Observation of Optical Band-Gap Narrowing and Enhanced Magnetic Moment in Co-Doped Sol–Gel-Derived Anatase TiO₂ Nanocrystals

Abstract

The magnetic behavior of TiO 2 and doped TiO 2 nanocrystals has been a challenge due to the unambiguous nature of defects present in oxide semiconductors. Here, a simple, low-temperature sol−gel method is developed for the synthesis of low-dimensional and highly efficient stable anatase TiO 2 nanocrystals. The X-ray powder diffraction pattern and Raman spectra confirm the formation of a single-phase anatase structure of TiO 2 . High-resolution transmission electron microscopy studies reveal the crystalline nature of the sol−gel-derived nanocrystals. The increase in lattice parameters together with the shifting and broadening of the most intense E g (1) mode in micro-Raman spectra of Codoped TiO 2 nanocrystals indicate the incorporation of Co in TiO 2 . Shifting of the absorption edge to the visible region in UV−visible spectra indicates narrowing of the band gap due to Co incorporation in TiO 2 . X-ray photoelectron spectra confirm the presence of Co 2+ and Co 3+ in Co-doped TiO 2 samples. Oxygen vacancy defects lead to the formation of bound magnetic polarons which induces a weak ferromagnetic behavior in air-annealed 3% Co-doped TiO 2 at room temperature. It is observed that irrespective of the dopant ion, whether magnetic or nonmagnetic, the overlapping of bound magnetic polarons alone can induce ferromagnetism, while the magnetic impurities give rise to an enhanced paramagnetic moment for higher Co concentrations. A detailed understanding on the variation of these magnetic properties by estimating the concentration of bound magnetic polarons is presented, which is in corroboration with the photoluminescence studies. The observed band-gap narrowing in Co-doped TiO 2 nanostructures and the mechanism underlying the magnetic interactions associated with the magnetic impurity concentration are advantageous from an applied perspective, especially in the field of spintronic and magneto-optic devices.

Edge‐Enriched Ultrathin MoS2 Embedded Yolk‐Shell TiO2 with Boosted Charge Transfer for Superior Photocatalytic H2 Evolution

“…In order to detect the surface state of the as‐formed photocatalysts, all of the samples were investigated by Raman scattering. Five phonon vibrational peaks of anatase TiO 2 are observed at 148, 200, 401, 518, and 644 cm −1 , that can be assigned to the E g 1 , E 2g 2 , B 1g 1 , A 1g + B 1g 2 , and E 3g 3 stretching modes (Figure c), respectively, which are similar to results from a previous report . Interestingly, a slight shift of the E g 1 mode to a higher wave‐number (147.8 to 153.5 cm −1 ) is found in the UMT‐0.14 hybrid, illustrating that embedding ultrathin MoS 2 may produce a robust interaction between MoS 2 and TiO 2 .…”

Diffusion doping of cobalt in rod-shape anatase TiO2 nanocrystals leads to antiferromagnetism

“…A number of reports of cobalt doping in TiO 2 nanomaterials do not include steps for removal of unbound or surface metal atoms such that the subsequent characterization and analyses may not correlate exclusively to cobalt-doped TiO 2 . [39][40][41] The Co (doped) -TiO 2 nanocrystals were characterized with TEM and XRD methods ( Fig. 3 and S2 †).…”

Doping effect and oxygen defects boost room temperature ferromagnetism of Co-doped ZnO nanoparticles: experimental and theoretical studies

“…To further analyze the effects of doping Co atoms and oxygen defects on the magnetization, all the M-H curves were tted by BMP's model, 10,[41][42][43] as shown in Fig. S1 and S2.…”

“…4 However, there are some overwhelming roadblocks related to its n-type conductivity and origin of intrinsic ferromagnetism for DMSs, which restrict its long-term development and applications. One feasible project is to regulate and control defects on the surface of materials, [5][6][7][8][9][10][11] the other is doping 3d magnetic transition-metal ions (such as Fe, Co, Ni, Mn etc.) into the ZnO lattice.…”