Shape Evolution Synthesis of Monodisperse Spherical, Ellipsoidal, and Elongated Hematite (α-Fe 2 O 3 ) Nanoparticles Using Ascorbic Acid Wen-Feng Tan,* ,†,‡ Ya-Ting Yu, † Ming-Xia Wang, † Fan Liu, † and Luuk K. Koopal †,§ † College of Resources and Environment, Huazhong Agricultural University, Wuhan 430070, P. R. China ‡ State Key Laboratory of Soil Erosion and Dryland Farming on Loess Plateau, Institute of Soil and Water Conservation, Chinese Academy of Sciences, Yangling, Shaanxi 712100, P. R. China § Laboratory of Physical Chemistry and Colloid Science, Wageningen University, Dreijenplein 6, 6703 HB Wageningen, The Netherlands * S Supporting Information ABSTRACT: Spherical, ellipsoidal, and elongated hematite particles have been obtained via a simple chemical precipitation reaction of FeCl 3 and NaOH in the presence of ascorbic acid (AA). The effects of pH, molar ratio of AA/Fe(III), and time on the formation and shape of the hematite particles were investigated. The optimal conditions to well obtain crystalline hematite are 0.1 mol/L FeCl 3 , 6 mol/L NaOH, pH 7, and AA/Fe(III) ratios of 0.5−2.0%. The presence of AA catalyzed the formation of hematite by reductive dissolution of ferrihydrite and the molar ratio of AA/Fe(III) determined the crystal structure and morphology of hematite. As the ratio of AA increased from 0.5 to 2%, the morphology changed from spherical to ellipsoidal particles and then to elongated particles. The dissolution of Fe(II) from the ferrihydrite precursor is enhanced by AA, and this leads to the formation of hematite by precipitation and crystallization. The effect of AA on the particle shape can be explained by the difference in AA adsorption on the various crystal planes. The hematite samples with different morphologies enhanced the photodegradation of methylene blue in an acid solution with peroxide; the elongated particles that had the highest specific surface area were most effective with the methylene blue degradation. ■ INTRODUCTION Iron oxides and hydroxides are widely present in nature and nanoparticles of iron minerals are distributed throughout the atmosphere, oceans, groundwater, surface waters, soils, and in and/or on most living organisms. 1,2 Natural iron oxides containing Fe(III) show excellent catalytic activity with the degradation of dissolved phenolic substances and organic dyes in polluted waters. 3−6 Synthetic iron oxides have been used as catalysts, photoelectrodes, battery electrodes, gas sensors, pigments, and magnetic materials. 7−11 Hematite (α-Fe 2 O 3 ) is the most stable iron oxide under ambient conditions and an n- type semiconductor with a band gap of 2.1 eV. 8 The α-Fe 2 O 3 nanoparticles show physical and catalytic properties that depend on the size and shape of the particles. 2,10,12−14 Preparation methods for monodisperse hematite nanoparticles include the sol−gel process, 15 calcination, 16,17 forced hydrol- ysis, 18 chemical precipitation, 19 the reflux method, 20 and hydrothermal/solvothermal synthesis. 13,21−23 Although mono- disperse hematite particles with controlled sizes and shapes can be obtained, it is difficult to scale up the syntheses because of the complex reaction processes with specific and time- consuming preparation conditions, high-energy consumption, and low product yields. Therefore, to find a simple synthetic pathway for the production of fairly monodisperse α-Fe 2 O 3 nanoparticles remains a challenge. The chemical precipitation method is relatively simple. Liu et al. 24−27 successfully obtained nearly spherical nanoparticles of α-Fe 2 O 3 with a diameter of 60 to 80 nm by employing FeCl 3 · 6H 2 O and NaOH in the presence of Fe(II). In accordance with Liu et al., 26 Fe(II) existing in the form of FeOH + at pH 7 will Received: September 5, 2013 Revised: November 21, 2013 Published: December 10, 2013 Article pubs.acs.org/crystal © 2013 American Chemical Society 157 dx.doi.org/10.1021/cg401334d | Cryst. Growth Des. 2014, 14, 157−164