Preparation of ZnO nanostructures by RF-magnetron sputtering on thermally oxidized porous silicon substrate for VOC sensing application

Highlights

• Porous structures were introduced to ZnO by using a thermally oxidized PS.

• The sensitivity of the ZnO/oxidized PS was 83.4% for 22 ppm of ethanol gases.

• High porosity, and abundant oxygen vacancies enhanced the sensing.

• The sensor showed favorable selectivity for ethanol over acetone.

Abstract

ZnO nanostructures were grown on thermally oxidized porous silicon (PS) substrates by RF-magnetron sputtering for volatile organic compounds (VOCs) sensing applications. PS was fabricated by electrochemical anodization of n-type crystalline silicon in an ethanoic hydrofluoric acid solution to produce a PS surface. X-ray diffraction (XRD) analysis confirmed that the ZnO nanostructure highly preferred an (0 0 2) phase orientation. The sensitivities of the ZnO/oxidized PS sensor investigated for 22 ppm of acetone, ethanol, iso-propanol, and toluene vapor at the optimum operating temperature of 250 °C were 79.6%, 83.4%, 69.3%, and 69%, respectively. The ZnO/oxidized PS sensor can be detect VOC vapor up to 2 ppm.

Introduction

The ZnO nanostructure has emerged as one of the most promising oxide materials because of its numerous industrial applications in the fields of medicine, pigments, catalysts, ceramics, and rubber additives. ZnO nanostructures also have potential applications in solar cells, electrodes, sensors, transparent UV protection films, UV light emission, surface acoustic waves, and magneto-optical devices [1], [2], [3], [4], [5], [6], [7], [8]. Several techniques can be used to synthesize ZnO nanocrystals, such as microwave-assisted chemical bath deposition, sol–gel, pulsed laser deposition, ultrasonic assisted chemical vapor deposition, RF-magnetron sputtering, molecular beam epitaxy, and vacuum arc deposition [4], [5], [9], [10], [11], [12], [13], [14]. Among these methods, RF-magnetron sputtering draws increased interest because of its high purity, low substrate temperature, high interfacial adhesion, uniformity, homogeneity, and nanostructural features [13], [15], [16]. Nanostructure crystallites produced by RF-magnetron sputtering are highly dense over the entire substrate area, indicating low porosity. Thus, these nanostructure crystallites have a low surface-to-volume ratio compared with one-dimensional nanorods, leading to poor sensitivity as well as long response and recovery time. To enhance the sensitivity of these nanostructure, porous structures are introduced into the ZnO nanostructure to increase its surface-to-volume ratio. The enlarged reaction area between the ZnO crystallite and gas molecules or incident photon corresponds to an increase in gas sensing, photoresponse, or any other sensing application of ZnO. PS substrate has been widely used to examine porous materials because of its adjustable roughness and structure as well as low cost [17], [18], [19], [20], [21]. Several reports suggested that the porous layer is a good substrate in lattice mismatch heteroepitaxy because of its special surface morphology [17]. Generally, the crystal quality of ZnO depends on growth temperature, thickness, annealing of the buffer layers, and passivation of the silicon substrate surface [17]. Kayahan [20] studied the effect of annealing on the photoluminescence (PL) spectra of ZnO thin films deposited on PS by RF-magnetron sputtering. Rajabi et al. [22] prepared ZnO tetrapods on silicon and thermally oxidized PS substrates with and without an Au catalyst layer by carbothermal reduction of ZnO powder through chemical vapor transport and condensation method and then studied its PL and structural properties. Kou et al. [23] fabricated photoelectric materials based on ZnO nanoflowers and nanosheets on PS at different applied potentials by electrodeposition. Most of these studies focused on characterization and rarely discussed or addressed the application of these structures, especially in the gas sensor field. However, volatile organic compound (VOC) vapors are among the primary sources of indoor environmental pollutants. VOC vapors are seriously harmful to the human body causing seasonal allergies, asthma, emphysema, and different types of cancers because of the rapid evaporation and toxic or carcinogenic nature of these compounds [3], [24]. A number of studies have focused on the preparation of ZnO gas sensor given the simplicity of its preparation and high chemical stability [3], [25], [26]. Al-Hardan et al. [3] prepared ZnO thin films by RF-magnetron sputtering on thermally oxidized silicon and found that the sensitivity [((R_a − R_g)/R_a) × 100] for 500 ppm of ethanol was approximately 100 at an optimal operating temperature of 400 °C. Zhao et al. [14] fabricated Al-doped ZnO nanofibers on ceramic tubes and obtained responses (R_a/R_g) equal to 8 and 2 for 500 ppm of ethanol and toluene, respectively at 250 °C. These ZnO nanostructures based sensors were found to have low response and/or high operating temperature. The present study investigates VOC sensors based on ZnO nanostructures grown on oxidized PS by RF-magnetron sputtering, and to the best of our knowledge, is the first to report on the subject.

In this work, ZnO nanostructures prepared by RF-magnetron sputtering on thermally oxidized PS substrate was examined for gas sensing application using low concentration (2 ppm to 87 ppm) of VOCs, including the following: acetone, C₃H₆O; ethanol, C₂H₆O; iso-propanol, C₃H₈O; and toluene, C₇H₈ vapor. PS was used to modify the surface morphology of the ZnO nanostructure to increase the surface-to-volume ratio. The structural, morphological, surface roughness, and PL were successfully analyzed. Sensitivity (S), response time, and recovery time were also studied at the optimal operating temperature of 250 °C.