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Performance modulation of α-MnO₂ nanowires by crystal facet engineering.

Li W, Cui X, Zeng R, Du G, Sun Z, Zheng R, Ringer SP, Dou SX - Sci Rep (2015)

Bottom Line: In the research on nanomagnets, it opens up new perspectives in the fields of nanoelectronics, spintronics, and quantum computation.First-principles density functional theory calculations confirm that both Mn- and O-terminated α-MnO2 (1 1 0) surfaces exhibit ferromagnetic ordering.The investigation of surface-controlled magnetic particles will lead to significant progress in our fundamental understanding of functional aspects of magnetism on the nanoscale, facilitating rational design of nanomagnets.

View Article: PubMed Central - PubMed

Affiliation: 1] Institute for Superconducting and Electronic Materials, University of Wollongong, NSW 2522, Australia [2] School of Materials Science and Engineering, Shanghai University, Shanghai 200072, PR China [3] Solar Energy Technologies, School of Computing, Engineering and Mathematics, University of Western Sydney, Penrith, NSW 2751, Australia.

ABSTRACT
Modulation of material physical and chemical properties through selective surface engineering is currently one of the most active research fields, aimed at optimizing functional performance for applications. The activity of exposed crystal planes determines the catalytic, sensory, photocatalytic, and electrochemical behavior of a material. In the research on nanomagnets, it opens up new perspectives in the fields of nanoelectronics, spintronics, and quantum computation. Herein, we demonstrate controllable magnetic modulation of α-MnO2 nanowires, which displayed surface ferromagnetism or antiferromagnetism, depending on the exposed plane. First-principles density functional theory calculations confirm that both Mn- and O-terminated α-MnO2 (1 1 0) surfaces exhibit ferromagnetic ordering. The investigation of surface-controlled magnetic particles will lead to significant progress in our fundamental understanding of functional aspects of magnetism on the nanoscale, facilitating rational design of nanomagnets. Moreover, we approved that the facet engineering pave the way on designing semiconductors possessing unique properties for novel energy applications, owing to that the bandgap and the electronic transport of the semiconductor can be tailored via exposed surface modulations.

No MeSH data available.


Light absorbance and bandgap properties of MnO2-110 and MnO2-210.(a), UV-Vis absorption spectra of the α-MnO2 nanowires. Inset shows the (αhv)1/2 vs. hv plots (α, absorption coefficient; hv, photon energy). Bandgap values of 0.98 eV and 0.84 eV can be deduced for the exposed (1 1 0) sample and the exposed (2 1 0) sample, respectively. (b), Sketch of possible bandgap alignments of MnO2-110 and MnO2-210.
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f4: Light absorbance and bandgap properties of MnO2-110 and MnO2-210.(a), UV-Vis absorption spectra of the α-MnO2 nanowires. Inset shows the (αhv)1/2 vs. hv plots (α, absorption coefficient; hv, photon energy). Bandgap values of 0.98 eV and 0.84 eV can be deduced for the exposed (1 1 0) sample and the exposed (2 1 0) sample, respectively. (b), Sketch of possible bandgap alignments of MnO2-110 and MnO2-210.

Mentions: MnO2 has been demonstrated to be a highly efficient photocatalyst37, either alone or in MnO2/TiO2 heterogeneous photocatalysts3839. Figure 4(a) shows the ultraviolet-visible (UV-vis) absorption spectrum of the α-MnO2 nanowires. Broad absorption bands ranging between 300 and 600 nm with peak positions of ~400 nm for MnO2-210 and ~450 nm for MnO2-110 are observed. The d−d transitions of Mn ions in the α-MnO2 nanowires is responsible for the absorption in the visible light range. The Mn 3d energy level splits into lower (t2g) and higher (eg) energy levels in the ligand field of MnO6 octahedra, and the energy difference between the eg and t2g states is responsible for the optical bandgap energy40. The bandgap energy Eg for the α-MnO2 nanowires was estimated using the Kubelka-Munk function to plot the product of the square root of the absorption coefficient and the photon energy against the incident photon energy (hv)41. A straight line in a photon energy range close to the absorption threshold can be fitted, as shown in the inset of Figure 4(a). α-MnO2 nanowires have an indirect electronic transition near the bandgap4142. The bandgap energy for the α-MnO2 nanowires can be derived as 0.98 eV for the sample with the exposed (1 1 0) planes, while it is 0.84 eV for the sample with exposed (2 1 0) planes, as derived from the intercept of the linear portion with the abscissa. Remarkable differences in the optical properties of nanostructured MnO2 materials were previously observed. For example, Pereira et al. found that the absorption of MnO2 colloid at longer wavelengths strongly decreases as the MnO2 particles become smaller43. Gao et al. observed a bandgap of 1.32 eV in α-MnO2 nanofibers with typical diameters of 20–60 nm and lengths of 1–6 μm44. Sakai et al. also reported that MnO2 nanosheets with a very small thickness of about 0.5 nm had bandgap energy of about 2.23 eV41. The shift in the bandgap to higher energies can be attributed to the carrier confinement in the small semiconductor particles. Figure 4(b) is a sketch of the possible bandgap alignment of MnO2. Selective surface engineering can be an effective tool to control the driving force of charge transport and charge separation.


