Controlling the Response of Gas-Sensitive Zinc Oxide Nanostructures to Water Vapor ^†

Abstract

Gas-sensitive devices have great potential for use in a variety of applications, including environmental monitoring, medicine, and various industries. The stability of gas sensors based on zinc oxide can be significantly affected by the presence of water vapors in the atmosphere. Gas-sensitive layers based on ZnO nanowires were synthesized by the hydrothermal method. The effect of different seed layers was studied in order to optimize the sensor properties of zinc oxide nanostructures. It was shown that layers consisting of zinc oxide nanowires synthesized using sacrificial doping on the seed layers of ZnO nanoparticles exhibited a moderate response to the vapor of volatile organic compounds, with almost no response to water vapor.

1. Introduction

Gas-sensitive devices have a wide range of applications, including environmental monitoring, biomedical equipment, and the pharmaceutical industry [1]. Their operation mechanism is based on changing the electrical resistance of oxide materials when gas is absorbed onto the surface of a heated oxide [2]. Recently, zinc oxide nanostructures are gaining attention as a promising material for highly sensitive and selective gas sensors [3]. ZnO has several advantages, including high biocompatibility, chemical stability, environmental safety, and low cost [4]. It can be used to detect various gasses, such as hydrogen (H₂), ammonia (NH₃), methane (CH₄), carbon monoxide (CO), nitrogen dioxide (NO₂), ethanol, and acetone [5].

Depending on the synthesis method, zinc oxide forms a wide variety of crystalline structures, typically with a hexagonal wurtzite arrangement in which each oxygen (O²⁻) ion is surrounded by four zinc (Zn²⁺) ions at the vertices of a tetrahedron (Figure 1). This tetrahedral and non-centrosymmetric arrangement with polar symmetry along the hexagonal axis is responsible for the anisotropic growth of ZnO crystals [6]. This leads to a variety of nanostructure morphologies, including nanorods, nanowires, nanofibers, nanotubes, quantum dots, nanoparticles, nanofilms, and three-dimensional hierarchical structures [7].

One of the most significant factors influencing the characteristics of semiconductor gas sensors is the presence of water vapors [8], which can competitively adsorb on the surface of metal oxide structures. The effect of humidity on the response of TiO₂ nanowires to ethanol vapors was studied in [9]. Authors found that as humidity increased, the sensor response decreased by almost 100-fold. Among all possible defects in ZnO, oxygen vacancies are crucial for enhancing the sensitivity of gas sensors [10,11]. In [12], it was demonstrated that zinc oxide nanorods with high oxygen vacancy content exhibit the highest response to ethanol vapor. An increased number of free electrons contribute to chemisorption of different oxygen particles, which participate in surface oxidation of ethanol. There are several approaches to escape the impact of humidity on the performance of semiconductor gas sensors. One of the simplest methods is surface treatment, which can be carried out by adding noble metals, alloying, modifying with hydrophobic materials, or combining with hydrophilic materials.

Subsequent processing of sensor materials can also improve their stability in humid environments. The proton beam surface treatment of zinc oxide (ZnO), described in [13], reduces response deviation when detecting nitrogen dioxide (NO₂) in a humid environment. This is due to the formation of vacancies in the ZnO structure, which have a higher adsorption energy for NO₂ molecules than for H₂O molecules. In addition, such processes as plasma fluorination of carbon nanotubes [14] and chemical etching [15] can also be used for processing. Water vapor exposure affects not only the baseline resistance and response of the gas sensor to the target gas, but also its response and recovery times. Humidity compensation can be achieved using mathematical models and neural networks to accurately calculate the concentration of the target gas.

In this paper, we have developed approaches to the synthesis of gas-sensitive zinc oxide layers that are resistant to the influence of water vapor. We have studied the influence of seed layer type on the sensor properties of zinc oxide nanostructures.

2. Experiment

In this study, the synthesis of gas-sensitive ZnO layers consisted of two stages. First, a seed layer was deposited onto substrates using the sol–gel method or precipitated from an aqueous solution of zinc salt. Finally, nanowires were grown using hydrothermal synthesis. The solutions were deposited on ceramic sensor platforms with gold electrodes in an interdigitated configuration [16]. The seed layer was prepared by the sol–gel method using zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O) and tetraethoxysilane (TEOS) [17,18,19]. Ethanol served as a solvent for the solutions. After dissolving the precursors in ethanol, the solution was deposited on the ceramic substrate by spin-coating. The substrate was then annealed at 350 °C for 30 min.

To form seed zinc oxide nanoparticles, five layers of an aqueous solution of zinc acetate dihydrate (Zn(CH₃COO)₂·2H₂O) were applied to the sensor platform using spin-coating with subsequent annealing for 15 min at 500 °C. Zinc oxide nanowires were then formed on the seed layers through hydrothermal synthesis [20,21,22,23], which was carried out in equimolar solutions of hexamethylenetetramine (C₆H₁₂N₄) and zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O). Sodium iodide (NaI) or sodium bromide (NaBr) were added to the solutions to realize sacrificial doping. The corresponding samples were labeled as ZnO(I) and ZnO(Br). The layers of ZnO nanowires were grown for one hour at 85 °C and were subsequently washed with distilled water before being annealed at 500 °C for 30 min.

