The effect of anodization conditions on the morphology of porous tungsten oxide layers formed in aqueous solution

Improving Photoelectrochemical Properties of Anodic WO3 Layers by Optimizing Electrosynthesis Conditions

“…It has been also reported, that WO 3 nanoplates can be synthesized by anodic oxidation of W in nitric acid [18] or in a mixture of sodium fluoride and sulfuric acid [17], while electrooxidation of tungsten in a NaOH solution leads to the formation of a hexagonally ordered nanobubble WO 3 structure [21]. Moreover, all other electrosynthesis conditions such as applied voltage [8,22], electrolyte composition (especially its pH and viscosity) [10,23], temperature [8], process duration [8,10], or even hydrodynamic conditions [24], can also have a significant impact on the morphology of anodic oxide layers.…”

“…Tungsten oxide (WO 3 ) is an n-type semiconductor that has been considered so far as one of the most promising materials for photoanodes for photoelectrochemical (PEC) water splitting due to its superior charge transport properties, moderate hole diffusion length and, mostly, a relatively narrow band gap (2.5-2.8 eV). Many different methods have been employed for the synthesis of WO 3 nanomaterials, including chemical vapor deposition (CVD) [1], hydrothermal methods [2,3], sol−gel processes [4], electrodeposition [5], anodic oxidation (anodization) [6][7][8], and many others [9]. Among these techniques, electrochemical oxidation of metallic tungsten has received considerable attention since it can be applied to synthesize nanostructured WO 3 with various morphologies such as nanoporous [6,8,[10][11][12][13][14][15] or nanotubular layers [10,16], compact films [8,12,14], nanoplates [17,18], nanowires [11], and others [11,14].…”

“…A great advantage of this method is its simplicity, versatility, and cost-effectiveness. Moreover, as-received anodic oxide films exhibit good adhesion to the conductive metallic substrate, which is another advantage in terms of its application in photoelectrochemical devices [8]. What is important, the type of the received morphology and geometrical features of the oxide film (e.g., pore/tube/wire sizes, anodic layer thickness) is strongly dependent on the conditions applied during electrolysis, in particular the electrolyte composition.…”

“…What is important, the type of the received morphology and geometrical features of the oxide film (e.g., pore/tube/wire sizes, anodic layer thickness) is strongly dependent on the conditions applied during electrolysis, in particular the electrolyte composition. For instance, nanoporous WO 3 layers can be received during anodization of tungsten in various electrolytes containing fluoride ions [6,8,[10][11][12][13][14], oxalic acid [15], and pure molten orto-phosphoric acid [19]. On the contrary, compact or almost compact oxide films can be obtained in electrolytes without fluoride ions [10,12] or when the F − content is insufficient [8,12,20].…”

“…For instance, nanoporous WO 3 layers can be received during anodization of tungsten in various electrolytes containing fluoride ions [6,8,[10][11][12][13][14], oxalic acid [15], and pure molten orto-phosphoric acid [19]. On the contrary, compact or almost compact oxide films can be obtained in electrolytes without fluoride ions [10,12] or when the F − content is insufficient [8,12,20]. It has been also reported, that WO 3 nanoplates can be synthesized by anodic oxidation of W in nitric acid [18] or in a mixture of sodium fluoride and sulfuric acid [17], while electrooxidation of tungsten in a NaOH solution leads to the formation of a hexagonally ordered nanobubble WO 3 structure [21].…”