S gas sensors based on the Hydrogen Sulfide (H 2 factors affecting sensory properties. However, sol-gel and

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1 NANOPARTICLES 21 H 2 S Gas Sensor Based on SnO 2 and CuO Nanoparticles AUTHORS: Mohammad Hossein Sarfi 1, Mohammad Ghadimi 1, and Arash Babaee 2 SCHOOLS: 1. Mofid High School, Tehran, Iran 2. Imam Khomeini International University Abstract There have been several reports of H 2 S gas sensors based on the Hydrogen Sulfide (H 2 S) is a toxic gas that has adverse effects two mentioned materials (2). Synthesis and deposition are of important factors affecting sensory properties. However, sol-gel and on human health as well as the oil and gas industry; hence, it is important to identify its characteristics appropriately. The present deposition using immersion method have not been used in order to research was aimed to study the sensing behavior of SnO 2 -CuO thin construct H 2 S gas sensor based on the nanoparticles. layers obtained by the sol-gel method. The working temperature in In the present research, sol-gel method was used to synthesize the present investigation was the environment temperature and the nanoparticles of CuO and their deposition on a conductive glass resistance curves related to the sensing behavior were obtained. (ITO) which eventually leads to better selectivity, more appropriate The microstructure of the covered layers was studied using Field return time, performance at room temperature and gas detection Emission Scanning Electron Microscope (FE-SEM), AFM and XRD at low concentrations. CuO is a p-type semiconductor and can be analysis method. The thin layer of SnO 2 -CuO optimized with sensory characteristics was coated using sol-gel method and were sensors and catalysts. In the present research, CuO nanoparticles used in variety of fields such as gas sensors, solar cells, biological heated for 5 hours at 400 C. The best response number was 1180 were used as a catalyst. CuO can identify gases such as CO and H 2 S (no unit), the response time was 43 seconds and the least recovery individually (3). time was 210 seconds. Moreover, it had a longer response time SnO 2 is an n-type semiconductor with 3.6 ev band gap and compared to the unheated sample, but the response number and can be used in different aspects such as gas sensors, solar cells the recovery time were improved due to the thermal treatment. and cathode in lithium ion batteries. In the present research, SnO 2 was chosen for its semiconducting and sensory properties. (4) As Keywords Nanostructure sensor, Thin layer, Gas sensors, H 2 S, SnO 2 -CuO, Sol-gel Introduction Hydrogen sulfide (H 2 S) is a toxic gas that has adverse effects on human health as well as the oil and gas industry. H 2 S gas is colorless with an annoying smell. Moreover, it is a polar and acidic gas and corrosion of metals is one of its side effects. H 2 S gas leak may cause mishaps such as human deaths, irritating disease, and massive explosions. Therefore, identification of H 2 S gas in oil fields, oil platforms, mines, etc. is important (1). Identification of toxic, hazardous and flammable gases is significant and vital for maintaining safety and environmental protection. Sensors based on bulk materials and semiconductors are used to identify some of these hazards. In sensors based on semiconductors, materials such as ZnO, SnO 2, Fe 2 O 3 and Ga 2 O 3 are used as base materials and materials such as CuO, Ag, Pd and Pt are chosen as catalyst in order to get better selectivity of gases. SnO 2 is a suitable identifier for a variety of gases. The selectivity of H 2 S gas is augmented by the addition of CuO nanoparticle catalyst (2-4). mentioned, there are different materials for sensing H 2 S gas. SnO 2 as the basic material and CuO as the catalyst bring about a better result than SnO 2 alone. However, there are superiorities of the two nanoparticles compared to other materials. Formation of a connection between n-type and p-type semiconductors is a gas sensing mechanism in composites (5). As stated, SnO 2 is an n-type semiconductor and its conductivity is due to free electrons. On the other hand, CuO is a p-type semiconductor and consists of a considerable number of holes. When the two semiconductors are placed next to each other, the connection between the p and n-type semiconductors takes place where a considerable number of the free electrons of SnO 2 is decreased. Consequently, decrease in electrical conductivity occurs (7-8). Presence of H 2 S gas in the environment leads to the in the formation of CuS (6): CuS is a metal compound and leads to the return of free electrons to SnO2 and consequently, its electrical conductivity increases. Many references have accepted this mechanism. Moreover, this mechanism is supported by X-Ray photoelectron spectroscopy (XPS) (9) /sfj (1) (2) 2015 journal.stemfellowship.org

