Gas dependent sensing mechanism in ZnO nanobelt sensor

Highlights

• ZnO nanobelts exhibit an appreciable response towards H₂S and NO.

• At room temperature, sensor recovers completely after exposure to NO (1 to 60 ppm).

• At room temperature, incomplete recovery observed on exposure to higher concentrations of H2S (> 5 ppm).

• Complete recovery on exposure to concentrations higher than 5 ppm H₂S is achieved by heating the sensor films at 250 °C.

• Incomplete recovery after exposure to higher concentrations of H₂S is due to formation of ZnS.

Abstract

Gas sensing properties of ZnO nanobelts synthesized using carbothermal reduction method has been investigated. At room temperature (28 °C), the sensor films exhibit an appreciable response towards H₂S and NO and response of these two gases were studied as a function of concentration. For NO the sensor films exhibit a complete reversible curve for the concentration range between 1 and 60 ppm. However, for H₂S a complete recovery was obtained for concentration <5 ppm and for higher concentration a partial recovery of the baseline resistance was observed. The reason for the incomplete recovery was investigated using X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) studies of the sample before and after the H₂S exposure. After exposure, appearance of an additional peak at 26.6° corresponding to the formation of ZnS was observed in XRD. Formation of additional phase was further corroborated using the results of XPS. H₂S exposure causes decrease in the intensity of O 1s peak and appearance of sulphide peaks at binding energies of 162.8 and 161.8 eV corresponding to S-2p peaks – 2p_3/2 and 2p_1/2, confirms the formation of ZnS upon exposure.

Introduction

Semiconducting metal oxides are widely used as commercial sensors as they exhibit a wide range of electronic, chemical, and physical properties that are often highly sensitive to chemical environment [1]. These electronic, chemical, and physical properties of oxides are created and can be tuned due to their two unique structural features: mixed cation valencies and an adjustable oxygen deficiency. Among the various oxides, zinc oxide (ZnO) is one of the promising material for gas sensing due to its non-toxicity, high chemical and physical stability and low cost. The structure of ZnO mainly comprises of number of planes containing tetrahedrally coordinated O²⁻ and Zn²⁺ ions, stacked alternately along the c-axis. The oppositely charged ions produce positively charged (0001)-Zn and negatively charged (000ī)-O polar surfaces, resulting in a normal dipole moment and spontaneous polarization along the c-axis, as well as a divergence in surface energy [2]. It has three fastest growth directions namely 〈0001〉, 〈0110〉, and 〈2110〉. Importantly, ZnO exhibit a wealth of interesting nanostructures that have demonstrated the potential to realize highly sensitive gas sensors. The different nanoforms of ZnO include nanoparticles, nanowires, nanobelts, nanosheets, nanoflowers, hexagonal discs, cones and much more. The surface area to volume ratio exhibited by these nanoforms has been utilized effectively to realize sensors for various gases. These include CH₃CH₂OH, CH₃COCH₃, O₃, NO₂, CO, H₂S, H₂, HCHO, NH₃, LPG etc [3], [4], [5], [6], [7], [8], [9], [10], [11]. Sensor response is reported to increase with decrease in grain size [12], [13], [14]. Traditionally, ZnO gas sensors are implemented as chemiresistive sensors, which work on the principle of change in electrical resistance due to interaction between the semiconductor and the test gas [15], [16].

Of the different nanoforms, nanobelts which have a rectangular cross section, in correspondence to a beltlike (or ribbonlike) morphology with width-to-thickness ratios of 5 to 10 are particularly promising for gas sensing applications due to their smaller thickness. Nanobelts are generally more defect rich in comparison to that of nanowire counterparts [17]. However there are only a few studies on investigation of gas sensing properties of ZnO nanobelts [18], [19], [20], [21], [22]. In the present work, H₂S and NO gas sensing properties of ZnO nanobelts sensor has been investigated.

Nitric oxide (NO) is readily converted in the air to nitrogen dioxide (NO₂) [23] with a formation rate constant of 1.2 × 10¹¹ ppm⁻² s⁻¹ [24]. The time to reach 5 ppm NO₂ from 20 ppm nitric oxide in 100% oxygen is 12 min, while in air it is more than 1 h.

Nitric oxide (NO) reacts with moisture and oxygen present in skin, eye and mucous membrane, and gets converted into nitric and nitrous acids, thus acting as an irritant. Inhalation of (NO) causes coughing and shortness of breath, along with a burning in the throat and chest [25]. Further NO makes the blood vessels expand, which would make people fainted [26].

At high concentration levels, nitrogen dioxide (NO₂) is potentially toxic to plants, it reduces growth and yield of leaves. In combination with either ozone (O₃) or sulphur dioxide (SO₂), nitrogen dioxide may cause injury at even lower concentration levels. As one of the components of smog, nitrogen dioxide (NO₂) is known to irritate the lungs and increase susceptibility to respiratory infections.

H₂S is one of the highly toxic and flammable gases being used in various industrial applications like oil and gas industries, pulp and paper industries, waste water treatment plants and nuclear reactors (heavy water plants). It has a characteristic rotten egg odor and can be detected at concentrations >0.13 ppm by human nose. However, repeated exposures or exposure to higher concentrations causes a drastic decline in the ability to smell. Importantly, it can interact with the enzymes in the blood stream inhibiting the cell respiration. This implies exposure to high concentrations can stop the functioning of lung and could be fatal. The long term (8 h) and short term (10 min) exposure limits of H₂S are 10 and 15 ppm, respectively [27]. Accordingly, it is crucial to detect H₂S at such concentration levels.

In the present work, gas sensing properties of ZnO nanobelts synthesized using carbothermal reduction method have been investigated in detail. At room temperature (∼28 °C) nanobelts were found to be sensitive towards H₂S and NO. In case of NO, a complete recovery was observed for concentration <60 ppm whereas for H₂S, complete recovery was observed for concentration <5 ppm. For higher concentrations, a complete recovery is achieved upon heating the sensor film at higher temperature of 250 °C. The slow or partial recovery at H₂S concentrations >5 ppm has been attributed to formation of ZnS. The results of x-ray photo electron and x-ray diffraction further confirmed the formation of ZnS.