The surface morphology of the seed layers and nanowire layers was analyzed using atomic force microscopy (AFM) and scanning electron microscopy (SEM). The sensor properties of synthesized nanostructures were studied. The experimental setup allows us to monitor the change in resistance of the sample as air and test gas are introduced in the cell. The air flow passes through silica gel for dehumidification and splits into two streams. The first stream of dry air is directed immediately toward the sample, while the second stream is saturated with vapors of studied substance, mixed with the dry air upon opening an electromechanical valve, and introduced into the cell with a fixed sample. Measurements were taken at 250 °C. The test gasses were isopropyl alcohol vapors at a concentration of 1000 ppm and water vapors at a concentration of 74,000 ppm. The graphs of the resistance dependence on time were built. To conduct a comparative analysis of the samples, the response values (S) to the vapors of the target gas were calculated as the ratios of the resistance of the gas-sensitive layer in air to its resistance in the presence of a given concentration of the target gas (vapors of isopropyl alcohol or water) in the atmosphere.

3. Results and Discussion

The AFM images of the synthesized seed layers are shown in Figure 2. As a result of sol–gel synthesis, porous net-like structures with a cross-section of branches in the micrometer range were formed. Nanoparticles with sizes ranging from several hundred nanometers to 1 micrometer were synthesized from an aqueous solution of a precursor salt. The results of the morphology study of the synthesized zinc oxide nanowires are shown in Figure 3. The diameter of the nanowires ranges from 20 to 120 nanometers, and the average length is approximately 200 nanometers.

As shown in our previous studies [16,21], the sacrificial doping of iodine and bromine leads to an increase in oxygen vacancies on the surface of zinc oxide nanowires. This, in turn, results in an increased gas-sensitive response.

The analysis of the interaction between samples of zinc oxide nanowires synthesized on different types of seed layers showed moderate response values, as shown in

Table 1. Sensor responses of samples formed on different types of seed layers to vapors of isopropyl alcohol and water.

Seed Layer	Nanowires Type	S(C3H7OH)	S(H2O)
ZnO-SiO2 nanocomposites	ZnO(I)	1.58	1.84
	ZnO(Br)	1.67	2.17
ZnO nanoparticles	ZnO(I)	2.97	1.21
	ZnO(Br)	4.03	1.18

.

The gas-sensitive layers grown on ZnO-SiO₂ nanocomposites exhibit similar response values to water vapors and isopropyl alcohol vapors. This can be explained by the presence of silicon dioxide (SiO₂) fragments on the surface of the composites, which act as sites for the predominant adsorption of water molecules [24]. In this case, the ZnO-SiO₂ nanocomposite layer controls the flow of the current through the sample. Changes in resistance occur due to changes in the thickness of conductive branches within the net-like structure, which is influenced by the Debye length. Under chosen conditions of sol–gel synthesis, branches with thicknesses ranging from hundreds of nanometers to micrometers are formed, resulting in moderate response values.

There are two mechanisms that can explain the effect of water vapors on the resistance of gas-sensitive layers. First, the adsorbed water vapors reduce the number of available adsorption sites, which in turn reduces the amount of oxygen that can be adsorbed on the surface of the semiconductor layer [25]. Second, the chemically adsorbed water molecule acts as a reducing agent and an electron donor thereby reducing the baseline resistance of the sensor [26].

At the same time, the response of the samples of zinc oxide nanowires synthesized on the seed layers of zinc oxide nanoparticles to isopropyl alcohol vapors significantly exceeds the response to water vapor. Moderate response values to isopropyl alcohol vapors may indicate that the current flows through the sample via a layer of seed ZnO nanoparticles. A high probability of this can be expected according to the results of the study on the morphology of the sample surface. The seed layer appears to have a large thickness, and the zinc oxide nanowires are not evenly distributed over the surface. Their length is also insufficient to form a continuous conductive network. A possible mechanism of the interaction between synthesized gas-sensitive layers and vapors of water and isopropyl alcohol is shown in Figure 4. When a gas-sensitive zinc oxide layer interacts with water vapor, adsorption occurs with the filling of oxygen vacancies on the surface. Nanowires can significantly increase the active area for water vapor adsorption, as they can act as “capture traps” for water molecules. However, when a current flows through the seed zinc oxide nanoparticle layer, the predominant adsorption of water vapors onto zinc oxide nanowires does not affect the resistance of the gas-sensitive layer. When isopropyl alcohol vapors appear, adsorption occurs on the surface sites of the seed layer, and interaction with negatively charged oxygen reduces the resistance of the gas-sensitive layer.

By choosing the synthesis conditions such that the zinc oxide nanowires are loosely located on the substrate, it is possible to achieve an almost complete absence of response of these gas-sensitive materials to water vapors.

The negative effect of water vapors on the response of gas-sensitive layers may be caused by the formation of hydroxyl (OH) groups from adsorbed water molecules. Unlike conventional interfering gasses, water molecules undergo a two-step adsorption process. As previously mentioned, water molecules are chemically adsorbed at the active sites of the gas-sensitive layer and reduced. After forming the first layer, as relative humidity increases, subsequent layers of water molecules physically adsorb [27]. Water molecules physically sorbed near the chemisorbed water layer can break down into H₃O⁺ and OH⁻ ions due to the strong electrostatic field in the chemisorbed layer [28]. In physically adsorbed water, charge transfer occurs through proton jumping along a series of hydrogen bonds between ions and water molecules [29]. Such proton conductivity dominates over electric current and results in an increase in the conductivity of the sensitive layer and a reduction in its resistance as relative humidity increases.