Cerium Oxide Nanostructures and their Applications

Due to excellent physical and chemical properties, cerium oxide (ceria, CeO2) has attracted much attention in recent years. This chapter aimed at providing some basic and fundamental properties of ceria, the importance of oxygen vacancies in this material, nano‐size effects and various synthesis strategies to form diverse structural morphologies. Finally, some key applications of ceria‐based nanostructures are reviewed. We conclude this chapter by expressing personal perspective on the probable challenges and developments of the controllable synthesis of CeO2 nanomaterials for various applications.


Introduction
In nanotechnology, nanomaterials play a vital role in various fields of science, such as physics, chemistry and materials sciences. As a key component of nanomaterials, nanoparticles are single particles or species whose diameter ranges from one to few tens of nanometres. Over the past few years, considerable efforts were made to develop many nanoparticles/nanocrystals for the development of new cutting-edge applications in communications, energy storage, sensing, data storage, optics, transmission, environmental protection, cosmetics, biology and medicine. Due to their limited size and a high density of corner or edge surface sites, the nanocrystals can exhibit unique physical and chemical properties, including optical, electrical and magnetic properties.
Due to reduced sizes, the nanoparticles possess high surface to volume ratios, which make them highly reactive with distinct characteristics. It is highly desirable to tune the properties of materials through various means such as shapes, morphologies and surface to volume ratios. Over the past few years, the researchers and scientists have made enormous efforts to develop nanoparticles with controlled morphologies, shapes and size. There are various possible synthesis routes, including liquid (chemical method), solid and gaseous media [3][4][5][6][7][8][9][10][11][12][13][14][15] for the synthesis of nanocrystals. However, chemical routes are considered as the most popular methods to synthesize nanoparticles, which can offer the advantages of low cost, eco-friendliness and reliability over other methods. Moreover, this method can also offer rigorous control on shape-and size-controlled synthesis of the nanoparticles.
It is generally believed that the reduced particle sizes may originate at high interface densities and hence may lead to enhanced non-stoichiometry levels [13]. Based on this, a lot of attention was paid to explore the nanostructures' ceria interfacial redox reactions and transport properties with respect to bulk ceria. This chapter is organised as follows. Section 2 presents cerium oxide (CeO 2 ) and material properties of CeO 2 . Section 3 describes imperfections/defect chemistry in CeO 2 . Section 4 describes the importance of oxygen vacancies in ceria. Section 5 presents nano-size effect. In Section 6, applications of CeO 2 in various fields are described. Finally, Section 7 presents concluding remarks and outlook.

Electronic structure
Cerium dioxide (CeO 2 /ceria) is considered the most stable oxide of the cerium. Cerium is the second member and the most reactive element in the lanthanide series. Being electropositive in nature, cerium exists in dual oxidation modes, Ce 3+ and Ce 4+ . The Ce 4+ oxidation state is usually considered more stable than 3+ due to Ce(4+) electronic structure [Xe]4f 0 being more stable state as empty than [Xe]4f 1 for Ce 3+ .Cerium usually has two types of oxides named cerium dioxide (CeO 2 ) and cerium sesquioxide (Ce 2 O 3 ), but in a larger context, CeO 2 is used as cerium oxide due to higher stability over Ce 2 O 3 .

Crystal structure
Cerium dioxide (CeO 2 ) has a fluorite structure (FCC) with space group Fm3m and it consists of a simple cubic oxygen sub-lattice with the cerium ions occupying alternate cube centres as shown in Figure 1. Figure 1 illustrates the structure of the stoichiometric CeO 2 with the four coordinated oxygen (represented by solid red big balls) and the eight coordinated cerium (represented by solid blue small balls). Cerium is at the centre of tetrahedron whose corners are occupied by oxygen atoms [14].

