This article reports on reversible manipulation of room temperature ferromagnetism (RTFM) in nondoped bulk CeO2. The magnetization measurements establish that paramagnetic CeO2 is driven to a ferromagnetic state, without change in structure, when vacuum annealed at 600 °C. The Ce ions transform from 4+ to 3+ state, accompanied by evolution of oxygen vacancies (VO) during the RTFM transition, as determined by x-ray photoemission. The F+ centers (the electrons in singly occupied oxygen vacancies) play key role in the exchange mechanism. The transition shows complete reversibility where the RTFM is removed by removing the vacancies through re-heating the vacuum-annealed CeO2 in air.
In this work, we demonstrate that room temperature ferromagnetism can be induced in pristine anatase TiO2 paramagnetic bulk powder through extended hydrogenation. Defect complexes, Ti3+–VO (Ti3+ ions accompanied by oxygen vacancies) are clearly identified in hydrogenated TiO2 by combining x-ray diffraction and photoemission spectroscopy. The observed ferromagnetic ordering is reversible that can be switched between “on” and “off” by inducing or removing, respectively, these defect complex. We convincingly elucidate that the factors (i) Ti 3d–O 2p hybridization (iii) F+ centers (the electrons in singly occupied oxygen vacancies), and (iii) oxygen vacancy assisted fragmentation of grains, compositely contribute to the ferromagnetic ordering.
Enhanced visible light photocatalytic activity of Gd‐doped CeO
2
nanoparticles (NPs) is experimentally demonstrated, whereas there are very few reports on this mechanism with rare earth doping. All‐pure and Gd‐doped CeO
2
NPs are synthesized using a coprecipitation method and characterized using X‐ray diffraction (XRD), absorption spectroscopy, surface‐enhanced Raman Spectroscopy (SERS), X‐ray photoelectron spectroscopy (XPS), and superconducting quantum interference device (SQUID). The effect of Gd‐doping on properties of CeO
2
is discussed along with defects and oxygen vacancies generation. The XRD confirms the incorporation of Gd
3+
at the Ce
3+
/Ce
4+
site by keeping the crystal structure same. The average particle size from transmission electron microscopy (TEM) images is in the range of 5–7 nm. The XPS spectra of Ce 3d, O 1s, and Gd 4d exhibits the formation of oxygen vacancies to maintain the charge neutrality when Ce
4+
changes to Ce
3+
. The gradual increase in hydrogen production is observed with increasing Gd concentration. The observed results are in good correlation with the characterization results and a mechanism of water splitting is proposed on the basis of analyses. The absorption spectra reveal optical band gap (2.5–2.7 eV) of samples, showing band gap narrowing leads to desired optical absorbance and photoactivity of NPs.
The electronic structure and magnetic properties of cobalt‐doped (7.5%) and manganese co‐doped (2.5%) ZnO polycrystalline samples have been investigated to understand the mechanism of room temperature ferromagnetism (RTFM) in dilute magnetic semiconductors. The samples in powder form were annealed in argon and hydrogen atmospheres followed by their repressing into pellets and reheating in air. Rietveld analysis of X‐ray diffraction patterns confirmed the single‐phase nature of the samples in the wurtzite type hexagonal (P63mc) ZnO structure. The X‐ray photoelectron spectroscopy (XPS) results indicate that the Co and Mn atoms are in +2 oxidation states, which incorporate at the Zn2+ site, with no signature of metallic clusters. The Co‐doped sample prepared in air displays a paramagnetic state while the sample annealed in Ar atmosphere shows a weak ferromagnetic ordering at 300 K. The co‐doping of Mn further enhances the ferromagnetic ordering, indicating that Co and Mn ions play an additive role in inducing the ferromagnetic ordering in the ZnO matrix. Interestingly, the Co‐ and (Co + Mn)‐doped ZnO samples annealed in hydrogen atmosphere show a huge increment in the magnetic moment, however, the Mn ions seem to stay passive towards the hydrogen induced magnetization. Notably, the samples reheated in air show suppression of the induced ferromagnetism (FM). The resistance measurements suggest that the additional carriers induced upon hydrogenation also play some role in mediating the exchange coupling. The O 1s XPS and the XRD results show clear evidence of oxygen depletion in the samples upon hydrogenation, followed by a recovery upon their reheating in air. The observed FM is explained in terms of composite effect of the oxygen vacancies and the carrier density. Our results point out that the ferromagnetic ordering could be switched between ‘on’ and ‘off’ by introducing (through hydrogenation) and by removing (through reheating in air) the oxygen vacancies in Co‐doped ZnO.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.