Enhanced Photoelectrochemical Performance of Anodic Nanoporous β-Bi₂O₃

Abstract

Bismuth oxide has well-dispersed valence bands that show enhanced mobility of charge carriers, high refractive index, and large dielectric constant. These properties are attractive for photo catalysis. Thin films of bismuth oxide containing vertically oriented and uniformly distributed nanopores of controllable dimensions were synthesized by a simple electrochemical anodization of a bismuth metal substrate in citric acid solution at 3-60 V. Annealing the anodic nanoporous Bi 2 O 3 at 200 • C for 2 h resulted in stabilization of the metastable β-Bi 2 O 3 phase at room temperature. The nanoporous anodic oxides showed an energy bandgap of 2.48 eV, and n-type semiconductivity. Scanning electron microscopy and electrochemical impedance spectroscopy results suggested a dual layered structure of the anodic oxide with a nanoporous outer layer and a planar inner layer. Thickness of the inner layer predominantly influenced the impedance of the anodic oxide. Under simulated 1-sun intensity, a maximum photo current density of 0.97 mA/cm 2 was observed in 1 M KOH at a potential of 1.53 VRHE for the sample anodized at 10 V. A positive shift in the flatband potential upon illumination suggested accumulation of photo generated holes due to low catalytic activity of the anodic bismuth oxide for oxygen evolution reaction. The observed photo current density of the nanoporous anodic oxide of bismuth was an order of magnitude higher than that of the planar anodic oxide, and twice that of thin film β-Bi 2 O 3 prepared by reactive sputtering.Synthesis of ordered arrays of nanoporous and nanotubular oxides of Al, Ti, and other valve metals has been investigated extensively because of their potential applications in solar energy conversion, energy storage, photocatalysts, sensors, and as templates for nanowire growth. 1-8 Enhanced photo catalytic, electrical, and electrochemical properties were reported for the nanoporous and nanotubular oxides, particularly TiO 2 and Fe 2 O 3 , when compared to their bulk crystalline counterparts. 9-14 Bismuth is different from regular valve metals such as Al, Ti, Nb etc., because of its semi-metallic nature and high atomic number with the electron configuration [Xe]4f 14 5d 10 6s 2 6p 3 . The large nuclear charge of bismuth increases the velocity of the 1s electrons to such an extent that the relativistic effect becomes significant which increases the mass of 1s electron by 26% and decreases the Bohr radius by 20%. Furthermore, the 6s electrons are less available for bonding. The normally degenerate p orbitals are split in to one spherically symmetric p 1/2 and two doughnut shaped p 3/2 orbitals because of the high degree of spin-orbit interaction. Since the p 3/2 orbitals have higher energy than p 1/2 , the p 3/2 electron is removed during first ionization of Bi resulting in an electron configuration of Bi(I) as 6s 2 6p 2 1/2 6p 0 3/2 . The major relativistic effects of bismuth are its lowest tendency to form a hybrid orbital and increasing tendency to form pyramidal structure. 15 The lone electron pa...

Enhanced Performance of β-Bi2O3by In-Situ Photo-Conversion to Bi2O3-BiO2-xComposite Photoanode for Solar Water Splitting

“…indicating the presence of β-Bi 2 O 3 based on the previous literature values. 26,27,39,40 The direct bandgap of the nanoporous oxide sample was 2.37 eV as observed from the Tauc plot given as Figure S1 (supporting information). The Bi 3+ intra-ionic electronic transitions and charge transfer transitions between oxygen ligands and Bi 3+ ions could be assigned to the observed band gaps.…”

“…25 Recently the present authors reported formation of nanoporous anodic oxide of bismuth by a facile anodization process. [26][27][28][29] The * Electrochemical Society Member. z E-mail: chit7390@vandals.uidaho.edu; kalyanchitrada@gmail.com nanoporous β-Bi 2 O 3 showed relatively high initial photocurrent density, yet the photocurrent decayed upon continuous illumination.…”

Bismuth Oxychloride Nanoplatelets by Breakdown Anodization

“…[14][15][16] However, significant efforts to anodize Bi to receive Bi 2 O 3 nanostructures were just carried out within the last decade. [17][18][19][20][21][22][23][24][25][26][27][28] The first article on the formation of Bi-based nanostructures upon the anodization of Bi was published by Yang et al in 2010. [17] The authors received BiPO 4 nanorods by anodization of Bi sheets in phosphoric acid containing various concentrations of HF, an electrolyte that was also used to produce TiO 2 nanotube layers.…”

“…[29,30] Nanoporous Bi 2 O 3 films were produced by anodization in glycol electrolyte containing (NH 4 ) 2 SO 4 and H 2 O, [18] as well as in citric acid based electrolytes. [19][20][21][22] Ahila et al reported on the anodization of Bi in diluted sodium hydroxide solution [23,24] to fabricate nanostructured Bi 2 O 3 films. The same authors also described the anodization in diluted oxalic acid for the fabrication of seaweed like Bi 2 O 3 nanostructures.…”

Preparation of porcupine-like Bi2O3 needle bundles by anodic oxidation of bismuth

“…More recently, several attempts have been made to anodize bismuth substrates to produce structures similar to Al 2 O 3 or TiO 2 . However, bismuth phosphate nanorods [19], non-organized Bi 2 O 3 nanoporous films [9,20,21] or thin films consisting of faceted Bi 2 O 3 nanoparticles were obtained [22], rather than organized nanoporous or nanotubular layers.…”