rus
Issue #2/2023
A.V.Smirnov
SYNTHESIS AND STUDY OF NICKEL OXIDE AND LINEAR-CHAIN CARBON FILM COMPOSITES
SYNTHESIS AND STUDY OF NICKEL OXIDE AND LINEAR-CHAIN CARBON FILM COMPOSITES
DOI: https://doi.org/10.22184/1993-8578.2023.16.2.132.137
This paper considers an experiment on the synthesis of semiconductor nickel oxide films and composites of nickel oxide films with linear-chain carbon as relative humidity sensor elements. Appropriate measurements have been carried out, and an analysis of NiO/LCC nanocomposites sensitivity mechanism, which describes increasing electrical conductivity (decreasing resistance) of film structures with humidity has been given.
This paper considers an experiment on the synthesis of semiconductor nickel oxide films and composites of nickel oxide films with linear-chain carbon as relative humidity sensor elements. Appropriate measurements have been carried out, and an analysis of NiO/LCC nanocomposites sensitivity mechanism, which describes increasing electrical conductivity (decreasing resistance) of film structures with humidity has been given.
Теги: lcc films nanomaterials ni films resistive humidity sensors наноматериалы пленки лцу пленки никеля сенсоры влажности резистивного типа
INTRODUCTION
Humidity is an important parameter in many different fields of science and technology. Humidity sensors are widely used in various fields (agriculture, medicine, industry) and are designed to control optimum conditions (storage of agricultural products 85–95%, living quarters 40–45%, etc.).
Several technologies are currently used to measure relative humidity, using property of different structures to change their physical parameters (capacitance, resistance, conductivity) depending on degree of saturation with water vapour. For existing moisture sensors, there are disadvantages such as long surface recovery times after several cycles of adsorption-desorption of water molecules and relatively low sensitivity.
In order to achieve excellent gas sensing properties, metal oxide nanomaterials with a high surface area to volume ratio, necessary for better adsorption/desorption phenomena of the analysed gas, are produced.
RESEARCH METHODS
Copper films were deposited on glass substrates by thermoresistive evaporation in vacuum at UVR-3M vacuum apparatus at pressure of 10–2–10–3 Pa, then the samples were annealed in MIMP-VM oven at 250 °C for 20 minutes and at 400 °C. For another group of samples to produce metal oxide-carbon systems, copper films were deposited on substrates and then linear-chain carbon (LCC) films were deposited by ion-plasma synthesis [1] and thermally oxidized in an oven at 400 °C. The structure of LCC films are the multiple layers, each consisting of chains of sp1-hybridized carbon atoms oriented normally to the layer surface. The chains are joined by Van der Waals forces into a hexagonal structure with a spacing between them of about 5 Å. The chains are curved were hydrogen atoms attached at the ends of the bends. The presence of delocalised electrons belonging to the whole LCC molecule ensures metallic conductivity along the chain. The lack of bonding between the chains makes the film a dielectric in perpendicular direction. This unique electrical property of the film is its record-breaking anisotropy of electro-physical properties.
The experimental samples were nickel oxide films on K8 glass substrates, some of them were coated with 1000 Å thick line-chain carbon. The NiO films were obtained in two stages. The first stage was the production of nickel thin films. The second stage was the nickel films heat treatment (oxidation).
To carry out the first stage the method of thermal deposition in vacuum was applied. "UVR-3M" and "U.V.N.I.P.A." units were used for vacuum deposition at a pressure of 10–2 to 10–3 Pa onto substrates and thin films of nickel were deposited. In the second stage, a MIMP-VM furnace was used for heat treatment.
RESULTS AND DISCUSSION
Resistance measurements were carried out as a function of relative humidity (RH %) at a fixed ambient temperature of 25 °C. All of the sensor samples studied indicated decreasing of resistance values with increasing relative humidity, indicating that conductivity occurs mainly on the grains surface and is determined by adsorbed water molecules [2]. Thus in [3] resistance of CuO–NiO changed by almost three orders of magnitude with an increase in relative humidity from 5% to 90%. Resistance changes in porous oxides with increasing levels of humidity due to adsorption and capillary condensation of water. At low levels of humidity chemisorption occurs leading to formation of two surface hydroxyls with charge transfer occurring by a stepwise mechanism [4]. At high levels of humidity, water is physically adsorbed on top of the chemisorbed layer (Fig.3, 4). When the initial water molecules are adsorbed, each one is hydrogen-bonded to two ionised hydroxyl groups OH- and the dominant charge carrier on the surface will be H3O+. When more water vapour is adsorbed, the water molecules cluster, forming a liquid-like multi-layer film of hydrogen-containing water molecules, where each water molecule is only separately bound to a hydroxyl group. Since the dissociation of H3O+ into H2O and H+ is energetically advantageous, H+ is the dominant charge carrier under high humidity conditions [5]. Given the model of carbon chain of LCC with inclusion of hydrogen atoms H at the bends of the chain (Fig.1), this would presumably give additional molecule adsorption centres and increase sensitivity of NiO/LCC film structures to humidity.
