Irregularly Shaped Mn₂O₃ Nanostructures with High Surface Area for Water Oxidation

Abstract

Water oxidation is an energy-consuming, four-electron-transfer reaction and is essential for solar fuel production from water. Catalysts based on precious metals such as RuO2 and IrO2 show high efficiency for oxygen evolution reaction. However, these catalysts are less abundant and expensive. To date, earth-abundant water oxidation catalysts still exhibit less activity for water oxidation. Herein, we report the synthesis of high surface area Mn2O3 nanomaterials for an efficient photocatalytic water oxidation catalyst. The synthesis process involves three simple steps. In the first step, CaMnO3 is synthesized by the citrate-gel method. In the second step, CaMnO3 is transformed into freestanding layers of ε-MnO2 by selective removal of Ca2+. In the third step, these layers are converted into irregularly shaped two-dimensional Mn2O3 flakes (AD-Mn2O3) by calcination at 550 °C. These AD-Mn2O3 nanostructures show 4 times higher surface area (127 m2 g–1) when compared to the irregularly shaped Mn2O3 nanoparticles (CG-Mn2O3) synthesized by the citrate-gel method at the same temperature. The AD-Mn2O3 nanostructures show super hydrophilicity with a contact angle of zero degree. This material exhibits excellent photocatalytic water oxidation activity with a turnover frequency of 1.53 × 10–3 s–1, which is twice the activity shown by CG-Mn2O3. This study can help in developing an earth-abundant, cost-effective, efficient catalyst for overall water splitting.

High surface area MnO2nanomaterials synthesized by selective cation dissolution for efficient water oxidation

“…The FT-IR spectra of AT-CMO-60 (3-MnO 2 ) having two distinctive vibrational bands at 363 and 500 cm À1 arising from the bending and stretching vibrations of the Mn-O bond of MnO 6 octahedra of the 3-MnO 2 phase. 39,42,43 With increasing the heating temperature of the AT-CMO-60 sample, shoulder vibrational bands started appearing and become clear till 400 C. The shoulder bands developed at 325, 550, and 650 cm À1 represent the characteristic Mn-O vibrational modes in the MnO 6 octahedral co-ordination of b-MnO 2 . [44][45][46] At 400 C a pure phase of b-MnO 2 formed.…”

“…On further increasing the heating temperature to 450 C, the vibrational spectrum matches with the Mn 2 O 3 phase. 39,47,48 Though, the XRD pattern of AT-CMO-450 shows that the major part of the sample is b-MnO 2 with a small amount of Mn 2 O 3 , the FT-IR spectrum shows that the major part of the sample consists of Mn 2 O 3 with a small Hence, it can be concluded that a major part of the sample is Mn 2 O 3 with a small amount of b-MnO 2 , which is further conrmed by the X-ray photoelectron spectroscopy (XPS).…”

“…CaMnO 3 microparticles were synthesized through an earlier reported method with minor modications. 39 In a typical synthesis, 10 mmol of Ca(NO 3 ) 2 $4H 2 O, 10 mmol of (CH 3 -COO) 2 Mn$4H 2 O, 60 mmol of citric acid, and 3 mL of ethylene glycol were dissolved in 200 mL deionized water and kept stirring at 80 C until a gel forms, which was then dried at 180 C for 12 hours. The dried gel was ground and calcined in air for 12 hours at 900 C. The heating rate is 5 C min À1 and then naturally cooled to room temperature.…”

“…Very recently, our group has synthesized high surface area Mn 2 O 3 nanostructures using this method. 39 Herein, high surface area porous MnO 2 layered nanomaterials with different crystal structures have been synthesized from the CaMnO 3 precursor by treating with dilute HNO 3 solution followed by heating at different temperatures. The structural, morphological, and textural properties of these materials were evaluated.…”

Boosting alkaline water splitting and the urea electrolysis kinetic process of a Co3O4 nanosheet by electronic structure modulation of F, P co-doping

“…4 It involves two reaction steps: a hydrogen evolution reaction (HER) and an oxygen evolution reaction (OER). [5][6][7] However, an anodic oxygen evolution reaction involves the transfer of four electrons, 8 and the reaction kinetics is very slow. Due to the inevitable overpotential generated by the two electrodes, the cell voltage of electrolytic water far exceeds the theoretical voltage of 1.23 V, which is unfavorable for energy saving hydrogen production.…”