Enhanced Performance of β-Bi₂O₃by In-Situ Photo-Conversion to Bi₂O₃-BiO_2-xComposite Photoanode for Solar Water Splitting

Abstract

Ordered nanoporous bismuth oxide layer consisting of metastable β-Bi 2 O 3 phase that showed n-type semiconductivity was synthesized by a simple electrochemical anodization of bismuth substrate. When the anodic nanoporous β-Bi 2 O 3 samples were tested as photoanodes for solar water splitting application in 0.5 M Na 2 SO 4 (pH:5.8), and 1 M KOH (pH:13.7), the photocurrent decayed continuously with time from an initial value of 0.95 mA/cm 2 under 1-sun illumination at a bias potential of 1.5 V RHE . Accumulation of photo generated holes at the photoanode/ electrolyte was attributed to the photocurrent decay. Addition of methanol as sacrificial hole scavenger was not found to be effective for the bismuth oxide photoanodes. When hydrogen peroxide was added to the 0.5 M Na 2 SO 4 electrolyte, the photocurrent density increased to ∼4 mA/cm 2 and was stable for more than 1 h of illumination of AM1.5 global light at 1.5 V RHE . Addition of hydrogen peroxide to the 1 M KOH solution showed a gradual increase in the photocurrent density for the first 300 seconds of light illumination to a maximum value of ∼10 mA/cm 2 at 0.65 V RHE and then it decreased continuously. The high photocurrent density of nanoporous β-Bi Bismuth based mixed oxides such as BiVO 4 , BiFeO 3 , BiWO 6 , Bi 3 NbO 7 , and Bi 2 MoO 6 are being actively investigated as promising high efficiency photocatalysts for solar water splitting applications. 1-11These bismuth based oxides show predominantly n-type semiconductivity and therefore function as photoanodes in the photoelectrochemical water splitting process. A significant amount of literature is available on the photoelectrochemical behavior of bismuth based ternary oxides, however very few studies have reported the performance of the binary Bi 2 O 3 which is the elementary constituent of the advanced bismuth mixed oxides. The photocatalytic behavior of the bismuth based oxides is well documented and the factors that limit their performance is well understood. Most importantly they exhibit low catalytic activity for oxygen evolution, which results in hole accumulation at the electrode/electrolyte interface. In order to overcome this limitation, a thin layer of oxygen evolution catalysts such as Co-Pi, FeOOH, Ni(OH) 2 or CoPO 4 is usually coated on the photoanodes.12-17 The binary and ternary bismuth oxides show more or less similar electronic structures, where the valence bands of both binary Bi 2 O 3 , and mixed bismuth oxides are considered to be formed by O-2p and Bi-6s orbitals. 18,19 The top of the Bi-6s orbital is positioned at more negative potential than that of O-2p. [20][21][22] The conduction band of Bi 2 O 3 is predominantly formed by Bi-6p orbitals. Whereas, the conduction band of mixed bismuth oxide is determined by the d-orbitals of transitions metals present in the mixed oxide. Bismuth oxide shows enhanced mobility of charge carriers because of its well-dispersed valence bands and it has high refractive index and large dielectric constant due to lone electron pairs of Bi(III) 6s 2 that...

Recent Advances in Bismuth‐Based Nanomaterials for Photoelectrochemical Water Splitting

“…However,t he electrode stabilityf or hydrogen production needs to be investigated. The PEC behavior of Bi 2 O 3 [170] is well documented, and the decrease in the catalytic activity for oxygen evolution is understood. To overcome the limitations, heterojunctions and loading with co-catalysts have been investigated.…”

Bismuth Oxychloride Nanoplatelets by Breakdown Anodization

“…Further investigations were carried out in the 1960's and 1970's . However, significant efforts to anodize Bi to receive Bi 2 O 3 nanostructures were just carried out within the last decade . The first article on the formation of Bi‐based nanostructures upon the anodization of Bi was published by Yang et al.…”

“…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 ,. Nanoporous Bi 2 O 3 films were produced by anodization in glycol electrolyte containing (NH 4 ) 2 SO 4 and H 2 O, as well as in citric acid based electrolytes …”

Desenvolvimento de fotocélulas de glicose/H2O2 usando fotoanodos à base de W-BiVO4/V2O5 para geração de eletricidade

“…• pode capturar outro buraco da superfície do fotoanodo (Equação 4.5) e minimizar a perda por recombinação de cargas, que por sua vez resulta no incremento da fotocorrente (CHITRADA et al, 2016). Todavia, alguns estudos que utilizam o TiO2 como fotoanodo demostram o oposto aos resultados obtidos, sob hipótese de que o papel de sequestrador de buracos do H2O2 é limitado, já que os elétrons fotogerados ficam retidos na superfície dos fotoeletrodos, reduzindo o H2O2 e diminuindo os elétrons fotogerados, existindo um processo de competição para liberar os elétrons que dependeria da cinética de redução do H2O2 (ZHU et al, 2017).…”

“…Por outro lado, no momento que o fotoeletrodo de W-BiVO4/V2O5 recebe os fótons de luz, começa a separação dos pares e -/h + em quantidade que seria limitada ao semicondutor. A geração de fotocorrente duplicada que foi relatada por diferentes autores (ANTHONY BYRNE et al, 1998;CHITRADA et al, 2016;DE TACCONI et al, 1998;KALAMARAS;LIANOS, 2015), aconteceria quando se adicionasse alguma solução, Glc ou H2O2 de forma individual, com capacidade de sequestrar um buraco (h + ) e de gerar um elétron (e -) que seria direcionado do fotoanodo até o circuito. No entanto, como foi demonstrado nos testes de cronoamperometria, quando se adiciona Glc ou H2O2 de forma individual na solução eletrolítica sob irradiação de luz, existe inicialmente separação dos pares e -/h + .…”