Gap energy values of CrxCe1-xPO4, BixCe1-xPO4 and Ce0.9Cd0.15-xLi2xPO4 nanomaterials.
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. The doping improves the properties of the compounds and can lead to new properties. It is the first time, that multi- ionic doping process is used in the CePO 4 matrix, in order to improve the ionic conductivity and the electrochemical stability. The low percentage of (Cd2+, Li+), Cr3+, Bi3+ dopant affect the structure showing a weak decrease in the lattice parameters compared to the CePO4. Impedance spectroscopy analysis was used to analyze the electrical behavior of samples as a function of frequency at different temperatures. The total electrical conductivity plots obtained from impedance spectra shows an increase of the total conductivity as Li, Cr-content increases. The determined energy gap values decrease with increasingly Li+, Cr3+ and Bi3+ doping content. Electrochemical tests showed an improved capacity when increasing the Li+, Cr3+ and Bi3+ content and a stable cycling performance.
- phosphate materials
- optical properties
- impedance spectroscopy
- electrochemical properties
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 . 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 , 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, 6, 7].
Most of the work has been done on the optical properties of the rare earth doped CePO 4 , so there are few studies on the effect of metal ion doping on CePO4. Additionally, CePO4 materials have been used in hydrogen fuel cells . 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.
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 . 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. . 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 .
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 . 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 , the generation of O2 [18, 19] and the decomposition of organic compounds [20, 21]. It was founded that Y 2 SiO 5 :Bi 3+ 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 . The broad absorption band of Bi 3+ improves the emission process which could be varied from the UV to the NIR, depending on its final valence in the compounds . 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, 27, 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 CePO 4 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 ].
Cerium orthophosphate has two crystalline phases [32, 33]. At low temperature this material crystallizes in the hexagonal system. At high temperature cerium 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 . 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 .
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.
3. Optical properties
The band gap energy of the as-prepared samples was calculated using the Kubelka-Munk plot. The Kubelka-Munk function for diffuse reflectance  is
where R is the reflectance. The optical band gap, Eg, can be determined using the Tauc relation:
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 . 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]2 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, 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+ .
4. Electrical conductivity
The dc-conductivity (σdc) of BixCe1-xPO4 could be calculated using the Formula’s
(A = area of the sample surface and t = sample thickness). The temperature dependence of dc-conductivity could be plotted based on the Arrhenius law with the following expression:
where A0 is the pre-exponential factor, Ea the activation energy and K the Boltzmann constant.
The activation energy of the undoped CePO4 nanorods (Ea = 1.08 eV) is comparable to that obtained for CePO4 nanosheets (Ea = 1.06 eV) . It seems that the change of the morphology and the synthesis route used weakly affect the activation energy of the cerium phosphates. The activation energy deduced from Log (σT) as a function of 103/T ( Figure 3 ) are summarized in Table 2 .
The effect of Cr3+, (Cd2+, Li+) substitutions decreases the activation energies with the increase in Cr, (Cd, Li)-concentration ( Table 2 ). Consequently, the dc-conductivity of the as-prepared samples increases with temperature and with doping concentration. Lattice defects and distortions in the phosphate structure produced by the substitution allow the increase of the DC conductivity. The enhancement of activation energy could be related to the mobility of oxygen ions (O2 −). This phenomenon has been observed by Nandini et al. . They show that with an appropriate ratio of magnesium and strontium, the ionic conductivity increases as compared to that exhibited by ceria singly doped with Mg.
The difference in the electrical transport process between the Cr, Cd, Li doped CePO4 and the Bi-doped CePO4 results from the difference in atomic weight of Bi and Cr, Cd, Li. The atomic weight affects the mobility of the ions and therefore the Bi3+ ions remain close to their initial positions.
5. Electrochemical measurements
In order to explore the potential application of nonmaterials as cathode materials, their electrochemical performance with respect to Li insertion/extraction was investigated. Cyclic voltammograms (CVs) for CePO4, Ce0.9Cd0.15PO4 and Bi0.02Ce0.98PO4 nanorods (examples) at 20 mV/s are shown in Figure 1 . For all the as-prepared compounds, the cyclic voltammograms are well superposed indicating the relative structural stability under these conditions. The same shape of the CV curves slightly is observed for Nanoplate-like CuO in the presence of LiClO4 in propylene carbonate .