Microstructure
The microstructure of CeO 2 thin films can be of various shapes depending on substrate material, deposition method, deposition parameters and composition contents. Therefore, the film growth can result in epitaxial films on the one hand or polycrystalline films with varying grain sizes from few nanometres up to several hundred micrometres on the other hand. Polycrystalline films consist of a certain number of grains, which can play a key role in the electrical conduction process in thin films by forming depletion layers with low conductivity similar to Schottky barriers in metal-insulator junctions [15].  shows the space-charge region and the reduced conductivity due to the immovable positive charges. For thin films with smaller grain sizes, the space-charge region can exceed the grain size, leading to entirely depleted films with locally distributed conductivities far below the bulk conductivity of the material [16].

Imperfections/defect chemistry in bulk ceria
For an ideal crystalline solid, atoms are periodically arranged in a regular and symmetrical way. The formation of a crystal structure is based on the combination of a basis and infinite space lattice. This space lattice can be further split into unit cells and the combination of identical cells forms the entire crystalline structure. Imperfections in crystal structures are occurred by displacing atoms from their lattice positions that lead to break symmetry of the perfect periodic crystal lattice. In ceria, the intrinsic and extrinsic defects can exist. The presence of intrinsic defect may be due to thermal disorderness in crystal, which can be formed by redox reactions between the solid and surrounding atmosphere. Frenkel and Schottky defects are considered to be more plausible crystalline intrinsic defects. The extrinsic defects in crystal may be formed by introducing foreign dopant or due to impurities. The most dominant and stable known defects in ceria are linked to the presence of oxygen vacancies under a wide range of conditions. A reversible transition in the oxidation state of two cerium ions from Ce 3+ to Ce 4+ may generate neutral oxygen vacancies in ceria. The process can be shown as ( ) Thus, a neutral species ½O 2 (g) is formed if an oxygen ion (O 2-) leaves the ceria lattice. The two electrons left behind were trapped at two cerium sites, i.e. they become localized at two cerium sites. At such cerium sites, the electron prefers to occupy an empty Ce4f state that splits the Ce4f band into two sub-bands: an occupied Ce4f full band and an empty Ce4f empty band as shown in Figure 3.
Practically, the reduction limit of non-stoichiometric ceria is Ce 2 O 3 , where all cerium ions are found in a Ce 3+ oxidation state. The electronic band structure of Ce 2 O 3 bears resemblance to that of partially reduced ceria, in which Ce4f empty and Ce5d bands have been merged together in the conduction band as shown in Figure 3(c).

Importance of oxygen vacancies in CeO 2
Cerium oxide can accommodate high oxygen deficiency by the substitution of lower valent elements on the cation sub-lattice. Due to this property, high oxygen ion conductivities are expected, which indicate towards its potential applications as a solid electrolyte in solid oxide fuel cells (SOFCs) [17]. At the same time, CeO 2 is also well known to release significant levels of oxygen at low oxygen partial pressures (PO 2 ) and elevated temperatures leading to a mixed ionic electronic conductivity. Due to ease in redox-based reactions, ceria can easily occupy multiple oxidation states such as Ce(3+) and Ce(4+); therefore, electrons in ceria can be thought to exist as small polarons. The motion of the electrons in the ceria lattice can be imagined as a thermally mediated hopping mechanism [18]. For carrier and transport properties, the concentration of vacancies that are more mobile and can contribute to oxygen-ion transport in the solid solutions should be taken into account [19].
Generally, three low-index lattice planes exist on the surface of CeO 2 nanocrystals: (100), (110) and (111). From Figure 4, the stability of all three planes is different and follows the sequence (111) > (110) > (100), whereas the activity follows the reverse order [20][21][22][23]. The formation energy of oxygen vacancies at (111) exposed facet is higher than (110) and (100) facets; therefore, there are more oxygen vacancies on the (110) and (100) planes than (111). For example, nanoparticles usually constitute octahedral or truncated octahedral shapes and they are mainly exposed to most stable (111) facets to keep surface energy as minimum as possible.
While 1D nanostructures such as nanorods and nanowires possess the (110) and (100) planes, 3D nanocubes can expose (100) planes. Therefore, nanorods and nanocubes should have more oxygen vacancies on their surfaces. Meanwhile, the concentration of oxygen vacancies in the crystal can also be influenced by many other internal or external factors, such as temperature and doping elements, etc. [25]. The existence of oxygen vacancies and their transportation in crystal are very important phenomena. Higher concentration of oxygen vacancies provides ease in the movement of oxygen atoms within crystal, which favour redox reactions on its surface for excellent catalytic activities.

