Nanoporous Anodic Bismuth Oxide Photo-Anodes

Metal-oxide semiconductors have received a wide attention as photo anodes since the demonstration of water splitting by TiO₂ electrode under UV illumination [1]. Significant research efforts were focused on developing various semiconductors with band-gap engineering for efficient photo water splitting. Bismuth oxide (Bi₂O₃) with a band gap of 2.8eV is a promising photo anode which exhibits good electrical conductivity, oxygen ion conductivity, and high dielectric permittivity. Due to its distinctive properties, Bi₂O₃ finds applications in a wide range of areas like gas sensors, optical coatings, microelectronics, photocatalysts, solid state electrolytes, superconductors etc. In spite being a non-toxic material with appropriate band gap and valence band edge position (+3.13 V vs. NHE), Bi₂O₃ demonstrates a poor hydrogen evolution due to its lower conduction band edge position (+0.33 V vs. NHE). The general strategies employed to overcome these limitations were simultaneous doping and nano-sizing of the material.

                 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 V_Ag/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 (I_illuminated-I_dark) recorded at an applied potential of 0.5 V_Ag/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 V_Ag/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.

References

1.    A. Fujishima and K. Honda, Nature 238 (1972) 37