CePO4, Cd0.15 Ce0.90PO4 and Bi0.02Ce0.98PO4 based electrode cyclic voltammogramm.
These voltammograms indicate the intercalation/de-intercalation process of Li+ ions. During the electrochemical redox processes, the intercalation/de-intercalation process of Li+ ions can be represented by the following reaction:
The lithium ion diffusion coefficients can be calculated from the Randles-Sevcik law :
where ip is the peak current (A), n is the number of electrons exchanged, A is the apparent surface area of the electrode (cm2), Dli and C are the diffusion coefficient (cm2/s) and the analyte concentration (in moles/cm3) respectively, and V is the potential scan rate (V/s). The lithium ion diffusion coefficients deduced are 2.5 × 10−9, 0.7 × 10−9, 4.6 × 10−9 cm2s−1 for CePO4, Ce0.9Cd0.15PO4 and Ce0.9Cd0.13Li0.04PO4, respectively. The structure, surface area, grain size and morphology affect the calculated lithium diffusion coefficient DLi of the electrode materials. For example, Bi doping with the appropriate amount improved the electrochemical performance of LiFePO4 cathode material, synthesized by the sol–gel method .
For as-prepared BixCe1-xPO4 (x = 0.00, 0.02, 0.08) electrodes, The lithium ion diffusion coefficient (DLi) values could be determined by using Nyquist plot through the relation :
Where: F, R and T indicate Faraday constant, gas constant and room temperature, respectively.
(1). DLi can be calculated as the Warburg impedance Zw is inversely proportional to the square root of the diffusion coefficient as shown in . The calculated lithium diffusion coefficient of the CePO4 and Bi0.02Ce0.98PO4 and Bi0.08Ce0.92PO4 electrodes is 3.3 × 10−16, 40 × 10−16 and 12.8 × 10−16 cm2.s−1 respectively. The DLi variation values n can be attributed to creating the defect and increasing disorder of the lattice in doped CePO4, drives to the improvement of the electrochemical performance. The structure of H-CePO4-type characterized by infinite tunnels provides fast ionic transport. The Li + ions can move quickly in an appropriate direction .
where ΔV is the potential window, m is the mass of active material in one electrode, I is the current, and s is the potential scan rate. The variation of the specific capacitance of two prepared simples versus cycle number is given in Table 3 . We show that the partial substitution of Ce by Cd increase the capacitance. The increasing of the capacitance can be attributed to the partial substitution and the small crystal size which improves the kinetics of electrochemical reactions and the structure which provides fast ionic transport.
The reason for the improvement of the discharge capacity can be explained as follows: with Bi-doping, the grain size of the particles decreases, which leads to the migration of the Li-ion.
Doped samples show better performance in terms of discharge capacity than undoped ones. These results could be attributed to the contribution of the nanorod shape and the particle size. Indeed, the reduction of the size allows a faradic reaction providing a short ion diffusion path and electron transport.
In summary, we have demonstrated a rapid and convenient hydrothermal method for the preparation of doped and undoped CePO 4 nanomaterials. The Cr3+, Bi3+, Cd2+ and Li+ ions substitution affects the optical, electrical and electrochemical properties. The band gap energies of the as-prepared CePO4 nanorods decreased with increasing doping-concentration showing a red-shift trend. Comparative experiments have witnessed that the doped-CePO4 electrode had the most excellent electrochemical properties in comparison with undoped CePO4 nanomaterials. The electrochemical results show that the specific capacity and the electrical conductivity increase with increasing doping content. The specific capacitance of the hybrid electrode materials presents a good cyclic stability. The improved specific capacitance is due to the surface morphology and the decrease of grain size of the particles. The lowering in the crystal size allows a fast faradaic reaction, giving a short ion diffusion path, which improves the electrochemical properties. This simple synthesis methodology together with the good optical and electronic properties makes this material scientifically; technologically interesting and could find a potential use in nanoelectronics.