1 Surface Functionality and Water Adsorption Studies of -Aluminium (III) Oxide Nanoparticles by Near Infrared Spectroscopy Peraya Hiranmartsuwan 1, a , Natthaya Siangdee 1, b and Alfred A. Christy 2, c 1 Department of Chemistry, Faculty of Science and Technology, Thammasat University, Pathumthani 12120, Thailand 2 Department of Science, Faculty of Engineering and Science, University of Agder, Sericeboks 422, Kristiansand 4604, Norway a peraya.ph@gmail.com, b natthaya.sci.tu@gmail.com, c alfred.christy@uia.no Keywords: Aluminium (III) Oxide nanoparticle, Near Infrared Spectroscopy, Second derivative, Water adsorption Abstract. The adsorption of water on aluminium (III) oxide nanoparticle surface was studied by near infrared (NIR) spectroscopy. The comparison of NIR spectra at 40% and 60% humidity were reported in this work and were analyzed using second derivative techniques. The second derivative spectra were used to understand the chemistry of adsorption of water molecules. Small amounts of samples were dried under vacuum at 230 °C before the analysis. The analysis of the spectra confirms the presence of three different hydroxyl groups on aluminium (III) oxide surface. The spectra acquired during the adsorption of water molecules show the characteristic peaks in the range of 5400-5100 cm -1 corresponding to the combination band of water molecules hydrogen bonded with hydroxyl groups. There is also evidence for the presence of free water in the bulk of aluminium oxide. Furthermore, the mass of water adsorption on Al 2 O 3 nanoparticle surface have been determined by gravimetric analysis. The gravimetric analysis confirms the adsorption of water molecules by aluminium (III) oxide surface. Introduction Aluminiun (III) oxide is the chemical compound that has been reported for a longtime as one of the important materials in several fields such as ceramics, pigments, cosmetics [1], and catalysis [2]. Nevertheless, it has also been used in the removal water in the gas purification process [2]. In all these applications, the hydroxyl groups on the aluminium oxide surface plays an important role. The study of aluminium (III) oxide surface was reported in previous researches that revealed the existence of hydroxyl groups on surface [3, 4, 5]. The Knözinger-Ratnasamy model explains the difference in the Al-OH groups and it is the most widely accepted model of the aluminium oxide surface [3]. This model mentions about the terminal hydroxyl groups on alumina surface. The hydroxyl polar surface of aluminium (III) oxide has a tendency to adsorb polar molecules such as water. Thus, the adsorption properties of aluminium (III) oxide lead to a large number of studies of its surface. Adsorption of water molecules on aluminium (III) oxide has been reported by various authors [5, 6]. These authors reported hydrogen bonding between water molecules and hydroxyl groups on aluminium (III) oxide surface and showed hexagonal structure of water molecules on the surface. Christy [7] has studied the adsorption of water molecules by different surfaces using near infrared spectroscopy. His contribution and insight in this regard have provided ways to identify different species of hydroxyl groups on different surfaces of materials. In this study, we investigated the surface functionality and water adsorption on α-aluminium (III) oxide nanoparticle surface by near infrared spectroscopy. The second derivative near infrared spectra were used to study the underlying features in the spectra and to reveal and resolve the types of hydroxyl groups on aluminium oxide surface. In addition, gravimetry was used in the determination of mass of water adsorbed on the surface.