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Semiconductor gas sensors based on metal oxide are widely used in a number of applications, from health and safety to energy effi-ciency and emission control. Nanomaterials including nanowires, nanorods, and nanoparticles have dominated the research focus in thisfield owing to their large number of surface sites that facilitate surface reactions. Recently, metal oxide hollow structures using soft tem-plates have been developed owing to their high sensing properties with large-area uniformity. Here, we provide a brief overview of metaloxide hollow structures and their gas-sensing properties from the aspects of template size, morphology, and additives. In addition, a gas-sensing mechanism and perspectives are presented. Keywords: Gas sensor, Soft-template method, Nanostructure, Hollow hemisphere, Metal decoration 1. INTRODUCTION The functional convergence of the Internet with radio frequencyidentification, sensors, and smart objects led to the era of theInternet of Things (IoT), which supplies and accesses all real-world information [1]. Sensor technology is the most importantfactor in IoT because sensors offer enormous amounts ofinformation for many types of environment. Accordingly, varioussensors such as gas, pressure, illumination, tilt, temperature, andmotion sensors have been heavily studied to more preciselymeasure and monitor changes in environments [2]. In particular,diverse applications of the gas sensor in broad fields including thedetection of air pollutants and gaseous hazards, homeland security,medical diagnosis, and fuel combustion control have acceleratedstudies for high-performance gas sensors [3-7]. Gas sensors for applications in the IoT should meet specialrequirements such as low cost, miniaturized size, ease ofintegration with electronic circuits, and high sensing performance[8]. Various gas sensors including optical, electrochemical, surfaceacoustic wave, and semiconductor metal oxide sensors have beenextensively studied to fulfill the needs of the IoT [9]. In particular,owing to their simplicity in operation, low cost, flexibility inproduction, small size, and easy integration with electroniccircuits, semiconductor gas sensors based on metal oxides arevery promising as sensing elements for IoT [10-12]. Generally, the gas-sensing properties of metal oxide aredetermined by three basic factors: utility factor, transducerfunction, and receptor function [13]. The first factor is related togas diffusion (porous structure), and the second factor is related tothe mechanism of electron transport between adjacent crystals(neck control). The receptor function is related to the ability of theoxide surface to interact with target gases (metal additives anddecoration). Over the past decade, metal oxide nanostructuressuch as nanoparticles, nanowires, nanotubes, and nanofibers withmetal catalysts have significantly improved the three basic factors,leading to high-performance gas sensors [14]. However, sincethese nanostructures are commonly synthesized using wetchemical methods or transfer methods, developing versatile andreproducible fabrication processes has remained a challenge [15]. An alternative method for achieving highly sensitive metaloxide gas sensors is a soft-template method consisting of polymermicrospheres, which is effective in fabricating long-range orderedsubmicron hollow structures of various metal oxides [16,17]. Inspite of their potential advantages, no review focused on the gas-sensing properties of metal oxide nanostructures fabricated by thesoft-template method has been published to the best of the

Design of Metal Oxide Hollow Structures Using Soft-templating Method for High-Performance Gas Sensors