Photoluminescence and Refractive Index Dispersion Properties of Zn_3−xM_x(PO₄)₂ (M=Co, Ni; x = 1) Nanoparticles ^†

Abstract

This study investigated the structural, optical, and photoluminescence properties of Zn_3−xM_x(PO₄)₂ (M=Co, Ni; x = 1) nanoparticles, synthesized via the solid-state method. The optical constants and dispersion energy parameters were determined using UV–visible spectroscopy. The optical band gaps were found to be 2.48 eV for Zn₃(PO₄)₂, 3.05 eV for Zn₂Ni(PO₄)₂, and 3.12 eV for Zn₂Co(PO₄)₂. The optical dielectric constant of Zn_3−xM_x(PO₄)₂ (M=Co, Ni; x = 1) was also determined. Photoluminescence spectra revealed peaks at 2.56 eV for Zn₃(PO₄)₂, 3.26 eV for Zn₂Co(PO₄)₂, and 2.47 eV for Zn₂Ni(PO₄)₂. These peaks correspond to the recombination of excitons and/or shallowly trapped electron–hole pairs, indicating band-edge emission.

1. Introduction

Current electronic technology emphasizes developing new functional materials and leveraging scientific advancements in electronics to design innovative devices, aiming to create a flexible, sustainable, and technologically advanced environment. Semiconducting materials are the backbone of modern electronics, enabling the creation of solid-state devices, transistors, and integrated circuits, which are essential for electronic communication and security systems. Phosphates are increasingly used as host lattices in the development of novel optoelectronic materials. The growth of the phosphor industry is propelled by the creation of new phosphor materials activated by transition metal ions or rare earth ions [1,2]. Recently, inorganic metal phosphate compounds have garnered significant attention due to their outstanding physical and chemical properties, including applications in ultraviolet (UV) nonlinear optics (NLO), electrode materials, fluorescent hosts, and ion exchange candidates [3,4,5,6,7,8,9,10]. NLO crystals are crucial for laser light frequency conversion, which is vital in solid-state laser applications [11]. Phosphates are traditional sources for NLO materials, such as the well-known commercial KTiOPO₄ (KTP) and KH₂PO₄ (KDP) crystals [12,13]. The presence of metal impurities can introduce ionic defects that affect the optical properties of KDP crystals. Recent studies have shown that nickel, as a transition metal, enhances low-energy charge transfer due to its open D-shell electronic configuration [14]. This characteristic makes nickel valuable for the development and modification of nonlinear optical crystals. Introducing small amounts of impurities, such as Ni²⁺ as a bimetallic dopant, significantly affects the growth rate, morphology, optical properties, spectral characteristics, and mechanical properties of ammonium dihydrogen phosphate (ADP) [15]. Transition metal compounds, such as those containing Ni and Co, are highly effective electrocatalysts for water splitting. Doping these metals can significantly enhance catalytic activity by increasing current density, reducing overpotential, and improving electrical conductivity and metal-to-metal charge transfer. Additionally, doping reduces surface adsorption energy [16]. Because Ni and Co share similar atomic structures with elements from the 8/VIII and 9/VIII groups, their incorporation helps maintain the original morphology and surface properties of transition metal compounds. Cobalt-based materials have been used as catalysts for water splitting for an extended period [17]. Keeping this in view, the present study focuses on the influence of Ni²⁺ and Co²⁺ doping on the crystal structure of Zn_3−xM_x(PO₄)₂ (M=Co, Ni) and its optical and photoluminescence properties. Zinc phosphate Zn₃(PO₄)₂ is a widely used, non-toxic, multifunctional semiconductor with properties like corrosion resistance and adhesiveness. It is used as dental cement and in chemically bonded ceramic (CBC) materials due to its low solubility in water and compatibility with biological environments. Its large energy band gap makes Zn₃(PO₄)₂ an ideal host lattice for luminescent materials, optical communications, and display devices [18].

2. Experimental Section

Powder products were synthesized by high-temperature solid state reactions. The raw materials were ZnO (Sigma-Aldrich (Saint Louis, MO, USA) 99.99%), analytical reagent (A.R.), (NH₄)₂HPO₄ (A.R.), CoO (Sigma-Aldrich 99.99%), and NiO (Sigma-Aldrich 99.99%). The concentration of Ni²⁺ or Co²⁺ ions was fixed with at a 1:2 molar ratio of the Zn ions in Zn₃(PO₄)₂. The mixtures of corresponding raw materials were thoroughly grounded and then fired at 500 °C for 3 h in air. After being reground, they were sintered at 950 °C in air for 5 h. The samples were characterized using X-ray diffraction (Cu-Kα radiation λCu = 1.5406Å). The optical measurements were determined by a UV–Vis spectrophotometer (PERKIN ELMER 1050). PL spectroscopy was employed to conduct photoluminescence (PL) research.

