**1. Introduction**

**Nanoscience and nanotechnology is a rapid-developing field which has demanded the technologist to innovate applicable nanomaterials with manipulated shape and size to explore their principal chemical and physical characteristics** [1]. In recent years, rare earth phosphates have attracted many researchers because of their technological applications [2, 3]. Cerium orthophosphate **nanomaterials** have important properties: high thermal stability [4], **very low solubility in water, their use in the production of moisture sensors for luminescent materials, a poison for automotive catalysts and a novel oxygen sensing material on the basis of its redox responsive reversible luminescence** [5–7].

**Most of the work has been done on the optical properties of the rare earth doped CePO4, so** there are few studies on the effect of metal ion doping on CePO4. Additionally, CePO4 materials have been used in hydrogen fuel cells [8]. To better understand the mechanism of conduction, information on the behavior and ionic conductivities of charge carriers located in phosphates, electrical studies have been carried out.

orthophosphate crystallizes in the monoclinic system. The hexagonal structure is characterized by the existence of large tunnels parallel to the c-axis in which the water present in the compound appears to be localized. The CePO4 produced in aqueous solution at room temperature crystallizes in a hexagonal form [34, 35]. After heat treatment at 650°C, the hexagonal phase (CePO4) started converting into

X-ray diffraction (XRD). All samples are single phase having a hexagonal structure similar to CePO4. **The** 2θ values of doped materials shift slightly higher angles with increasing Cr, Bi, Cd and Li content, confirming the complete dissolution of dopants (**Figure 1**). The same behavior was observed when Fe3+ ion substitutes La3+ ion in LaPO4 [36]. The average crystallite size of all samples decreases with increasing the amount of doping. The main reason for the decrease of the grain size may be due to the fact that doping introduced defects and the defects prevent

Many parameters affecting the morphological characteristics of the hexagonal cerium phosphate nanocrystals such as the cerium concentration, the treatment temperature, the reaction time, the nature of the surfactant, the pH value of the solution and the synthesis method. The materials take on a similar shape to the

The band gap energy of the as-prepared samples was calculated using the Kubelka-Munk plot. The Kubelka-Munk function for diffuse reflectance [38] is

*<sup>f</sup>*ð*R*Þ ¼ <sup>1</sup> � <sup>R</sup><sup>2</sup>

½*F*ð*R*Þ*:hν*� ¼ *A*½*hν* � Eg�

The determined energy gap values decrease with increasing Cr, Bi, Cd and Li-doping content in CrxCe1-xPO4 (x = 0.00, 0.08, 0.10 and 0.20), BixCe1-xPO4 (x = 0.00, 0.02 and 0.08), Ce0.9Cd0.15-xLi2xPO4 (x = 0 and 0.02) nanorods,

from the corresponding intercept of the tangents to the plots of [F(R)\*hν]

*X-ray diffraction pattern of CePO4, Bi0.02Ce0.98PO4 and Li0.06Cd0.12Ce0.90PO4.*

where R is the reflectance. The optical band gap, Eg, can be determined using the

where A is an energy-independent constant, Eg is the optical band gap and n can take values of 0.5, 1.5, 2 and 3 depending on the mode of transition [39]. The band gap energies can be estimated by extrapolating the linear portions to the hν axis and

<sup>2</sup>*:<sup>R</sup>* (1)

*<sup>n</sup>* (2)

<sup>2</sup> vs. hν.

nanorod morphology with the size depending on the dopant-content.

), Cr3+, Bi3+ doped CePO4 materials were characterized by

*), Cr3+, Bi3+ Doped CePO4 Materials Optical…*

a monoclinic structure. The ions (Cd2+, Li+

*Designing and Synthesis of (Cd2+, Li+*

*DOI: http://dx.doi.org/10.5772/intechopen.91330*

grain to grow [37].

