Study on the Ozone Gas Sensing Properties of rf-Sputtered Al-Doped NiO Films

Round 1

Reviewer 1 Report

The article describes an ozone sensor operating in the low-temperature range based on rf-sputtered Al-doped NiO films. Ozone sensors are of interest in the field of air quality monitoring, especially sensors with low operating temperatures. In the article, variously fabricated Al-doped NiO functional layers are investigated in the temperature range from RT to 150°C. The morphological properties of the layers are investigated using appropriate methods and the characteristic properties of an ozone sensor are determined and analyzed. Nevertheless, some points should be considered before publication:
1. In line 101 is written that the films are treated with synthetic air. Is dry synthetic air or humid synthetic air used for the investigation of the sensing properties?
2. In line 148-149, Fig. 3(c) is described. However, the increased number of defects in the surface is not clearly and unambiguously visible in the figure. The image should be optimized. 
3. In chapter 3.2. the gas sensor properties are discussed. A time-dependent sensor signal, i.e. I vs. time, should be displayed to show the sensor signal when the ozone concentration changes for one temperature and one layer. Please insert a corresponding figure.
4. What is the influence of humidity on the properties of the ozone sensor? For an air quality sensor working at low temperatures, the investigation of the influence of humidity is very important. The authors should comment on this and provide results.
5. Fig. 5 shows the sensor characteristics as a function of ozone concentration. However, there is no description about the relationship of sensitivity versus concentration, i.e. linear, semi-logarithmic, double-logarithmic. Is the correlation expected according to the sensor mechanism?
6. The response and recovery times are shown in Fig. 7. The scaling of the y-axis should be identical to make the difference between the layers clearer. Additonally it is not clear how the shown values of response and recovery times were determined: in line 173-175 it was written that ozone was added for 5 minutes but the response time is greater than 300 seconds? Please explain in more detail how response times of 600-800 sec were determined when ozone was only added for 5 minutes. The same is for the recovery times.

Author Response

Reply to Reviewer 1

The article describes an ozone sensor operating in the low-temperature range based on rf-sputtered Al-doped NiO films. Ozone sensors are of interest in the field of air quality monitoring, especially sensors with low operating temperatures. In the article, variously fabricated Al-doped NiO functional layers are investigated in the temperature range from RT to 150°C. The morphological properties of the layers are investigated using appropriate methods and the characteristic properties of an ozone sensor are determined and analyzed. Nevertheless, some points should be considered before publication:

1. In line 101 is written that the films are treated with synthetic air. Is dry synthetic air or humid synthetic air used for the investigation of the sensing properties?

Reply: Thanks very much for your comment. The synthetic was dry. It is referred in the manuscript now (highlighted).

2. In line 148-149, Fig. 3(c) is described. However, the increased number of defects in the surface is not clearly and unambiguously visible in the figure. The image should be optimized.

Reply: Thanks very much for your comment. It is done in the manuscript (highlighted).

3. In chapter 3.2. the gas sensor properties are discussed. A time-dependent sensor signal, i.e. I vs. time, should be displayed to show the sensor signal when the ozone concentration changes for one temperature and one layer. Please insert a corresponding figure.

Reply: Thanks very much for your comment. A new Figure (Fig.5) was inserted in the text showing the electrical current variation as a function of time for different O₃ concentrations (highlighted).

4. What is the influence of humidity on the properties of the ozone sensor? For an air quality sensor working at low temperatures, the investigation of the influence of humidity is very important. The authors should comment on this and provide results.

Reply: Thanks very much for your comment. All measurements were taken under the room relative humidity which is about 60%, in order to investigate the response of NiO:Al films to ozone at low temperatures. Since the films were able to detect ozone even at room temperature, further measurements will be done to optimize the sensing performance.

5. 5 shows the sensor characteristics as a function of ozone concentration. However, there is no description about the relationship of sensitivity versus concentration, i.e. linear, semi-logarithmic, double-logarithmic. Is the correlation expected according to the sensor mechanism?

Reply: Thanks very much for your comment. New graphs (Fig.6) that describe the relationship between the sensitivity and the ozone concentration are presented in the manuscript.

6. The response and recovery times are shown in Fig. 7. The scaling of the y-axis should be identical to make the difference between the layers clearer. Additionally it is not clear how the shown values of response and recovery times were determined: in line 173-175 it was written that ozone was added for 5 minutes but the response time is greater than 300 seconds? Please explain in more detail how response times of 600-800 sec were determined when ozone was only added for 5 minutes. The same is for the recovery times.

Reply: Thanks very much for your comment. It is corrected to the right one. In specific, the exposure time to ozone was 5 min for films prepared with 4% O₂ in plasma and 10 min for the films prepared with 2% and 2.8% O₂ in plasma, while the recovery of the sensors was carried out by exposing them to synthetic air for 10 min and 15 min, respectively. The y-axis of the graphs of Fig.8 was changed as you proposed.

 

Author Response File: Author Response.docx

Reviewer 2 Report

In Figure 1 the indications in the inset are not clear

 

Authors should write the details of the reaction between nickel oxide and ozone at the basis of the sensing action of the metal oxide in the section Gas Sensing (starting from row 93)

 

In the introduction authors should mention the use of nickel oxide as active material in photovoltaic devices (an important field of application), namely perovskite solar cells and dye-sensitized solar cells. At this regard authors could add the five new refs. with doi:  10.1039/D0SC02859B; 10.1039/D0SC02859B; 10.1016/j.solmat.2019.110288; 10.3389/fchem.2018.00601; 10.1149/2.0051704jes

 

It is known that nickel oxide based materials in the configuration of thin film can undergo structural reorganization after the occurrence of redox processes (see ref. 10.1002/slct.201800827). Do the authors have verified an analogous phenomenon after the interaction of nickel oxide with ozone?

 

Can the authors evaluate the porosity of their films of nickel oxide? It would be a useful datum for the gas-sensing purpose.  

Author Response

Reply to Reviewer 2

In Figure 1 the indications in the inset are not clear.

Reply: Thanks very much for your comment. It is corrected to the manuscript.

Authors should write the details of the reaction between nickel oxide and ozone at the basis of the sensing action of the metal oxide in the section Gas Sensing (starting from row 93)

Reply: Thanks very much for your comment. The gas sensing mechanism for the p-type NiO:Al semiconductor against to the O3, which is a strongly oxidizing gas, is described in the manuscript in more details, now. The corresponding reactions are also added in the manuscript.

In the introduction authors should mention the use of nickel oxide as active material in photovoltaic devices (an important field of application), namely perovskite solar cells and dye-sensitized solar cells. At this regard authors could add the five new refs. with doi:  10.1039/D0SC02859B; 10.1039/D0SC02859B; 10.1016/j.solmat.2019.110288; 10.3389/fchem.2018.00601; 10.1149/2.0051704jes

Reply: Thanks very much for your comment. The application of NiO on photovoltaic devices as well as the proposed references were added in the manuscript, in the section of Introduction.

It is known that nickel oxide based materials in the configuration of thin film can undergo structural reorganization after the occurrence of redox processes (see ref. 10.1002/slct.201800827). Do the authors have verified an analogous phenomenon after the interaction of nickel oxide with ozone?

Reply: Thanks very much for your comment. The authors did not verified such a phenomenon.

Can the authors evaluate the porosity of their films of nickel oxide? It would be a useful datum for the gas-sensing purpose.

Reply: Thanks very much for your comment. At the moment it is not possible to measure the porosity of the films. However, the films deposited by rf-sputtering technique are usually of low porosity, which is also confirmed by the relatively high values of response and recovery time.

 

Author Response File: Author Response.docx