**3. Optical properties**

**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

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 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

Divalent cations were doped in monophosphates, giving variations in the electrical

Cerium orthophosphate has two crystalline phases [32, 33]. At low temperature

this material crystallizes in the hexagonal system. At high temperature cerium

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**

the dependence on humidity and the effect of H/D isotopic exchange [13].

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

expected to improve its optical and electrical properties.

electrical properties for application in electronic devices.

**size, low-temperature and less-complicated techniques [31].**

**2. Characterizations**

**70**

carried out.

*Electrochemical Impedance Spectroscopy*

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

$$f(\mathbf{R}) = \frac{\mathbf{1} - \mathbf{R}^2}{2R} \tag{1}$$

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

$$\left[F(\mathbf{R})\,h\nu\right] = A\left[h\nu - \mathbf{E\_g}\right]^n\tag{2}$$

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 from the corresponding intercept of the tangents to the plots of [F(R)\*hν] <sup>2</sup> vs. hν.

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,

**Figure 1.** *X-ray diffraction pattern of CePO4, Bi0.02Ce0.98PO4 and Li0.06Cd0.12Ce0.90PO4.*

**Figure 2.**

*[f(R)* � *hν]2 versus the hν (eV) plots of: (a) CePO4; (b): Ce0.9Cd0.13Li0.04PO4; and (c) Cr0.20Ce0.80PO4.*


**Table 1.**

*Gap energy values of CrxCe1-xPO4, BixCe1-xPO4 and Ce0.9Cd0.15-xLi2xPO4 nanomaterials.*

respectively, showing a red-shift trend when the doping- substitution percentage increases (**Figure 2)**. **Table 1** summarizes the gap energy values of nanomaterials.

The size, morphology and substitution of crystallites affect the energy of the band gap. The substitution of Ce3+ by a transition metal could induce the formation of several structural defects, creating different energy levels below the conduction band. The same behavior has been observed in Cr-doped Ni3(PO4)2 where the band gap decreases when Cr3+ replaces Ni2+ [43].
