Highly Visible Light Activity of Nitrogen Doped TiO 2 Prepared by Sol–Gel Approach LE DIEN THAN, 1 NGO SY LUONG, 2 VU DINH NGO, 1 NGUYEN MANH TIEN, 1 TA NGOC DUNG, 3 NGUYEN MANH NGHIA, 4 NGUYEN THAI LOC, 5 VU THI THU, 6 and TRAN DAI LAM 7,8,9,10 1.—Viet Tri University of Industry, 9 Tien Son street, Phu Tho, Viet Tri, Viet Nam. 2.—Hanoi University of Science, 19 Le Thanh Tong Road, Ha Noi, Viet Nam. 3.—Ha Noi University of Science and Technology, 1 Dai Co Viet, Ha Noi, Viet Nam. 4.—Hanoi National University of Education, 136 Xuan Thuy, Ha Noi, Viet Nam. 5.—Asian Institute of Technology, Klong Luang, PO Box 4, Pathumthani, Bangkok 12120, Thailand. 6.—Hanoi University of Science and Tech- nology, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet Road, Ha Noi, Viet Nam. 7.—Graduate University of Science and Technology, Vietnam Academy of Science and Technology, 18 Hoang Quoc Viet Road, Ha Noi, Viet Nam. 8.—Duy Tan University, 182 Nguyen Van Linh Road, Da Nang, Viet Nam. 9.—e-mail: trandailam@gmail.com. 10.—e-mail:tdlam@gust-edu. vast.vn A simple approach was explored to prepare N-doped anatase TiO 2 nanopar- ticles (N-TiO 2 NPs) from titanium chloride (TiCl 4 ) and ammonia (NH 3 ) via sol– gel method. The effects of important process parameters such as calcination temperatures, NH 3 /TiCl 4 molar ratio (R N ) on crystallite size, structure, phase transformation, and photocatalytic activity of titanium dioxide (TiO 2 ) were thoroughly investigated. The as-prepared samples were characterized by ultraviolet–visible spectroscopy, x-ray diffraction, transmission electron mi- croscopy, energy dispersive x-ray spectroscopy, and x-ray photoelectron spectroscopy. The photocatalytic activity of the samples was evaluated upon the degradation of methylene blue aqueous solution under visible-light irra- diation. The results demonstrated that both calcination temperatures and NH 3 /TiCl 4 molar ratios had significant impacts on the formation of crystallite nanostructures, physicochemical, as well as catalytic properties of the ob- tained TiO 2 . Under the studied conditions, calcination temperature of 600°C and NH 3 /TiCl 4 molar ratio of 4.2 produced N-TiO 2 with the best crystallinity and photocatalytic activity. The high visible light activity of the N-TiO 2 nanomaterials was ascribed to the interstitial nitrogen atoms within TiO 2 lattice units. These findings could provide a practical pathway capable of large-scale production of a visible light-active N-TiO 2 photocatalyst. Key words: TiO 2 , anatase, visible-light activity, photocatalyst, interstitial nitrogen, sol–gel INTRODUCTION In recent years, photocatalytic detoxification of water and air has attracted considerable atten- tion. 1,2 Among several photocatalysts being investi- gated, titanium dioxide is highly preferred due to its low-cost of production, strong catalytic activity, stability, and nontoxicity. 3,4 However, the large band gap (3.2 eV) of TiO 2 restricts its applications mainly to the ultraviolet (UV) ranges, which account for only 3–5% of sunlight energy. 3 Photo- catalytic efficiency of TiO 2 could be enhanced by generating mid-gap states or narrow its band gap. 5 The most effective method is to dope TiO 2 with impurities such as metal [iron (Fe) and copper (Cu)] or non-metal elements [boron (B), carbon (C), nitro- gen (N), sulfur (S), and fluorine (F)]. 6–10 However, (Received January 21, 2016; accepted August 19, 2016) Journal of ELECTRONIC MATERIALS DOI: 10.1007/s11664-016-4894-6 Ó 2016 The Minerals, Metals & Materials Society