Nano-size effects
By decreasing particle size, ceria nanoparticles demonstrate the formation of more oxygen vacancies. The large surface area to volume ratio existing in a nanoparticle enables CeO 2 to react differently resulting in unique properties. For instance, two orders of magnitude high catalytic activity for CO oxidation was observed by synthesizing 3-4 nm sized CeO 2 nanoparticles supporting Au over a regular bulk cerium oxide support medium [26,31]. Therefore, it is feasible to tune the specific reactivity of ceria nanoparticles by controlling their size. Furthermore, nano-sized CeO 2 can have enhanced electronic conductivity, size lattice relaxation and many other effects as compared to bulk ceria.
Additionally, for determining particle reactivity, building nano-sized particle is very crucial in a way that the lattice of nanoceria expands if the particle size is reduced. The expansion of lattice would decrease its oxygen release and reabsorption capabilities. A detailed electron diffraction study on nanoceria was conducted previously [27] in which a systematic lattice expansion was observed by reducing the size of ceria particles down to nanoscale over bulk ceria. Their findings suggested that by having ultra-small-sized (1.1 nm) ceria nanoparticle, a large fraction of the cerium atoms occupy fully reduced state, even though the ceria nanoparticles retain a cubic-shaped lattice rather than hexagonal lattice [27]. In another theoretical study on ceria nanoparticles, the simulation calculations revealed that by increasing ceria particle size the formation energy of oxygen vacancies is reduced [28].

Applications of ceria
Being technologically important functional material, ceria has remarkable applications in many diverse fields. In this section, a brief discussion on some of the most widely known commercial and industrial applications of ceria and nanoceria is presented.

Solid oxide fuel cells
Solid oxide fuel cells (SOFCs) are considered to have great potential in providing clean and reliable electric power; therefore, research in this area attracts great attention in recent years. Many reports suggest that ceria-based ion conductors possess huge resistance to carbon deposition and have potential to allow unstoppable supply of dry hydrocarbon fuels to the anode [29,30].
Gorte and Vohs [31] first demonstrated a direct electrochemical oxidation of hydrocarbons such as methane and toluene using SOFC at 973 and 1073 K with copper and ceria composite. The anode was designed as Cu/CeO 2 /YSZ, with Cu primarily utilized as the current collector and the function of CeO 2 was to impart catalytic activity for the required oxidation reactions. These anodes were found to have inherently high redox stability, large sulphur tolerance and were operative with sulphur contents up to 400 ppm without losing its significance performance [32].
Ceria-based ceramics are well known to exhibit mixed ionic and electronic conductivity in a reducing atmosphere due to reversible redox transition between Ce 3+ and Ce 4+ . Additionally, their excellent catalytic activities also correspond to ease in an oxygen-vacancy formation mechanism. Another effective method to increase the reforming reactions of hydrocarbon, i.e. to facilitate C-H bond breaking process is the addition of noble metals, such as Pt, Rh, Pd and Ru. Gd-doped CeO 2 -based electrolyte films having Ru and Ni as incorporated metals were fabricated to evaluate the role of metal catalysts in anode reactions by Hibino et al. [33,34].
The anodes were directly operated on various hydrocarbons at 600°C and the results demonstrated that the presence of Ru catalyst favours the reforming reactions of unreacted hydrocarbons by steam and carbon dioxide. This further avoids the gas-phase diffusion of fuels by restricting the interference of steam and CO 2 . The obtained power density in this case was 750 mW cm -2 with dry methane, which was quite analogous to the power density of 769 mW cm −2 achieved with wet hydrogen [33,34].
The reduction of activation over potential for hydrogen oxidation by having ceria-based anodes for SOFC has been exemplified in many reports. Moreover, well-defined geometry and having porous or composite electrodes structure based on ceria were also described. In a report, Chueh et al. [35] demonstrated well-defined interferences occupied ceria-metal structures, in which near-equilibrium H 2 oxidation reaction pathway was subjugated by electrocatalysis at the oxide-gas interface with a minimal contribution that was observed from oxide-metal-gas triple-phase boundaries as shown in Figure 5. The similar phenomenon was observed even for structures with reaction site densities approaching those of commercial SOFCs [35].
By keeping the ceria-catalysed reaction site density fixed as 2PB, the density of metal-catalysed reaction site (3PB) was varied by a factor of 16; no substantial change was observed in the electrochemical activity. Later on, the addition of metals did not put much impact on the electrochemical activities, respectively. They conclude that the rational design of ceria nanostructures promote electrochemical activities and thus provide a new route to achieve high performance [35].