2 22 NANOPARTICLES Materials and Methods The experimental method in the present study is divided into two sections. The first section is the preparation of CuO sol and sample preparation, and the second section refers to the issues related to gas test. Specifications of the substrate Indium tin oxide (ITO) coated glass was used as conductive substrate. SnO 2 nanoparticle was located in the conductive glass surface using physical vapour deposition (PVD). Preparation of CuO sol CuO sol used in the present research is based on a copper nitrate trihydrate (Cu(NO 3 ) 2 3H 2 O), nitric acid (HNO 3 ) and distilled water, which are all purchased from Merck Company. In order to prepare CuO sol, the Cu(NO 3 ) 2 and distilled water were combined with each other in different molar concentrations (1.5, 1, 0.5, 0.3). Afterwards, the 1 molar nitric acid (HNO 3 ) solution was added drop wise to the above solution to adjust the ph to 2. Following acidification, aging treatment was carried out for 24 hours at 70 C (solution was stirred 24 hours at 70 C). Coating method Dip-coating (deposition using immersion method) was used to apply coating. Using this method, a layer of copper oxide (CuO) nanoparticles is placed on the surface of the conductive glass (ITO). H2S gas release A test was designed in order to investigate the performance of the sensor in which H2S gas is produced with a very low concentration (10 ppm) by the reaction of citric acid solution and the Na2S solution. Afterwards, the produced gas is driven to the sensor and the electrical resistance changes are recorded. Eventually, the H2S gas is driven towards the NaOH solution to neutralize this dangerous gas. (3) A cover is put on the surface of the sensor in order to isolate it in the first 50 seconds. Afterwards, the coating is removed and the sensor is in contact with the gas for 150 seconds. In this case, the resistance is decreased for a short period of time and the H 2 S gas is removed from the chamber after increasing the resistance. Finally, the prepared sample is in contact with the environment for 14 minutes and changes in the resistance are recorded at the end. Atomic Force Microscope. Femto Scan V.1.1, was used for mapping and study the topography of the surface structure. Vega Mira Field Emission Scanning Electron Microscope (FE-SEM) was used for surface topography and quantitative compositional analysis. Results Study of coatings FE-SEM Results Due to faster magnification and higher resolution, pictures obtained by the FE-SEM microscope were chosen to be studied. Figure 1 indicates a micro structure of SnO 2 made with PVD (Physical Vapor Deposition). In general, figure 1 shows deposited SnO 2. Figure 1(A) shows the good dispersion of SnO 2. Measurement of the particles was carried out in magnification (B). According to the following picture, the particle size is between 41 and 130 nm. Figure 1: FE-SEM from SnO 2 made with PVD (Physical Vapor Deposition) A) 35,000 times magnification; B) 200,000 times magnification. STEM Fellowship Journal vol.1 issue 12

3 NANOPARTICLES 23 Figure 2 refers to the shell structure with low porosity. Copper oxide nanoparticles were produced by sol-gel method and were coated on the surface of the conductive glass using immersion deposition technique, which the FE-SEM of the mentioned sample is figured in the following. According to figure 2(B), the copper oxide nanoparticles, which were deposited, have an almost spherical form with a diameter of 60 nm. AFM Results In order to investigate the undulation of the surface of SnO 2, a test was carried out using an Atomic Force Microscope (AFM), which the surface roughness is shown in figure 3. The AFM test was carried out to study the undulation of CuO deposited on SnO2, which figure 3(B) refers to the uneven coating due to the lump of the material particles. XRD Results The crystal structure of the mentioned samples was studied using XRD because crystalline modification and the particles size have a significant impact on the quality of sensing. The three samples were investigated using XRD test, which are expressed as follows. 