Synthesis of tungsten oxide nanoparticles using a hydrothermal method at ambient pressure
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Reza Younesi Department of Energy Conversion and Storage, Technical University of Denmark, Roskilde 4000, Denmark
Maxime J-F. Guinela) Department of Physics, College of Natural Sciences, University of Puerto Rico, San Juan, Puerto Rico 00936-8377, USA; and Department of Chemistry, College of Natural Sciences, University of Puerto Rico, San Juan, Puerto Rico 00936-8377, USA (Received 3 April 2014; accepted 13 June 2014)
Tungsten oxide (WO3) nanostructures receive sustained interest for a wide variety of applications, and especially for its usage as a photocatalyst. It is therefore important to find suitable methods allowing for its easy and inexpensive large scale production. Tungstite (WO3H2O) nanoparticles were synthesized using a simple and inexpensive low temperature and low pressure hydrothermal (HT) method. The precursor solution used for the HT process was prepared by adding hydrochloric acid to diluted sodium tungstate solutions (Na2WO42H2O) at temperatures below 5 °C and then dissolved using oxalic acid. This HT process yielded tungstite (WO3H2O) nanoparticles with the orthorhombic structure. A heat treatment at temperatures at or above 300 °C resulted in a phase transformation to monoclinic WO3, while preserving the nanoparticles morphology. The production of WO3 nanoparticles using this method is therefore a three step process: protonation of tungstate ions, crystallization of tungstite, and phase transformation to WO3. Furthermore, this process can be tailored. For example, we show that WO3 can be doped with cesium and that nanorods can also be obtained. The products were characterized using powder x-ray diffraction, transmission electron microscopy (including electron energy-loss spectroscopy and electron diffraction), and x-ray photoelectron spectroscopy. I. INTRODUCTION
Tungsten oxide (WO3) is a transition metal oxide semiconductor with a widely tunable band gap (Eg), in the range of 2.5–2.8 eV at room temperature. Interest was recently put on WO3 thin films and nanoparticles1 for a wide variety of applications in microelectronics and optoelectronics,2 dye-sensitized solar cells,3 colloidal quantum dot light-emitting diodes,4 photocatalysis5 and photoelectrocatalysis,6 water splitting photocatalysis7–13 and methanol oxidation catalyst.14 Environmental applications may also benefit with the use of WO3 as a visible light photocatalyst to generate OH radicals in the wastewater treatment,15 bacteria destruction,16 and photocatalytic reduction of CO2 into hydrocarbon fuels.17 Yin et al.18 have reported high hydrophobic properties and improved performances of WO3 as anode materials in lithium-ion batteries. WO3 has also been used in the so-called smart windows19 for energy-efficient buildings, flat-panel displays, optical a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2014.155 1424
J. Mater. Res., Vol. 29, No. 13, Jul 14, 2014
http://journals.cambridge.org
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memory, and writing–reading–erasing devices. Moreover, WO3 shows functional a
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