The samples obtained were tested for their gas-sensitive properties: in air atmosphere at varying humidity, in ethanol vapour, ammonia and carbon monoxide. The observed increase of conductivity (and hence sensitivity) is due to a combination of chemisorption, physical sorption and/or capillary condensation of water when relative humidity increases from 25 to 85 %. All of the tested sensor samples indicated decreasing of resistance values with increasing relative humidity, it is shown that conductivity occurred predominantly on the grain surface, which was regulated by adsorbed water molecules. The surface of most metal oxides is coated with hydroxyl groups, and affects humid atmosphere in such a way that water molecules are further adsorbed on it by means of hydrogen bonding (Fig.2).
Figure 3 presents sensitivity and diagram of the change in electrical resistance of NiO and NiO/LCC film structures as a function of relative humidity.
Sensitivity S = (Rk–R0/R0)·100% for NiO sample (t-o 400 °C) at low humidity levels is slightly higher (Fig.4) than that of Ni + LCC sample (t-o 400 °C ), at higher humidity levels sensitivity of Ni+LCC (t-o 400 °C) is significantly greater. Correspondingly, the response k = Rk/R0 for the non-LCC samples for a relative humidity level of 40...85 RH % was 1.9...12.2; for the samples with LCC it was 1.6...54.
Study of operating cycles (Fig.5, 6) of the samples obtained at 80% humidity, were obtained the response time of Ni sample (t-o 400 °C) is 6 s, which exceeds the experimental results of Ni+LCC sample (t-o 400 °C) by 3 s. Considering recovery time, the Ni+LCU sample (t-o 400 °C) showed a two-fold superiority over the Ni sample (t-o 400 °C), values of 35 seconds and 63 seconds respectively (Table 2). This suggests that linear-chain carbon has little effect on the response time and is very important for recovery time LCC has vacancies for adsorption and desorption of OH- ions (hydrogen atoms on the bends of the carbon chains).
CONCLUSIONS
All the relative humidity sensors studied present decreasing of resistance values with increasing relative humidity of the medium. It was shown, that conductivity occurred predominantly on the grain surface, which was regulated by adsorbed water molecules. The surface of most metal oxides was covered with hydroxyl groups when exposed to the humid atmosphere in such a way that hydrogen bonding further adsorbed the water molecules. The use of LCC films in the composite improves the sensory characteristics of NiO films. The response and recovery times for the sensitive NiO/LCC films were 10 and 35 seconds, respectively.
PEER REVIEW INFO
Editorial board thanks the anonymous reviewer(s) for their contribution to the peer review of this work. It is also grateful for their consent to publish papers on the journal’s website and SEL eLibrary eLIBRARY.RU.
Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Humidity is an important parameter in many different fields of science and technology. Humidity sensors are widely used in various fields (agriculture, medicine, industry) and are designed to control optimum conditions (storage of agricultural products 85–95%, living quarters 40–45%, etc.).
Several technologies are currently used to measure relative humidity, using property of different structures to change their physical parameters (capacitance, resistance, conductivity) depending on degree of saturation with water vapour. For existing moisture sensors, there are disadvantages such as long surface recovery times after several cycles of adsorption-desorption of water molecules and relatively low sensitivity.
In order to achieve excellent gas sensing properties, metal oxide nanomaterials with a high surface area to volume ratio, necessary for better adsorption/desorption phenomena of the analysed gas, are produced.
RESEARCH METHODS
Copper films were deposited on glass substrates by thermoresistive evaporation in vacuum at UVR-3M vacuum apparatus at pressure of 10–2–10–3 Pa, then the samples were annealed in MIMP-VM oven at 250 °C for 20 minutes and at 400 °C. For another group of samples to produce metal oxide-carbon systems, copper films were deposited on substrates and then linear-chain carbon (LCC) films were deposited by ion-plasma synthesis [1] and thermally oxidized in an oven at 400 °C. The structure of LCC films are the multiple layers, each consisting of chains of sp1-hybridized carbon atoms oriented normally to the layer surface. The chains are joined by Van der Waals forces into a hexagonal structure with a spacing between them of about 5 Å. The chains are curved were hydrogen atoms attached at the ends of the bends. The presence of delocalised electrons belonging to the whole LCC molecule ensures metallic conductivity along the chain. The lack of bonding between the chains makes the film a dielectric in perpendicular direction. This unique electrical property of the film is its record-breaking anisotropy of electro-physical properties.