3. Results and Discussions

3.1. X-ray Diffraction Analysis

The crystal structure of Zn_3−xM_x(PO₄)₂ (M=Co, Ni) nanoparticles was analyzed from the XRD pattern, shown in Figure 1a. All samples crystallize in an orthorhombic space group of Pnma.

The average crystallite size ($\mathit{D}$) was calculated using Scherrer’s formula given by the following:

$$\mathit{D}={\displaystyle \frac{\mathit{k}\mathit{\lambda }}{\mathit{\beta }\mathbf{cos}\left(\mathit{\theta }\right)}}$$

(1)

where $\mathit{\lambda }$ represents the X-ray wavelength, $\mathit{\beta }$ is the full width at half maximum (FWHM) in radians, and $\mathit{\theta }$ is the Bragg diffraction angle. The dislocation density ($\mathit{\delta }$) is defined as follows:

$$\mathit{\delta }={\displaystyle \frac{\mathit{1}}{{\mathit{D}}^{\mathit{2}}}}$$

(2)

The strain ($\mathit{\epsilon }$) is expressed using the following relation:

$$\mathit{\epsilon }={\displaystyle \frac{\mathit{\beta }}{4\mathbf{tan}\left(\mathit{\theta }\right)}}$$

(3)

The average crystallite size, the dislocation density, and the strain of Zn_3−xM_x(PO₄)₂ (M=Co, Ni) nanoparticles were listed in

Table 1. Some structural parameters of Zn_3−xM_x(PO₄)₂ (M=Ni, Co; x = 1) nanoparticles.

Samples	Lattice Parameter	Crystallite Size (nm)	Dislocation Density × 104 Lines/m2	Strain ε × 10−3
Zn3(PO4)2	a = 10.620 Å b = 18.429 Å c = 5.020 Å	109	8.417	5.36
Zn2Co(PO4)2	a = 10.568 Å b = 18.257 Å c = 5.025 Å	95	11.080	5.94
Zn2Ni(PO4)2	a = 10.562 Å b = 18.224 Å c = 5.013 Å	87	13.211	6.21

. As can be seen, the crystallite size decreased with the Ni and Co addition, and an increase in dislocation density was observed. Generally, dislocations occur as a result of internal stress within the material.

3.2. Luminescence Analysis

Figure 2a–c display the excitation and emission spectra of the Zn_3−xM_x(PO₄)₂ (M=Ni, Co; x = 1) nanoparticles. The optimal excitation spectrum of Zn₃(PO₄)₂ occurs at 575 nm. When excited at 460 nm, Zn₃(PO₄)₂ shows strong emission peaks at 575 nm, with CIE chromaticity coordinates of x = 0.4787 and y = 0.5202, attributed to ligand-to-metal charge transfer transitions [19]. Compared to Zn₂Ni(PO₄)₂, the latter exhibits a green-shifted emission spectrum at 527 nm when excited at 440 nm. For Zn₂Co(PO₄)₂, the emission spectrum, shown in Figure 2c, is obtained with an excitation wavelength of 290 nm, and its optimal emission wavelength is 458 nm, with CIE chromaticity coordinates of x = 0.1469 and y = 0.0266. This shift occurs due to the interaction bonds between Zn²⁺–Ni²⁺ or Zn²⁺–Co²⁺, which enhance the rigidity of the planar structure.

3.3. Optical Study

Figure 3a illustrates the variation in optical absorbance as a function of photon wavelength in the range of 350 nm to 700 nm. Generally, the absorption edge is located in the UV region and shifts towards higher wavelengths.

The optical band gap (E_g) is a crucial parameter for determining the optoelectronic properties of a material. It can be calculated as a function of photon energy (hν) using the following relation:

$$\mathit{\alpha }\mathit{h}\mathit{\nu }={{\mathit{\alpha }}_0\times (\mathit{h}\mathit{\nu }-{\mathit{E}}_{\mathit{g}})}^{\mathit{m}}$$

(4)

In the formula, hν represents photon energy, m equals 1/2 or 2 for direct or indirect transitions, respectively, and α₀ stands for an optical constant. As shown in

Table 2. Some optical parameters Zn_3−xM_x(PO₄)₂ (M=Ni, Co; x = 1) nanoparticles.

