _sample is the mass of the sample, m _substrate is the mass of the glass substrate, ρ _{tin oxide} is the density of cassiterite, taken as 7 g/cm3, S _substrate is the area of ​​the glass substrate.

Reference for the source has been cited in paper.

Point 6: On page 3, line 76, who means Q – 2Q configuration?

Response 6: Thank you for pointing out this important issue. It was a typo. Correct way of writing - θ -2θ configuration.

Point 7: The thickness of the film was measurement after heat treatment?

Response 7: Sample annealing is envisaged at the stage of deposition each layer (400°C), and after deposition of 4 layers (1 h, 400 °C in an air atmosphere). The micro-weighing method was used to determine the film thickness after its deposition of 4 layers and subsequent annealing.

Point 8: About discussion of figure 1, It is unclear which film is being discussed

Response 8: Figure 1 shows the surface morphology of films obtained from a film-forming system without the addition of an aqueous ammonia solution (pH = 1.40).

Point 9: In Figure 1, you are talking about which temperature?

Response 9: Thank you for pointing this out. This sample was synthesized by the sol-gel method and deposited on the substrate by the dip-coating method. The current process involves annealing at the stage of deposition each layer (400°C in air atmosphere) and after deposition of 4 layers (1 h, 400 °C in an air atmosphere). These procedures were carried out for this sample.

Point 10: 1)On page 7, about figura 4, how these multilayers were made and heat treated. It would be interesting to put the values of all thickness of the films.

2)About figure 11, what’s about the thickness of these films

Response 10: Thank you for this excellent observation. All samples were coated by 4 layers with an overall thickness of about 166 ± 7 nm.The formation of a complex surface structure of a given sample influences inconsiderably on the thickness of the sample.

 

Author Response File: Author Response.docx

Reviewer 2 Report

Review of the manuscript»The effect of pH solution in the sol-gel process on the structure and
properties of thin SnO2 films»  

The work is devoted to the study of the effect of the pH level of the solution on the structure and properties of tin oxide films. The work is very voluminous, many experimental methods are involved. The film structure is analyzed depending on the synthesis conditions. The optical and electrophysical properties of the resulting layers are being studied. The differences between the obtained estimates of the band gap from the classical ones are carried out taking into account the Burstein-Mohs effect.

The work is well-organized, the research is systematic. Study methods are valid and reliable. The used units are appropriate. Tables are relevant and clearly presented.

The research topic is relevant to the Processes journal scope.

 

It is suggested to solve the following problems before publishing:

• The introduction could be improved, it is not very consistent. It is not very clear why the transparency properties of films are important for gas sensors. The purpose of this study is unclear.
• In Part 2, I would suggest splitting this paragraph into two: 2.1 - Materials and Synthesis, 2.2 - Characterization methods.
• Typo on line 77: in the notation θ-2 θ.
• Figure 1 a) is not informative. According to Fig. 1 it is not obvious that a film is formed, it is more like the formation of individual particles on the substrate. In this case, it is not very correct to discuss the film for this sample.
• The green texts on Fig. 1 b) and Fig. 5 are not readable. Please use other colors.
• It would be useful to highlight the novelty of this work, since a lot of papers have been published in the field of sol-gel synthesis of tin oxide layers. In conclusion, it would be useful to make recommendations.

Author Response

Response to Reviewer 2 Comments

 

Point 1: The introduction could be improved, it is not very consistent.

Response 1: Thanks to the reviewer for this comment. We have edited the introduction text from the language point feature and made it shorter without losing the sense.

Point 2: It is not very clear why the transparency properties of films are important for gas sensors.

Response 2: The combination of great optical and electrophysical properties determines a wide range of the material applications, such as gas sensors and solar cells. The optical transparency of the film is essential for the formation of transparent conductive coatings (based on SnO₂) on the solar cell surface.

Point 3: The purpose of this study is unclear.

Response 3: The purpose of this research is to determine the pH range in which the changes of volumetric structure formation transfer to surface formation, and in this case the process of optical characteristics modifies as well as surface resistance. We have inserted the goal of the study into the corrected version of the article.

Point 4: In Part 2, I would suggest splitting this paragraph into two: 2.1 - Materials and Synthesis, 2.2 - Characterization methods.

Response 4: Thank you for pointing out this important issue. We have divided the text of this section in accordance with your suggestion.

Point 5: Typo on line 77: in the notation θ-2 θ.

Response 5: Thank you for pointing this out. We have corrected the typo. The XRD measurements were performed in the θ -2θ configuration using a DRON-6 system.

Point 6: Figure 1 a) is not informative. According to Fig. 1 it is not obvious that a film is formed, it is more like the formation of individual particles on the substrate. In this case, it is not very correct to discuss the film for this sample.

Response 6: Thank you for your comment. Indeed, at the initial moment, individual particles are formed on the surface of the substrate.

1)We have changed the text in the article:

Figure 1 shows the morphology of the sample surface, on which a film-forming system is applied without the addition of an aqueous ammonia solution (pH = 1.40). It can be seen the flat surface of the sample shown in figure 1(a). At a magnification of 30,000 times (figure 1(b)), sol particles are observed formed during the maturation of the film-forming system. They have the shape of a unit cell SnO₂, shown in figure 1(c). The geometric parameters of the sol particles are 496.21 nm by 340.72 nm and their ratio is 1.46. This correlates well with the ratio of the unit cell parameters of tin oxide a=4.64 Å, c=3.15 Å, a/c = 1.47 [20]. Sol particles are fixed in a xerogel formed in accordance with the reactions (2,6,7).

2) In addition, we changed the caption to figure 1:

The surface morphology of the sample surface, on which a film-forming system is applied without the addition of an aqueous ammonia solution.

Point 7: The green texts on Fig. 1 b) and Fig. 5 are not readable. Please use other colors.

Response 7: Thank you so much for catching these errors. We changed the color of the highlights, moreover, we increased the font of the captions on the figures.

Point 8: It would be useful to highlight the novelty of this work, since a lot of papers have been published in the field of sol-gel synthesis of tin oxide layers.

Response 8: Thank you for this important observation. We highlighted the novelty of the study:

The novelty of the current observation comprises the consideration of the properties of SnO₂ films which are investigated when the pH changes are below the level of precipitation of tin acid by adding NH₃ H₂O to the solution.

 

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

All observations have been made

for me now , the paper is accepted