Environmentally Friendly γ‐MnO₂ Hexagon‐Based Nanoarchitectures: Structural Understanding and Their Energy‐Saving Applications

Abstract

Although about 200,000 metric tons of gamma-MnO(2) are used annually worldwide for industrial applications, the gamma-MnO(2) structure is still known to possess a highly ambiguous crystal lattice. To better understand the gamma-MnO(2) atomic structure, hexagon-based nanoarchitectures were successfully synthesized and used to elucidate its internal structure for the present work. The structural analysis results, obtained from the hexagon-based nanoarchitectures, clearly show the coexistence of akhtenskite (epsilon-MnO(2)), pyrolusite (beta-MnO(2)), and ramsdellite in the so-called gamma-MnO(2) phase and verified the heterogeneous phase assembly of the gamma-MnO(2) state, which violates the well-known "De Wolff" model and derivative models, but partially accords with Heuer's results. Furthermore, heterogeneous gamma-MnO(2) assembly was found to be a metastable structure under hydrothermal conditions, and the individual components of the heterogeneous gamma-MnO(2) system have structural similarities and a high lattice matches with pyrolusite (beta-MnO(2)). The as-obtained gamma-MnO(2) nanoarchitectures are nontoxic and environmentally friendly, and the application of such nanoarchitectures as support matrices successfully mitigates the common problems for phase-change materials of inorganic salts, such as phase separation and supercooling-effects, thereby showing prospect in energy-saving applications in future "smart-house" systems.

Excellent catalytic and electrochemical properties of the mesoporous MnO2 nanospheres/nanosheets

“…Composite electrodes based on the MnO 2 -modified carbon powder proved to be useful for detection of H 2 O 2 , ascorbic acid and nitrite ions [10]. Recently, much effort has gone into the investigation of MnO 2 nanostructures because of their promising applications in high performance nanodevices [11][12][13]. Compton and coworkers [14] have reported the ␣-and ␤-MnO 2 nanorods produced significantly lower limits of detection and greater sensitivity towards H 2 O 2 than the MnO 2 microparticles, which was likely attributed to an increased surface area.…”

Achieving MnO2 Nanosheets through Surface Redox Reaction on Nickel Nanochains for Catalysis and Energy Storage

“…The corresponding TEM and SEM images of Ni and Ni@MnO 2 can also be found in Figures S3–S6 in the Supporting Information. The HRTEM image and the fast Fourier transform (FFT) pattern (Figure e) illustrates the existence of hexagonal symmetry in the Ni@MnO 2 product, which may be related to the presence of some ϵ ‐MnO 2 phase in the γ‐MnO 2 sheets . The EDS mapping images reveal the existence and uniform distribution of Ni, Mn, and O elements (Figure f).…”

Synthesis, properties, and performance of nanostructured metal oxides for supercapacitors

“…Manganese dioxide MnO 2 occurs in many forms in nature (for details see e.g., [108]). Among them are hollandite α-MnO 2 , pyrolusite β-MnO 2 (rutile structure), birnessite (Na 0.3 Ca 0.1 K 0.1 )(Mn 4+ ,Mn 3+ ) 2 O 4 ·1.5 H 2 O forming together with many other layered manganese dioxides the group of δ-MnO 2 with a layered structure [109] particularly useful for ion intercalation, akhtenskite (ε-MnO 2 ), ramsdellite or nsutite γ-MnO 2 or R-MnO 2 (this assignment seems to be under dispute, see e.g., [110][111][112][113]). Of interest as a precursor is also hausmannite Mn 2+ Mn 3+ Mn 3+ O 4 , although it is itself electrochemically inactive.…”

“…α-, β-, and R-MnO 2 show hexagonal closest packing with tunnel structures possibly useful for ion ingress/egress. γ-MnO 2 has been claimed to the electrochemically most active one [112,114] for alkaline battery applications. It shows a thickness change of a film prepared by electrodeposition of up to 150 μm thickness by about 10 % upon reduction as measured by ellipsometry [112].…”