Synchrotron X-ray absorption of iron oxides synthesised by ultrasound assisted coprecipitation: effects of temperature and surfactant W. Tangwatanakul 1 , C. Sirisathitkul* 1 , P. Jantaratana 2 and W. Limphirat 3 Iron oxide nanoparticles are synthesised by the coprecipitation of Fe(NO 3 ) 3 .9H 2 O, FeCl 2 .4H 2 O and NaOH under ultrasonic irradiation with comparisons on the temperature and surfactant used. Synchrotron X-ray absorption near edge structure (XANES) indicates that the predominant phase is Fe 2 O 3 , whereas extended X-ray absorption fine structure (EXAFS) confirms the presence of Fe– O and Fe–Fe bonding. Effects of synthesis temperature and surfactant are also revealed by the fitting of EXAFS spectra. The largest structural distortions compared to the Fe 2 O 3 standard is obtained in superparamagnetic particles synthesised at 60uC. The magnetisation is enhanced with the temperature increasing from 60 to 80uC. The addition of oleic acid as a surfactant leads to the ferromagnetic particles with increased magnetisation and a local structure closer to that of the Fe 2 O 3 standard. Keywords: Coprecipitation, Ultrasound, Fe 2 O 3 , EXAFS, XANES Introduction Iron oxide nanoparticles have been synthesised and investigated by several methods for their magnetic and other interesting properties. 1 Hematite (a-Fe 2 O 3 ) and maghemite (c-Fe 2 O 3 ) are known as materials for informa- tion storage, pigments and catalysis. Amorphous Fe 2 O 3 with large surface area to volume ratio is proposed for higher sorption and catalytic activity. 2 Furthermore, functionalised biocompatible maghemite (c-Fe 2 O 3 ) as well as magnetite (Fe 3 O 4 ) are implemented in magnetic separation, drug delivery, hyperthermia treatment and magnetic resonance imaging enhancement. 1–3 Magnetic resonance imaging contrast agents and ferrofluids based on c-Fe 2 O 3 are commercially available. To obtain desirable morphology and magnetic properties, copreci- pitation, thermal decomposition, microemulsion, hydro- thermal, electrochemical and sonochemical synthesis have been used. 1,4 Furthermore, these methods could be combined for greater effect and ultrasound could be used to assist the precipitation technique. For example, Wang et al. obtained catalytic Fe 3 O 4 from the advanced reverse coprecipitation with ultrasonic irradiation. 5 Ray et al. employed high power sonication in the precipitation to obtain pure phase c-Fe 2 O 3 at room temperature. 6 It was reviewed by Wu et al. that the temperature during the coprecipitation affected the formation of iron oxide and the surfactant controlled the size of nanoparticles. 1 Oleic acid is commonly used to obtain Fe based nanoparticles with narrow size distribution. The formation of the surface layer prevents the agglomeration and stabilises magnetic nanoparticles. X-ray absorption spectroscopy (XAS) incorporating extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) is a powerful characterisation technique for nanoparticles. In addition to X-ray diffractometry, XANES is capable of phase identification and Espinosa et al. successfully employed XANES spectra in fluorescence mode to distinguish between Fe 3 O 4 and c-Fe 2 O 3 phases. 7 The difference in XANES spectra of iron oxides were also analysed by Okudera et al. 8 EXAFS was used in structural investigations of Fe 3 O 4 by Gilbert et al. 9 and c-Fe 2 O 3 by Corrias et al. 10 Exchange bias in iron oxide nanoclusters whose a-Fe 2 O 3 was the dominant phase was studied by both XANES and EXAFS. 11 In addition, Fe–Pt nanoparticles have been characterised by XAS. 12–15 Fe 2 O 3 formation in the reaction between Fe(acac) 3 and Pt(acac) 2 using the modified polyol process was revealed by XANES. 16 Furthermore, the phase and structure of annealed Fe–Pt based nanopar- ticles were respectively investigated by XANES and EXAFS. 17 The objective of this work is to investigate the effect of temperature and surfactant used in the ultrasound assisted coprecipitation synthesis of iron oxide nanoparticles by means of synchrotron XAS and the fitting of EXAFS with the iron oxide model. 1 Molecular Technology Research Unit, School of Science, Walailak University, Nakhon Si Thammarat, Thailand 2 Department of Physics, Faculty of Science, Kasetsart University, Bangkok, Thailand 3 Synchrotron Light Research Institute, Nakhon Ratchasima, Thailand *Corresponding author, email schitnar@wu.ac.th ß W. S. Maney & Son Ltd. 2014 Received 15 September 2013; accepted 12 December 2013 DOI 10.1179/1432891714Z.000000000543 Materials Research Innovations 2014 VOL 18 SUPPL 2 S2-547