Composition	Zn3(PO4)2	Zn2Co(PO4)2	Zn2Ni(PO4)2
Exact band gap [eV]	2.48	3.12	3.05
Linear refractive index, no	2.55	2.36	2.38
Optical dielectric constant, ɛo	6.50	5.59	5.67
Linear susceptibility, χ1 [esu]	0.437	0.363	0.371
Nonlinear susceptibility, χ3 × 10−12 [esu]	6.25	2.97	3.22
Nonlinear refractive index, n2 × 10−11 [esu]	9.239	4.744	5.100

, the optical band gap increased with the addition of Ni and Co. The values of (E_g$)$ were determined to be 2.48 eV, 3.05 eV, and 3.12 eV for Zn₃(PO₄)₂, Zn₂Ni(PO₄)₂, and Zn₂Co(PO₄)₂, respectively. These findings indicate that Zn₃(PO₄)₂-based materials, functioning as advanced n-type semiconductors, are garnering considerable interest due to their potential applications in areas such as light-emitting diodes and lithium-ion batteries [20].

The linear refractive index ${\mathit{n}}_0$ is linked to the optical band gap according to the Dimitrov–Sakka expression [21], and the optical dielectric constant is calculated from the refractive index (ε_o= n₀²).

$${\mathit{n}}_0={\left[6\sqrt{{\displaystyle \frac{5}{{\mathit{E}}_{\mathit{g}}}}}-2\right]}^{{\displaystyle \frac{1}{2}}}$$

(5)

Table 2 represents the values of the refractive index and the dielectric constant. The refractive index values are observed to decrease with the Co and Ni addition.

The polarization density P of a material is expressed by the following formula:

$$\mathit{P}={\mathit{\chi }}^1\mathit{E}+{\mathit{\chi }}^2\mathit{E}+{\mathit{\chi }}^3\mathit{E}+\dots$$

(6)

In the equation, χ¹ represents the linear susceptibility, while χ² and χ³ denote the second- and third-order nonlinear susceptibilities, respectively, and E stands for the electric field. The second-order susceptibility χ² is zero for glasses and centrosymmetric crystals.

Also, the refractive index n is described by the following equation:

$$\mathit{n}={\mathit{n}}_0+{\mathit{n}}_2{\mathit{E}}^2$$

(7)

where n₂ represents the nonlinear refractive index. The nonlinear refractive index n₂ is related to the third-order nonlinear susceptibility χ³ as follows:

$${\mathit{n}}_2={\displaystyle \frac{12\mathit{\pi }}{{\mathit{n}}_0}}{\mathit{\chi }}^3$$

(8)

and

$${\mathit{\chi }}^3={\left[{\mathit{\chi }}^1\right]}^4\times 1.7\times {10}^{-10}={\left[{\displaystyle \frac{{{\mathit{n}}_0}^2-1}{4\mathit{\pi }}}\right]}^4\times 1.7\times {10}^{-10}\mathit{e}\mathit{s}\mathit{u}$$

(9)

The linear susceptibility χ¹, the third-order nonlinear susceptibility χ³, and the nonlinear refractive index are represented in Table 2.

4. Conclusions

Zn_3−xM_x(PO₄)₂ (M=Co, Ni; x = 1) nanoparticles were successfully grown by the solid-state method. The XRD analysis confirmed the orthorhombic structure of these compounds. UV–visible spectroscopy revealed that doping increased the optical band gaps, which are 2.48 eV for Zn₃(PO₄)₂, 3.05 eV for Zn₂Ni(PO₄)₂, and 3.12 eV for Zn₂Co(PO₄)₂. Additionally, the non-linear refractive index values decreased with doping. Compared with the compound Zn₃(PO₄)₂, the optimal excitation and emission wavelengths of the compound Zn₂Ni(PO₄)₂ were green-shifted, with the excitation wavelength of 440 nm and the emission wavelength of 527 nm. The special luminescent properties and simple synthesis method of the Zn_3−xM_x(PO₄)₂ (M=Co, Ni; x = 1) make it a promising candidate in the field of white-light-emitting diodes.