Surface Magnetism of Cobalt-Doped Anatase TiO₂ Nanopowders

Abstract

Cobalt-doped anatase Ti1–x Co x O2 (0 < x ≤ 0.04) nanopowders (with a particle size of 30–40 nm) were produced by the hydrothermal synthesis method. Morphology, structure, and thermal stability of the synthesized compounds were examined using transmission electron microscopy, infrared spectroscopy, and X-ray diffraction analysis. Using X-ray photoelectron spectroscopy, cobalt ions are shown to have an oxidation state of 2+, with titanium ions having a tetravalent state of Ti4+. In the as-prepared state, all investigated compounds of Ti1–x Co x O2 are paramagnetic, with the value of paramagnetic susceptibility growing in proportion to cobalt content; with the spin of cobalt ion equal to S = 3/2. Analysis of the electron paramagnetic resonance spectra reveals that doping TiO2 with cobalt (up to 2%) is accompanied by a significant increase in the concentration of F+ centers. Further growth of the cobalt content results in a relatively wide line (nearly 600 Oe) in the spectrum, with a g-factor of about 2.005, demonstrating exchange-coupled regions being formed, the fraction of which increases with cobalt content, while the intensity of F+-center signals is reduced appreciably. Annealing of Ti0.96Co0.04O2 in vacuum at 1000 K is shown to have resulted in the substantial localization of cobalt atoms in the subsurface layers, resulting in an approximately 3-fold increase in the Co atoms content on the surface of nanoparticles as compared with that in the bulk. This is shown to be accompanied by appearance of spontaneous magnetization at room temperature, the value of which depends on the cobalt content in TiO2 nanopowders. The value of magnetic moment per Co atom decreases monotonically down to a value of ≃1 μB with cobalt content increasing. A core–shell model proposed to be the most adequate for describing the magnetic properties of TiO2:Co after the reducing annealing. A hypothesis is put forward suggesting that the defect surface enriched with Co atoms and vacancies is described with itinerant type magnetism, allowing for the delocalized nature of electrons near vacancies.

UV/Vis Light Induced Degradation of Oxytetracycline Hydrochloride Mediated by Co-TiO2 Nanoparticles

“…Among various transition metals (i.e., Pt, Ag, Au, Ru, and Pd) [28][29][30][31] and transition metal oxides (i.e., Cu 2 O, α-Fe 2 O 3 ) [32][33][34], cobalt [35,36] and cobalt oxides [37][38][39][40] as an interesting low-cost and earth-abundant mineral have attracted tremendous attention for the development of visible light active TiO 2 materials. Cobalt-doped TiO 2 photocatalysts have been synthesized by various methods, such as the sol/gel technique [36,[41][42][43][44][45][46][47][48][49][50], hydrothermal treatment [51][52][53][54], impregnation method [55,56], precipitation process [57] for the photocatalytic degradation of methylene blue [57,58], rhodamine B [58], methylene orange [57], phenol [41], 2-chlorophenol [41], and so on.…”

“…In contrast, other scientific groups reported that cobalt-modified TiO 2 synthesized by wet impregnation methods seems to increase the photocatalytic activity [55,56]. Additionally, earlier studies have been mostly done on Co-doped TiO 2 thin films, and the main focus has commonly been made on the ferromagnetic properties of these catalysts [45,53,[62][63][64].…”

Conformal Titanyl Phosphate Surface Passivation for Enhancing Photocatalytic Activity

“…The positive shift of Ti 2p1/2 and Ti 2p3/2 peaks of TP-TiO2 were observed in the Ti 2p XPS spectra (Figure 2a). The shift in Ti 2p can be attributed to the ameliorated surface states [32,33]. This positive shift of Ti 2p demonstrates that the Ti-O-P-O bonds were successfully formed on the surface of the TiO2 core particle, and resulted in the reduction of trap states originated from Ti 3+ defects.…”

Loading of Co3O4 onto Pt-modified nitrogen-doped TiO2 nanocomposites promotes photocatalytic hydrogen production

“…1d). 33,34 No other crystal was found in the HRTEM image of PCNT(S)-2, indicating that Co 3 O 4 were probably dispersed in the N-TiO 2 particles rather than loaded onto the surface of N-TiO 2 . No Pt particle was found in either sample, suggesting the low content of Pt in the composites.…”