The experimental samples were nickel oxide films on K8 glass substrates, some of them were coated with 1000 Å thick line-chain carbon. The NiO films were obtained in two stages. The first stage was the production of nickel thin films. The second stage was the nickel films heat treatment (oxidation).
To carry out the first stage the method of thermal deposition in vacuum was applied. "UVR-3M" and "U.V.N.I.P.A." units were used for vacuum deposition at a pressure of 10–2 to 10–3 Pa onto substrates and thin films of nickel were deposited. In the second stage, a MIMP-VM furnace was used for heat treatment.
RESULTS AND DISCUSSION
Resistance measurements were carried out as a function of relative humidity (RH %) at a fixed ambient temperature of 25 °C. All of the sensor samples studied indicated decreasing of resistance values with increasing relative humidity, indicating that conductivity occurs mainly on the grains surface and is determined by adsorbed water molecules [2]. Thus in [3] resistance of CuO–NiO changed by almost three orders of magnitude with an increase in relative humidity from 5% to 90%. Resistance changes in porous oxides with increasing levels of humidity due to adsorption and capillary condensation of water. At low levels of humidity chemisorption occurs leading to formation of two surface hydroxyls with charge transfer occurring by a stepwise mechanism [4]. At high levels of humidity, water is physically adsorbed on top of the chemisorbed layer (Fig.3, 4). When the initial water molecules are adsorbed, each one is hydrogen-bonded to two ionised hydroxyl groups OH- and the dominant charge carrier on the surface will be H3O+. When more water vapour is adsorbed, the water molecules cluster, forming a liquid-like multi-layer film of hydrogen-containing water molecules, where each water molecule is only separately bound to a hydroxyl group. Since the dissociation of H3O+ into H2O and H+ is energetically advantageous, H+ is the dominant charge carrier under high humidity conditions [5]. Given the model of carbon chain of LCC with inclusion of hydrogen atoms H at the bends of the chain (Fig.1), this would presumably give additional molecule adsorption centres and increase sensitivity of NiO/LCC film structures to humidity.
The samples obtained were tested for their gas-sensitive properties: in air atmosphere at varying humidity, in ethanol vapour, ammonia and carbon monoxide. The observed increase of conductivity (and hence sensitivity) is due to a combination of chemisorption, physical sorption and/or capillary condensation of water when relative humidity increases from 25 to 85 %. All of the tested sensor samples indicated decreasing of resistance values with increasing relative humidity, it is shown that conductivity occurred predominantly on the grain surface, which was regulated by adsorbed water molecules. The surface of most metal oxides is coated with hydroxyl groups, and affects humid atmosphere in such a way that water molecules are further adsorbed on it by means of hydrogen bonding (Fig.2).
Figure 3 presents sensitivity and diagram of the change in electrical resistance of NiO and NiO/LCC film structures as a function of relative humidity.
Sensitivity S = (Rk–R0/R0)·100% for NiO sample (t-o 400 °C) at low humidity levels is slightly higher (Fig.4) than that of Ni + LCC sample (t-o 400 °C ), at higher humidity levels sensitivity of Ni+LCC (t-o 400 °C) is significantly greater. Correspondingly, the response k = Rk/R0 for the non-LCC samples for a relative humidity level of 40...85 RH % was 1.9...12.2; for the samples with LCC it was 1.6...54.
Study of operating cycles (Fig.5, 6) of the samples obtained at 80% humidity, were obtained the response time of Ni sample (t-o 400 °C) is 6 s, which exceeds the experimental results of Ni+LCC sample (t-o 400 °C) by 3 s. Considering recovery time, the Ni+LCU sample (t-o 400 °C) showed a two-fold superiority over the Ni sample (t-o 400 °C), values of 35 seconds and 63 seconds respectively (Table 2). This suggests that linear-chain carbon has little effect on the response time and is very important for recovery time LCC has vacancies for adsorption and desorption of OH- ions (hydrogen atoms on the bends of the carbon chains).
CONCLUSIONS
All the relative humidity sensors studied present decreasing of resistance values with increasing relative humidity of the medium. It was shown, that conductivity occurred predominantly on the grain surface, which was regulated by adsorbed water molecules. The surface of most metal oxides was covered with hydroxyl groups when exposed to the humid atmosphere in such a way that hydrogen bonding further adsorbed the water molecules. The use of LCC films in the composite improves the sensory characteristics of NiO films. The response and recovery times for the sensitive NiO/LCC films were 10 and 35 seconds, respectively.
PEER REVIEW INFO
Editorial board thanks the anonymous reviewer(s) for their contribution to the peer review of this work. It is also grateful for their consent to publish papers on the journal’s website and SEL eLibrary eLIBRARY.RU.
Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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