Vol.:(0123456789) 1 3 Applied Physics A (2018) 124:609 https://doi.org/10.1007/s00339-018-2036-0 Efect of defect states and oxygen vacancies on optical transitions due to Co 2+ substitution in CeO 2 Saurabh Tiwari 1  · Nasima Khatun 2  · Parasmani Rajput 3  · Dibyendu Bhattacharya 3  · S. N. Jha 3  · Chuan‑Ming Tseng 4  · Shun‑Wei Liu 5  · Sajal Biring 5  · Somaditya Sen 1,5 Received: 11 May 2018 / Accepted: 11 August 2018 © Springer-Verlag GmbH Germany, part of Springer Nature 2018 Abstract CeO 2 has cubic fuorite structure which is modifed due to oxygen content as well as external substituents. The oxidation state of Ce plays an important role in strain and related physical properties. Ce 3+ being larger in size than the Ce 4+ ion, one expects a change in band structure due to changes in bond length; substitution of Ce 4+ by Co 2+ in sol–gel prepared, homogeneous, single-phase Ce 1−x Co x O 2 (x ≤ 10) nanopowders. The lower valence states of Co 2+ induces oxygen vacancies which transforms some Ce 4+ to Ce 3+ . A careful study of oxygen vacancies, strain, bond length and related band structure changes, have been targeted in this study. The possibility of phonon participation in electronic transition has been discussed using Tauc plot. Ce 3+ forms defect states, between valence and conduction bands. Lattice parameters decrease, but strain increases with substitution. 1 Introduction Fluorite-structured cerium oxide (CeO 2 ) has attracted atten- tion from various felds in materials engineering, chemis- try and physics. It has versatile applications in water–gas shift reaction [1], gas sensor [2], biomedicines [3], catalysis [4], glass polishing [5], fuel cells [6], ultraviolet absorbents [7], etc. Recently many researchers have worked on aliova- lent (especially transition element) ion-doped CeO 2 , with enhanced functionalities such as oxygen storage [8], ferro- magnetism [9], photocatalysis [10, 11], diferential cytotox- icity against neuroblastoma cancer cells [12], and catalytic activity for oxidation of CO [13]. The last two functionalities are related to bandgap of these ceramic materials. CeO 2 has bandgap ~ 6 eV which is due to O2p, valance band (VB) and Ce 5d and 6s, conduction band (CB) but in between these bands an empty Ce 4f band is present which with partially flling Ce 4f 1 causes bandgap ~ 3 eV [14]. With substitu- tion, defects and disorder in lattice increase which leads to the formation of some new states between VB and CB, and thereby reduces the bandgap [15]. However, band structure and bandgap of CeO 2 have always been a controversial sub- ject. Both direct and indirect bandgaps have been reported with a range of values of bandgaps [16–21]. However, in both types only allowed transitions have been discussed in literature, whereas forbidden transitions have not been dis- cussed much. This work describes synthesis of Ce (1−x) Co x O 2 solid solution and their subsequent characterization in terms of electronic structure and optical properties. Efect of Co sub- stitution on crystallite size, bandgap and defects has been investigated. Electronic band structure transitions have been discussed. 2 Experimental procedure Ce 1−x Co x O 2 nanoparticles (hereafter referred as CCO) for x = 0, 0.025, 0.05 and 0.1 were synthesized by Pechini sol–gel technique, hereafter referred to as CCO0, CCO2, CCO5 and CCO10, respectively. Cerium (III) nitrate * Somaditya Sen sens@iiti.ac.in 1 Metallurgy Engineering and Material Sciences, Indian Institute of Technology Indore, Simrol, Indore, India 2 Department of Physics, Indian Institute of Technology Indore, Simrol, Indore, India 3 Atomic and Molecular Physics Division Bhabha Atomic Research Centre, Mumbai, India 4 Materials Engineering, Ming Chi University of Technology, New Taipei City, Taiwan 5 Electronic Engineering, Ming Chi University of Technology, New Taipei City, Taiwan