Nanoporous Anodic Bismuth Oxide Photo-Anodes
In the present investigation, Bi2O3 nanoporous films were synthesized by electrochemical anodization in the electrolyte solutions containing citric acid, ethylene glycol and glycerol. Nanoporous bismuth oxide films were formed by anodizing bismuth circular discs of 3 mm thick and 12.7 mm diameter. Anodization was carried out at various potentials ranging from 3 V to 60 V for different time durations ranging from 0.5 to 2 h. After anodization, the samples were thermally annealed at 200 °C for 2h. The influences of anodization time, electrolyte concentration and applied voltages on morphology have been investigated in this study. Potentiodynamic, potentiostatic, electrochemical impedance spectroscopy (EIS), and Mott-Schottky analysis studies were carried out with and without illuminated conditions.
All the depositions were examined under a FEI Quanta 200F scanning electron microscope. Figure 1 shows the nanoporous morphology of the oxide layer formed at 3V for 30minutes in citric acid electrolyte. The diameters of the pores were in the range of 20 -50 nm and total thickness of the film was about 500nm. It was observed that pore diameter and film thickness changed with the change in applied potential, time and electrolyte concentration. Photo electrochemical studies were carried out using a potentiostat (Gamry, Reference 600) with platinum as the counter electrode. Potential Vs current plots were constructed by scanning the potential of the sample from the open circuit potential to 0.5 V at a scan rate of 5mV/s in 1 M KOH solution. Potentiostatic measurements were carried out in 1 M KOH solution at 0.2 V and 0.5 VAg/AgCl. A solar simulator with an AM 1.5 filter was used for illuminating the samples. The thickness of the oxide layer increased with the increase in the anodization potential. The photo current density of the nanoporous bismuth oxide increased with increase in the thickness of the oxide. The dark current density decreased with the increase in thickness of the oxide layer. The maximum photo current density (Iilluminated-Idark) recorded at an applied potential of 0.5 VAg/AgCl was 1 mA/cm2 for the sample anodized at 20 V. The sample anodized at 60 V showed about 20 µA/cm2 dark current density and 0.8 mA/cm2 photo current density at 0.5 VAg/AgCl. The photo activity of the nanoporous bismuth oxide is comparable to that of nanotubular TiO2 oxide photo anodes. The nanoporous Bi2O3 contained a defect concentration in the range of 7x1016 – 4x1018 cm-3 under dark condition for various anodized conditions. Upon illumination, the defect density increased to values in the range of 4x1017 to 2x1019 cm-3. A detailed discussion will be provided in the final presentation on the photo electrochemical behavior of the Bi2O3 nanoporous structure as a function of morphology, and defect and electronic structures.
- A. Fujishima and K. Honda, Nature 238 (1972) 37