Surface Morphological Control of Nanostructured Single-Phase Fe₃O₄, α-FeOOH and γ-FeOOH Electrodeposited Thin Films

Abstract

Nanostructured single-phase magnetite, goethite, and lepidocrocite thin films with variable surface morphology and degree of cristallinity were anodically electrodeposited in a potentiostatic mode onto titanium substrates under different deposition conditions. Dependence of phase composition, degree of cristallinity, preferred crystallographic orientation and surface morphology of these films on applied potential, electrodeposition electrolytic bath temperature and pH was conducted. X-ray diffraction analysis confirmed that the applied potential have a crucial effect on the phase composition of these films so that on degree crystallinity and prefererred crystallographic orientations. Scanning electron microscopy micrograph images indicated that electrolytic bath pH and temperature play an important role on the surface morphology and on the grain size of the deposited materials.Iron oxides and oxyhydroxides are abundant in nature and are the interest subject within the area of many applied sciences such as geochemistry, mineralogy, corrosion science, soil science, medicine, biology, metallurgy, and environmental protection. 1,2 In addition to the mentioned interest of iron oxides as a bulk material, the formation of thin films of these materials is being investigated for their remarkable properties from a scientific and technological point of view. 3,4 For instance, magnetite thin films have attracted great attention for spintronic applications, 5 hematite thin films have been extensively studied as photoelectrodes for solar driven water splitting 6-8 and there have been encouraging results reporting on the utilization of goethite and lepidrocrocite thin films as electrodes for rechargeable lithium-ion batteries and supercapacitors. 9 Moreover, it has recently been reported that goethite containing semiconductor materials enhance the photocatalityc activity efficiency of photocatalysts. 10 For these applications, iron oxide thin films have been obtained by different techniques: chemical vapor deposition (CVD), 11 sputtering, 12 plasma-enhanced chemical vapor deposition, 13 reactive sputter deposition, 14 dc reactive magnetron sputtering, 15 metallorganic chemical vapor deposition, 16 spray pyrolysis 17 and electrodeposition. 18,19 Among them, electrodeposition is presented as a soft-solution processing technique of thin-film preparation which enables to control the composition and morphology of deposited materials by the adjustment of the electrochemical parameters. 20-23 This ability to tune the shape of inorganic crystals is of extraordinary importance because their electronic structure, bonding, surface energy and chemical reactivities are directly related to their surface morphology. 24 In this work we are interested into the preparation of nanostructured thin films of magnetite (Fe 3 O 4 ), goethite (α-FeOOH) and lepidocrocite (γ-FeOOH) by electrochemical deposition. The anodic deposition from ferrous ions in aqueous solution was studied in the past in relation to the corrosion of iron. 25 Synthesis generally...

Understanding the Enhancement of the Catalytic Properties of Goethite by Transition Metal Doping: Critical Role of O* Formation Energy Relative to OH* and OOH*

“…Only the Cr-doped goethite was not able to be synthesized from the electrodeposition because the deposition potential of Cr was significantly different from other elements. The shape of the pristine goethite is similar to the rhombic-shaped pure goethite which is seen in another reference . The columnar structure of the synthesized pure goethite is consistent with the preferred crystallographic orientation of (002) from both calculation and XRD data.…”

“…The shape of the pristine goethite is similar to the rhombic-shaped pure goethite which is seen in another reference. 53 The columnar structure of the synthesized pure goethite is consistent with the preferred crystallographic orientation of (002) from both calculation and XRD data. The morphology of the TM-doped goethites was nearly the same as the pristine goethite.…”

“…Therefore, purging is maintained during the electrodeposition. The temperature is fixed at 60 °C, which is an appropriate condition to achieve a goethite phase among various iron oxyhydroxide phases. , In the case of pH, as mentioned above, the precipitation of iron­(III) (oxy)­hydroxides consistently lowers the pH of the solution during the electrodeposition. It is possible to slow down the change of pH with nitrogen purging, but as time goes on, the pH value becomes far from the initial value of 6.5.…”

“…Then it is covered with ketone tape to make a deposition area of 1 cm 2 . The deposition was carried out at a voltage of −0.2 V vs Ag/AgCl (3 M KCl), as mentioned in other references, , which gave a current density of 300–400 μA/cm 2 . For TM dopant-doped goethites, similar current density ranges of 300–400 μA/cm 2 were observed.…”