1. SnO 2 prepared by PVD method 2. CuO layer placed on the conductive glass 3. Powder of CuO Figure 4 shows the pattern of XRD of mentioned samples. In the carried out XDR test, the peaks labeled 1, 2 and 3 shows good agreement with the relevant standard patterns. Considering there is not any standard XRD patterns for the CuO - SnO 2 multilayer and relevant overlapping peaks, a XRD test was carried out on the dried sol of CuO (calcinated CuO in 400 ºC for 5 hours) and compared with the existing standard XRD patterns. Gas Test Results Figure 2: FE-SEM from CuO layer deposited on SnO 2 A) 100,000 times magnification; B) 200,000 times magnification. Figure 3: AFM topography of the: A) Conductive glass surface (deposited SnO 2 with PVD); B) AFM topography of deposited CuO on the conductive glass surface (deposited SnO 2 with PVD The resistances of the prepared samples were calculated in the presence of H2S gas in order to investigate their performance. In this regard, the made sample was connected to a multimeter using silver-filled adhesive and the designed sample was located in a chamber in order to being exposed to the H2S gas. The three samples were tested as explained in the following in order to investigate and compare their reversibility. 1. Layer of SnO 2 (prepared by PVD method) 2. Multilayer CuO and SnO 2 (CuO was prepared using sol-gel method and immersion; SnO 2 was prepared using PVD method) (without heat treatment) 3. Multilayer CuO and SnO 2 (CuO was prepared using sol-gel method and immersion; SnO 2 was prepared using PVD method) (with heat treatment). Heating after sol-gel preparation leads to crystallization of the particles and consequently, improvement of the sensing mechanism. Heating process refers to heating of the sample at 400 C for 5 hours. The fol journal.stemfellowship.org

4 24 NANOPARTICLES Figure 4: XRD patterns of: A) SnO 2 prepared by PVD method; B) CuO-SnO 2 multilayer (without heating); C) dried sol of CuO (calcinated in 400 ºC for 5 hours) lowing diagram indicates the changes in electrical resistance over Discussion time in the existence of 10 ppm H 2 S gas. The curves available in the Because of the semiconducting behavior of the CuO primary resistance increase. In the presence of the H 2 figure 5 were obtained by the mentioned method in experimental S gas, the CuO layer reacts with the gas and forms a CuS layer that is a higher conducting part. According to the figure 5, sample 1 has less primitive resistance layer. This CuS layer acts as a catalyst for increasing the chemical (R a). Since the sample 1 was prepared through PVD, the obtained reaction in the substrate tin dioxide layer. Thus, by this mechanism particles were in the form of plate and in a large size. Therefore, the secondary resistance (Rg) decreases. In SnO 2 samples, the sensor resistance decrease from Ra to Rg, is due to the interaction of their initial resistance was very low. However, resistance is reduced significantly in the sol-gel method according to the small size of the target gas molecules with the adsorbed oxygen on the surface of particles. SnO 2 layers. The desorbed oxygen releases the trapped electrons Re (response), is one of the most important criteria in determining the sensor efficiency that shows the sensitivity of sensor. is observed. It was mentioned that using a catalyst improves sen- and therefore a decrease in the sensor resistance from Ra to Rg Response of different samples are shown in table 1. Re is the ratio sory properties and table 1 supports this idea. CuO-SnO 2 multilayer of primitive resistance (Ra) to secondary resistance (3). result in much higher decrease in the sensor resistance from Ra to Rg.so that multilayer sample has better result than SnO 2 (PVD). (4) Cracks on the surface can be considered as a useful pathway for the diffusion of the sensing gas According to the table 1 with obtained result produced multilayer Table 1: Investigation of the response time, response and recovery time of the prepared samples. easier and cheaper and using this method leads to better sensory CuO-SnO 2 has some benefits such as: the production method is properties. STEM Fellowship Journal vol.1 issue 12