Catalytic applications
The tendency of oxygen uptake and release of ceria due to reversible transition between Ce 3+ and Ce 4+ makes this material a key ingredient for catalytic applications and reactions. Various nano-micro ceria structures have been fabricated, and their catalytic applications are extensively reported in recent years.
Carbon monoxide (CO) oxidation is the most exclusively studied reaction in this category. During CO oxidation, ceria nanomaterials can elevate the oxygen storage capability of catalysts at low temperatures. Generally, high surface area occupied nanomaterials are considered ideal for this application. The high surface area provides a greater tendency to active species to contact with reactants, and therefore an enhanced catalytic performance can be expected. There are different reports that discuss various morphologies to evaluate the CO oxidation performance. For instance, nanotubes exhibit inner and outer surfaces that unadventurously provide active sites for the reactants adsorption that leads to better catalytic performance in CO oxidation [36]. On the other hand, nano-sized particles also exhibit high surface area, but demonstrate poor catalytic performance; while nanorods, with low surface area and larger diameter, were found to be more active in CO oxidation [37][38][39][40]. This unusual behaviour of diverse morphologies is mainly attributed to the exposed planes. For example, the nanoparticles have dominant (111) exposed planes, whereas the nanorods' edges are terminated by (110) and (100) planes. The (110) and (100) planes are considered to be more active than the (111) plane and hence are found to express significantly enhanced redox properties required for excellent catalytic activities in CO oxidation.
In a study, Kayama et al. [41] introduced a new concept of fabricating ceria particles' aggregates around central silver metal (rice-ball-like structure) which is quite different than conventional core-shell-like configuration. The catalytic activities of fabricated structure were evaluated, and it was found that below 300°C CeO 2 -Ag oxidizes soot much more efficiently than pure CeO 2 and Ag-CeO 2 that oxidize soot above 300°C as shown in Figure 6. In the absence of rice-ball nanostructure, the oxidation by the catalyst was inefficient at low temperatures below 300°C. Therefore, it was suggested that the synthesis mechanism of the rice-ball-like structure can potentially be extended to other compositions. Doping foreign elements into ceria matrix can considerably tune the inherent physical and chemical properties of ceria. To keep this in mind, Y-, La-, Zr-, Pr-and Sn-doped ceria were reported by Xiao et al. [42]. In their work, they fabricated a flower-like structure, and the catalytic activities towards CO oxidation were recorded for all dopant elements as shown in Figure 7.
From the figure, two main observations were recorded. (i) The doping of Sn and Pr enhanced the catalytic activities of ceria-based nanoflower structure and this behaviour was attributed to the variable valence states of Pr and Sn. (ii) The doping of Y, La and Zr in CeO 2 did not express considerable catalytic activities, and this trend was explained on the basis of the stable valence states of dopants. Also, they suggested that the doping cations having stable valence state partially suppress the redox reactions, i.e. Ce 4+ to Ce 3+ conversion which ultimately suppress the oxygen storage capacity in ceria.

