1 Chemical Sensors Chemical Sensors 2014, 4: 19 Cognizure www.cognizure.com/pubs © Cognizure. All rights reserved. Advances in synthesis of nanostructured metal oxides for chemical sensors Ramireddy Boppella, P. Manjula, S. Arunkumar, Sunkara V. Manorama * Nanomaterials Laboratory, Inorganic & Physical Chemistry Division, Council of Scientific and Industrial Research-Indian Institute of Chemical Technology (CSIR-IICT), Hyderabad-500 007, Andhra Pradesh, India * Author for correspondence: Sunkara V Manorama, email: manorama@iict.res.in Received 01 Nov 2013; Accepted 30 Jan 2014; Available Online 30 Jan 2014 1. Introduction Since historical times, the development of new synthesis procedures for the design and fabrication of nanoscale materials with controlled shape and size has been an exciting field. Significant advances have been made in several directions followed by an understanding of the basic principles underlying the methods to obtain materials with desired properties and subsequently fine tuning and tailoring the procedures to obtain the required product with desired morphology. The selected approach would also be modulated appropriately to meet the requirements of energy conservation and stipulated green technology principles with the expediency of up-scaling. Adopting these practices centre around economic viability and meeting the industrial demand which practically would lead to simpler techniques and versatility to be routinely adopted for similar materials leading to generic procedures. Synthesis of material for a selected application entails imparting the necessary characteristics which leads to render the materials with the desired properties. Today the globe is in need of clean air and environment which is crucial for sustenance of life on earth that is at great risk because of soaring levels of atmospheric pollution and all the associated problems that are primarily impacting health and safety of life on earth. Although our nose serves as a highly sophisticated sensor that can differentiate between hundreds of smells, there are limitations when it comes to sensing toxic, inflammable and hazardous gases. Further the detection of odourless gases and quantification of the gases by the human nose is almost impossible [1]. Therefore, efficient and cost-effective sensor systems are required for the detection and quantification of explosive and toxic gases. This requires the development of suitable materials by various methodologies that are economically viable, and environmentally benign. It has been realized that the advances in nanoscience and nanotechnology have propelled the development of fast and sensitive gas sensors to detect inflammable and toxic gases with high sensitivity [2, 3]. A wide range of nanomaterials have been utilized for sensor fabrication including metals, metal oxides, carbon nanomaterials, chalcogens and many other composite materials [4-12]. Among them semiconductor metal oxides have been proven to have a great potential as sensor materials that are expected to play a major role in alleviating environmental concerns [6, 9]. A great deal of interest is directed towards nanocrystalline semiconductor materials primarily because of the quantum-size effects in the associated optical, magnetic and electrical properties, which account for their high catalytic activity and improved performance [13]. The unique physical properties of nanostructured metal oxides such as large surface to volume ratio, defined morphology, size, dimension, and dielectric environment would greatly influence the device properties. The thermal stability and high mobility of electrons in semiconductors are the desired properties for applications in gas sensing, because these features provide the necessary change of electrical conductivity upon changes in concentration of gas molecule when heated to typical working temperatures [14-16]. The most important attributes of a gas sensor are its sensitivity and selectivity, which are improved by incorporating porosity to the sensing material, reducing the grain size comparable to Debye length, better dispersion of additives on metal oxide and creating test gas specific sites by dopants [17-21]. Most of these properties can be conveniently accomplished by optimizing the sensor materials using the standard nanotechnology processing routes. The existing inadequacy coupled with the demand to innovate better devices is the primary motivation for the research activity on gas sensors to be pursued at several laboratories worldwide. Progress in sensors development requires a combination of skills in materials synthesis and understanding the chemical, physical, thermal and electronic properties of materials to improve the sensing properties, in particular, sensitivity, selectivity, lower operating temperature for practical convenience and long-term stability at typical operating temperatures [22, 23]. Several recent articles have reviewed the importance of grain size, pore size, morphology, structural Abstract Semiconductor gas sensors have received much attention because of their ability to detect most of the inflammable and toxic gases. This review article presents succinctly an overview of the recent trends in materials synthesis methodologies being adopted to realize smart materials with special emphasis on highlighting the requirement for materials suitable for chemical sensing. We begin with the fundamental principle governing the gas sensing mechanism on semiconductor surface followed with prerequisite that would need to be satisfied by the materials to become suitable as ideal gas sensor material. Then a brief description of methods that are currently being adopted for the synthesis of nanostructured materials with a focus on their application as gas sensors is presented. Keywords: Synthesis methods; Metal oxides; Chemical sensor; Sensor mechanism; Deposition techniques; Template directing method