**3. Optical properties**

Tauc relation:

**Figure 1.**

**71**

Generally, the doping process improves the properties of the compounds and can lead to new properties [9, 10]. Trivalent elements have been known as doping elements, improving the physico-chemical properties of cerium phosphate-based materials [11]. In order to improve the electrical and optical properties, the cerium phosphate was partially substituted by divalent transition metal ions. The doping with Ca and Sr. has improved the electrical conductivity of (La, Ce) PO4 [12, 13]. The high conductivity of the Sr-doped CePO4 under wet oxidizing conditions due to electronic and ionic conduction is shown by Moral et al. [12]. Norby et al. studied the effect of the substitution of lanthanum by calcium and strontium on the conductivity, described by the dependence on humidity and the effect of H/D isotopic exchange [13].

The substitution effect depends on the nature of the doping elements. Chromium shows the stability of the valence state (+ III) in conductive p-type SOFC interconnection materials [14]. Numerous reports show that substitution with Cr3+ ions introduces interesting properties in ferrites [15, 16]. Cr-doping CePO4 is expected to improve its optical and electrical properties.

Bismuth-based materials have been studied because of their excellent photocatalytic activities in the reduction of NO [17], the generation of O2 [18, 19] and the decomposition of organic compounds [20, 21]. **It was founded that Y2SiO5:Bi3+ gives rise to three emission bands centering at: 355, 408, and 504 nm upon UV excitation possibly from three types of bismuth emission centers in the compound, respectively** [22]**. The broad absorption band of Bi3+ improves the emission process which could be varied from the UV to the NIR, depending on its final valence in the compounds** [23]. The Bi3+ ions combined with rare earth ions such as cerium, Ce3+, can improve the optical properties of CePO4 nanomaterials. The study of the effect of doping with Bi3+ ions on the structural and electrical properties of CePO4 is virgin. This leads to new optical and electrical properties for application in electronic devices.

Divalent cations were doped in monophosphates, giving variations in the electrical properties of these doped materials. The aim is to study the combined effect of monovalent Li+ and divalent Cd2+ ions on structural, electrical and optical properties. Indeed, the electrical and electrochemical properties of cadmium allow it to be used in mobile phone batteries [24, 25]. Also lithium Li+ ions associated with the divalent Fe2+, Mn2+ and Co2+ ions favor the increase of the capacity, the lifetime, **diffusion process** and the electrochemical stability of a phosphate-based electrode [26–28]. **The adjustment of the size, shape, density, optical, electrical and dielectric properties of nanoparticles could help tune their broad spectral resonance wavelength [29]. Microemulsion approach associated to the hydrothermal conditions could be used to fabricate single crystalline CePO4 nanowires with controlled aspect ratios [30]. Hydrothermal process has emerged as a powerful tool due to some significant advantages such as cost-effective, controllable particle size, low-temperature and less-complicated techniques [31].**

## **2. Characterizations**

Cerium orthophosphate has two crystalline phases [32, 33]. At low temperature this material crystallizes in the hexagonal system. At high temperature cerium

*Designing and Synthesis of (Cd2+, Li+ ), Cr3+, Bi3+ Doped CePO4 Materials Optical… DOI: http://dx.doi.org/10.5772/intechopen.91330*

orthophosphate crystallizes in the monoclinic system. The hexagonal structure is characterized by the existence of large tunnels parallel to the c-axis in which the water present in the compound appears to be localized. The CePO4 produced in aqueous solution at room temperature crystallizes in a hexagonal form [34, 35]. After heat treatment at 650°C, the hexagonal phase (CePO4) started converting into a monoclinic structure.

The ions (Cd2+, Li+ ), Cr3+, Bi3+ doped CePO4 materials were characterized by X-ray diffraction (XRD). All samples are single phase having a hexagonal structure similar to CePO4. **The** 2θ values of doped materials shift slightly higher angles with increasing Cr, Bi, Cd and Li content, confirming the complete dissolution of dopants (**Figure 1**). The same behavior was observed when Fe3+ ion substitutes La3+ ion in LaPO4 [36]. The average crystallite size of all samples decreases with increasing the amount of doping. The main reason for the decrease of the grain size may be due to the fact that doping introduced defects and the defects prevent grain to grow [37].

Many parameters affecting the morphological characteristics of the hexagonal cerium phosphate nanocrystals such as the cerium concentration, the treatment temperature, the reaction time, the nature of the surfactant, the pH value of the solution and the synthesis method. The materials take on a similar shape to the nanorod morphology with the size depending on the dopant-content.