Photocatalysis
Globally, efficient visible light photocatalytic water splitting is an active area of research for renewable energy as well as water and air purification. A novel and efficient Au-supported CeO 2 nanoparticle-based photocatalysts were fabricated, and its visible light activities were reported by Primo et al. [43]. Excellent photocatalytic activity of ceria nanoparticles was observed for oxygen generation from water as shown in Figure 8.  Figure 8 shows the evolved oxygen over time for visible-light illumination for various Au loadings on CeO 2 catalysts. The sample, with 1.0 wt% loading of Au, renders a more profound photocatalyst than 3.0 wt% Au. Under UV light illumination, the ceria sample (A) prepared by the novel biopolymer template procedure having smaller particle size was found to be more efficient than the large particle-sized occupied commercial CeO 2 (B) sample. Their observations were quite different from other reports on TiO 2 and WO 3 , in which under UV light, the photocatalytic activity was poor and reduced, while Au-supported CeO 2 delivers outclass photocatalytic response under visible light. Recently, our group has successfully synthesized ultra-small-sized ceria nanocubes. The effect of indium doping on oxygen vacancies' concentration and subsequently on photocatalytic response was investigated in detail [44]. Various In doping concentrations ranging from 0, 5 to 15% were fabricated and their photocatalytic responses were evaluated. By increasing Indium doping concentration, the photocatalytic activities were found to be enhanced up to a certain extent (10% In doping). After this doping concentration, the photocatalytic activities were suppressed. The possible reason may be due to the presence of In 3+ , which may serve as an electron acceptor and electron donor simultaneously (from In 3+ to In 2+ and/or from In 3+ to In 4+ ) to localize the charge carriers. This localization may further enlarge the separation of electron-hole pairs by trapping at energy levels in close approximity to the valence and conduction bands [44]. This effect is schematically described in Figure 9.
Ceria nanomaterials have also a potential application for environmental remediation. The photocatalytic activities of CeO 2 nanotubes and nanoparticles were investigated and compared with commercial TiO 2 (P25) as shown in Figure 10 [45]. A well-known toxic pollutant named 'aromatic benzene' that is commonly found in urban ambient air and is of substantial concern regarding environmental health was used in photocatalytic measurements.
The results in Figure 10 demonstrate that at the reaction time of 22 h, the CeO 2 -NT expressed excellent photocatalytic performance towards the gas-phase degradation of benzene. The amount of CO 2 over CeO 2 -NT was maintained at 29 ppm even after 10 h of reaction time. On the contrary, ceria nanoparticles and P25 nanoparticles demonstrate quite uneven photocatalytic behaviour of benzene degradation.

Concluding remarks and future outlook
In this chapter, some fundamental physical and chemical properties of ceria were addressed in the beginning. In the later half, recent progress in the shape-controlled synthesis and morphology-dependant performance of ceria nano-microstructures was highlighted. Although, the microstructures, morphologies, characterization approaches and theoretical studies of ceria-based nanostructured materials are extensively investigated in recent years with the achievement of some exceptional encouraged results. However, there are still numerous contests such as cost-effective synthetic processes for morphology-dependent catalytic and biomedical applications that required more efforts to tackle the key fundamental issues.
Another important fatal issue in the preparation of ceria-based materials is 'Sintering'. Particles can easily agglomerate at high temperatures, which is a big reason for sudden drop in specific surface area of nanomaterials and reduced crystal defects. Although, the addition of rare-earth elements somehow facilitates anti-sintering phenomenon; however, this restricts the limitation of special additives in ceria-based nanomaterials. Therefore, to spread the span of nanoceria applications, this problem needs to be addressed comprehensively. Finally, vast scale production of ceria nanomaterials is also challenging. It is essentially required to develop and improve more valuable preparation of technologies for industrial and commercial applications combined with deepening comprehension of the formation mechanisms.