Performance modulation of α-MnO₂ nanowires by crystal facet engineering.

Li W, Cui X, Zeng R, Du G, Sun Z, Zheng R, Ringer SP, Dou SX - Sci Rep (2015)

Light absorbance and bandgap properties of MnO2-110 and MnO2-210.(a), UV-Vis absorption spectra of the α-MnO2 nanowires. Inset shows the (αhv)1/2 vs. hv plots (α, absorption coefficient; hv, photon energy). Bandgap values of 0.98 eV and 0.84 eV can be deduced for the exposed (1 1 0) sample and the exposed (2 1 0) sample, respectively. (b), Sketch of possible bandgap alignments of MnO2-110 and MnO2-210.
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f4: Light absorbance and bandgap properties of MnO2-110 and MnO2-210.(a), UV-Vis absorption spectra of the α-MnO2 nanowires. Inset shows the (αhv)1/2 vs. hv plots (α, absorption coefficient; hv, photon energy). Bandgap values of 0.98 eV and 0.84 eV can be deduced for the exposed (1 1 0) sample and the exposed (2 1 0) sample, respectively. (b), Sketch of possible bandgap alignments of MnO2-110 and MnO2-210.
Mentions: MnO2 has been demonstrated to be a highly efficient photocatalyst37, either alone or in MnO2/TiO2 heterogeneous photocatalysts3839. Figure 4(a) shows the ultraviolet-visible (UV-vis) absorption spectrum of the α-MnO2 nanowires. Broad absorption bands ranging between 300 and 600 nm with peak positions of ~400 nm for MnO2-210 and ~450 nm for MnO2-110 are observed. The d−d transitions of Mn ions in the α-MnO2 nanowires is responsible for the absorption in the visible light range. The Mn 3d energy level splits into lower (t2g) and higher (eg) energy levels in the ligand field of MnO6 octahedra, and the energy difference between the eg and t2g states is responsible for the optical bandgap energy40. The bandgap energy Eg for the α-MnO2 nanowires was estimated using the Kubelka-Munk function to plot the product of the square root of the absorption coefficient and the photon energy against the incident photon energy (hv)41. A straight line in a photon energy range close to the absorption threshold can be fitted, as shown in the inset of Figure 4(a). α-MnO2 nanowires have an indirect electronic transition near the bandgap4142. The bandgap energy for the α-MnO2 nanowires can be derived as 0.98 eV for the sample with the exposed (1 1 0) planes, while it is 0.84 eV for the sample with exposed (2 1 0) planes, as derived from the intercept of the linear portion with the abscissa. Remarkable differences in the optical properties of nanostructured MnO2 materials were previously observed. For example, Pereira et al. found that the absorption of MnO2 colloid at longer wavelengths strongly decreases as the MnO2 particles become smaller43. Gao et al. observed a bandgap of 1.32 eV in α-MnO2 nanofibers with typical diameters of 20–60 nm and lengths of 1–6 μm44. Sakai et al. also reported that MnO2 nanosheets with a very small thickness of about 0.5 nm had bandgap energy of about 2.23 eV41. The shift in the bandgap to higher energies can be attributed to the carrier confinement in the small semiconductor particles. Figure 4(b) is a sketch of the possible bandgap alignment of MnO2. Selective surface engineering can be an effective tool to control the driving force of charge transport and charge separation.

Bottom Line: In the research on nanomagnets, it opens up new perspectives in the fields of nanoelectronics, spintronics, and quantum computation.First-principles density functional theory calculations confirm that both Mn- and O-terminated α-MnO2 (1 1 0) surfaces exhibit ferromagnetic ordering.The investigation of surface-controlled magnetic particles will lead to significant progress in our fundamental understanding of functional aspects of magnetism on the nanoscale, facilitating rational design of nanomagnets.

View Article: PubMed Central - PubMed

Affiliation: 1] Institute for Superconducting and Electronic Materials, University of Wollongong, NSW 2522, Australia [2] School of Materials Science and Engineering, Shanghai University, Shanghai 200072, PR China [3] Solar Energy Technologies, School of Computing, Engineering and Mathematics, University of Western Sydney, Penrith, NSW 2751, Australia.

ABSTRACT
Modulation of material physical and chemical properties through selective surface engineering is currently one of the most active research fields, aimed at optimizing functional performance for applications. The activity of exposed crystal planes determines the catalytic, sensory, photocatalytic, and electrochemical behavior of a material. In the research on nanomagnets, it opens up new perspectives in the fields of nanoelectronics, spintronics, and quantum computation. Herein, we demonstrate controllable magnetic modulation of α-MnO2 nanowires, which displayed surface ferromagnetism or antiferromagnetism, depending on the exposed plane. First-principles density functional theory calculations confirm that both Mn- and O-terminated α-MnO2 (1 1 0) surfaces exhibit ferromagnetic ordering. The investigation of surface-controlled magnetic particles will lead to significant progress in our fundamental understanding of functional aspects of magnetism on the nanoscale, facilitating rational design of nanomagnets. Moreover, we approved that the facet engineering pave the way on designing semiconductors possessing unique properties for novel energy applications, owing to that the bandgap and the electronic transport of the semiconductor can be tailored via exposed surface modulations.

No MeSH data available.