Rapid and controlled electrochemical synthesis of crystalline niobium oxide microcones
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esearch Letter
Rapid and controlled electrochemical synthesis of crystalline niobium oxide microcones Basamat S. Shaheen, Energy Materials Laboratory, School of Sciences and Engineering, The American University in Cairo, New Cairo 11835, Egypt; Materials Science, California Institute of Technology, Pasadena, California 91125 Timothy C. Davenport, Materials Science, California Institute of Technology, Pasadena, California 91125; Present address: Department of Materials Science and Engineering, Northwestern University, 2200 Campus Dr., Evanston, Illinois 60208 Hanadi G. Salem, Energy Materials Laboratory, School of Sciences and Engineering, The American University in Cairo, New Cairo 11835, Egypt Sossina M. Haile, Materials Science, California Institute of Technology, Pasadena, California 91125; Present address: Department of Materials Science and Engineering, Northwestern University, 2200 Campus Dr., Evanston, Illinois 60208 Nageh K. Allam, Energy Materials Laboratory, School of Sciences and Engineering, The American University in Cairo, New Cairo 11835, Egypt; Materials Science, California Institute of Technology, Pasadena, California 91125 Address all correspondence to N. K. Allam at [email protected] (Received 8 May 2014; accepted 10 June 2015)
Abstract We demonstrate the fabrication by anodization of niobium oxide microcones, several microns long, from aqueous solutions of 1 wt% hydrogen fluoride (HF) with varied sodium fluoride (NaF) concentration (0–1 M). Raman spectroscopy and x-ray diffractometer analysis revealed the as-grown microcones to be crystalline Nb2O5−x with preferred (1 0 0) and (0 1 0) orientations. The overall Nb2O5−x formation rate increased with the increasing NaF concentration, and structures as tall as 20 μm were achieved in just 20 min of anodization at 1 M NaF. Rapid formation of niobia microcones was even observed in the absence of HF at this NaF concentration. Photocatalytic activity for water oxidation was highest for microcones grown under the highest NaF concentration.
Introduction Niobium oxides offer rich chemistry, existing in many chemical forms with variable stoichiometries, leading to a large variety of structures, properties, and applications.[1–3] Anodization is an attractive method for fabricating oxides, including those of niobium, because of the possibility of creating self-organized structures with a high degree of order. While far less studied than aluminum, anodization of niobium has been reported to result in structures ranging from random porous films[4] and sunflower formations[5] to granular forms[6] and membranes with smooth, linear pores[7] or with rough, vein-like pores.[8] A particularly intriguing morphology is that of microcones, which appears to be unique to anodized niobium. The niobia microcone morphology was first reported by Karlinsey, resulting from anodization in an aqueous electrolyte of HF[9] or HF plus NaF.[10] Habazaki and co-workers have subsequently shown that microcones can also be generated from anodization in a hot glycerol electrolyte contain
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