5 NANOPARTICLES 25 Conclusion The results demonstrated that synthesis of CuO through sol-gel and deposition on the surface of the conductive glass leads to a soft surface together with several cracks. This information was obtained using atomic force microscope (AFM). Moreover, these cracks on the surface improved the sensing properties. In addition, heating the samples resulted in crystallization of the particles and in fact improved the recovery time compared to the previous samples. The response time of 1180 was also as one of the strengths of this sensor. Acknowledgements Authors are grateful to Mr. Babaei and Mr. Shokati and also thankful to the Mofid high school. References 1. Amosa MK, Mohammed IA, Yaro SA. Sulphide scavengers in oil and gas industry A review. NAFTA. 2010;61(2): Verma MK, Gupta V. A highly sensitive SnO2 CuO multilayered sensor structure for detection of H2S gas. Sensors and Actuators B: Chemical. 2012; : Chowdhuri A, Sharma P, Gupta V, Sreenivas K, Rao KV. H2S gas sensing mechanism of SnO2 films with ultrathin CuO dotted islands. Journal of Applied Physics. 2002;92(4): Delgado RD. Tin oxide gas sensors: an electrochemical approach. Spain: Universitat Autònoma de Barcelona; Zhang Q, Zhang K, Xu D, Yang G, Huang H, Nie F, Liu C,Yang S. CuO nanostructures: Synthesis, characterization, growth mechanisms, fundamental properties, and applications. Progress in Materials Science. 2014;60: Xu C, Tamaki J, Miura N, Yamazoe N. Grain size effects on gas sensitivity of porous Sn02-based elements Sensors and Actuators B. 1991;3: Wang C, Yin L, Zhang L, Xiang D, Gao R. Metal oxide gas sensors: sensitivity and influencing factors. Sensors. 2010;10(3): Shao F, Hoffmann MWG, Prades JD, Zamani R, Arbiol J, Morante JR, et al. Heterostructured p-cuo (nanoparticle)/n-sno2 (nanowire) devices for selective H2S detection. Sensors and Actuators B: Chemical. 2013;181: Wang S, Xiao Y, Shi D, Liu HK, Dou SX. Fast response detection of H2S by CuO-doped SnO2 films prepared by electrodeposition and oxidization at low temperature. Materials Chemistry and Physics. 2011;130(3): STEM 2015 Fellowship Journal journal.stemfellowship.org vol.1 issue 1

6 26 NANOPARTICLES REVIEW by Dr. Kelley Barsanti Assistant Professor of Chemical Engineering, Bourns College of Engineering, University of California Riverside This manuscript was a pleasure to read. The language is, for the most part, quite clear, and the figures present well. The subject matter (metal oxide-based gas sensors) is a very important technology and will be of interest to your readers. Mofid High School is apparently a top-notch school and must be proud of the authors, who are evidently on their way to accomplishing good things in Iran. I recommend this paper for publication and make the following suggestions for improvement. The authors state (p. 2), Moreover, it can be expressed that sol-gel and deposition using immersion method have not been used in order to construct H2S gas sensor based on the nanoparticles. I am not immersed in the literature of metaloxide gas sensors, but that statement does not ring true for me. The sol-gel method and dip coating are very common in this field, and H2S is a common analyte for sensors. It is very likely that someone has made an H2S sensor using those methods. The authors would have to do a much more exhaustive search of the literature before making such a claim. Figure 1 shows electron micrographs of SnO2 as deposited on the ITO glass substrate. However, there is no control micrograph showing the ITO substrate alone. ITO substrates tend to be rough, polycrystalline surfaces, and at least some of the roughness would be due to the substrate particles. There was no description of PVD in the methods section. The process should be described fully. Some of the other methods need more fulsome descriptions, as well. I have annotated the manuscript where I thought additional detail was warranted. The authors are kindly asked to read the notes on other subjects as well and consider them before submitting the final paper. Panels A and B in fig. 3 appear to be reversed. The micrograph in fig. 2A shows a bare patch of SnO2 not covered by CuO. The higher roughness of fig. 3B is consistent with that. It is also more consistent with the text. The authors should review fig. 3 and ensure that the panels are correctly displayed. Just two more (minor) criticisms: Names of compounds (e.g. copper oxide) should not be capitalised and chemical formulae should not be italicised STEM Fellowship Journal journal.stemfellowship.org vol.1 issue 12