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Synthesis and Microstructural Properties of Cu-Doped Goethite and Cu-Doped Hematite Nanoneedles

Goethite (α-FeOOH) and hematite (α-Fe2O3) are the most stable and the most ubiquitous iron oxyhydroxide and iron oxide, respectively. A number of favourable properties (nontoxicity, stability, low cost, band gap in the visible range, etc.) make these compounds promising candidates for different applications (catalysts, photocatalysts, gas sensors, battery electrodes, supercapacitors, etc.). Properties of goethite and hematite can be improved and tuned by modification of particle size and shape, as well as by incorporation of various metal cations into their crystal structure. For example, elongated (1D) hematite nanoparticles (nanorods) showed higher visible-light photocatalytic activity compared to hematite nanoplates (2D) or nanocubes (3D). Also, smaller hematite and goethite 1D nanoparticles (nanorods) showed higher visible-light photocatalytic activity compared to larger hematite and goethite 1D particles (microrods). Substitution of Fe3+ ions in iron oxides with divalent metal cations can significantly modify their electronic properties. As a consequence of modified electronic properties, Cu-doped goethite and hematite showed improved catalytic, photocatalytic and gas sensing properties compared to pure phases. In this work Cu-doped goethite and hematite 1D nanoparticles (nanoneedles) were prepared and studied. Cu-doped goethite nanoneedles (with Cu content up to 4 mol%) were synthesized by precipitation in a highly alkaline medium. Cudoped hematite nanoneedles were prepared by calcination of Cu-doped goethite samples at 500 °C. Phase composition of prepared samples was determined using X-ray powder diffraction (XRPD) and Mössbauer spectroscopy. Cu2+-for-Fe3+ substitution in the goethite crystal structure was confirmed by the determination of the unit cell expansion using XRPD and by the measurement of the hyperfine magnetic field reduction using Mössbauer spectroscopy. An increased Cu2+-for-Fe3+ substitution in goethite caused a gradual elongation and narrowing of nanorods with formation of nanoneedles at 3 and 4 mol% Cu. The shape and size of nanoneedles were not changed by goethite to hematite thermal transformation. Thermal analysis showed a decrease in the temperature of goethite dehydroxylation by Cu doping. Cu2+-for-Fe3+ substitution in goethite and hematite affected an increased absorption of visible and near IR radiation compared to pure phases, as well as a decrease in the optical band gap energy.

iron oxides, Cu-doping, XRPD, 57Fe Moessbauer spectroscopy

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