Optical properties of Mg and Ni- doped Ag2S colloidal nanoparticles
Jamil K. Salem1, Talaat M. Hammad2*, Aowda M. Shallah2
1 Chemistry Department, Faculty of Science, Al-Azhar University, P.O. Box 1277, Gaza, Palestine
2 Physics Department, Faculty of Science, Al-Azhar University, P.O. Box 1277, Gaza, Palestine
In this work we report optical properties of Mg and Ni- doped Ag2S colloidal nanoparticles. Mg and Ni -Ag2S nanoparticles were prepared using a wet chemical method. The influence of doping on the optical properties of Mg and Ni-doped Ag2S nanoparticles was investigated. The TEM images showed the shape of samples is spherical of average particle size of about 6–18 nm for all pure and doped Ag2S nanoparticles. The absorption spectra of the Mg -doped samples are red shifted from 2.42 to 2.20 eV, however the UV–vis spectra of Ni-doped Ag2S showed a blue shift from 2.42 to 2.60 eV. The observed blue shift in the band gap of Ni-doped Ag2S may be due to the substitution of Ni to Ag2S lattice. The Pl intensity of Mg-doped Ag2S nanoparticles increased as Mg concentration was increased. However, the Pl intensity of Ni-doped Ag2S reduced as the concentration of Ni is enhanced.
Keywords: doped- Ag2S, optical, photoluminescence
* Corresponding author. Tel.: +9722876672.
E-mail address: [email protected] (T.M.hammad).
It is well known that the chemical composition, shape and size controlled the properties of semiconductor nanostructured materials 1–6. Semiconductor nanostructured revealed a good electric, magneto-optical and photochemical properties and greatly differing from those observed in the exact bulk materials due to quantum size effects, resulting from predominant number of surface atoms in nanosize materials 7,8. Transition metal chalcogenides are very important semiconductor materials, especially in nanosize because of their excellent photoelectron transformation properties and potential application in physics, chemistry, biology, medicine and materials science and their different interdisciplinary fields, for instances solar cells, sensitive sensor, photon computer, and slow release medicament 9. The Ag2S is found to be amongst the most important chalcogenides and because of its good optoelectronic properties. Ag2S nanoparticles have been widely investigated due to its many valuable applications in optical and electronic devices 10–14. Ag2S has a direct band gap (0.9–1.05 eV), mutually large absorption, useful optical limiting and considerable chemical stability properties 15,16. Different synthetic methods have been explored to prepare Ag2S nanoparticles, such a microemulsions 17, sol–gel, ion implantation techniques 18, template 19, sonochemical way 20, gamma-irradiation 21 and organic–metallic precursor 22. Semiconductor nanocrystals doped with metals turn out new opportunities for luminescent 23 because of the formation of the additional electronic levels among the band gaps and also the modification of the band structure. There are few reports on the investigation of the optical properties of Mg and Ni- doped Ag2S nanoparticles in the literature; Ali Fakhri etal 24 synthesized the Cu doped Ag2S nanoparticles by the aids of simple chemical co- precipitation method. The TEM images showed the products are spherical shape in with diameter size of 30 nm and the Pl consequence confirmed that the change of emission wave length is almost between 456 and 477 nm. E.S. Aazam prepared Ni-doped Ag2S by using a hydrothermal method and he studied the impact of Ni dopant material on the photocatalytic pastime of Ag2S 25.
However, to our information Mg and Ni-doped Ag2S nanoparticles synthesized through a wet chemical methodology and their optical properties have been stated for the first time on this study.
2. 1. Materials and methods
Silver sulfate (Ag2S), magnesium sulfate (MgSO4• 7H2O), nickel sulfate (NiSO4.H2O) and sodium sulfide (Na2S. xH2O) were obtained from Merck and used as precursors. The chemical reagents were of analytical reagent grade and used without further purification. All the glass wares used in this experimental work were acid washed. Distilled water was used for all dilutions and sample preparations. Pure colloidal solution of Ag2S nanoparticle was prepared by a wet chemical method. Initially 0.1 mmol of AgNO3 was dissolved in 50 ml of distilled water. The obtained solution was added drop wise into 50 mL of 0.1 M Na2S solution with stirring until a transparent pale yellow color solution is obtained. The Mg and Ni-doped Ag2S colloidal solution were prepared by adding 25 ml aqueous solution of 0.001M Na2S drop wise to a mixture solution of 25 ml of 0.001M solution of Ag2SO4 and 25 ml of 0.001M solution of MgSO4• 7H2O or NiSO4.H2O with stirring until transparent clear solution is obtained. The colors of solutions depend on the amount and type of dopant. Finally, the prepared colloidal solutions of Ag2S nanoparticles were used for all measurements.
UV–vis absorption spectra of colloidal solutions were recorded with a UV–vis spectrophotometer (Shimadzu, UV-2400). The photoluminescence spectra (PL) measured at room temperature with a spectrofluorometer (JASCO, FP-6500) and with 300 nm excitation wavelength for Mg-doped Ag2S and 350 nm for Ni-doped Ag2S nanoparticles. The transmission electron microscopy (TEM) analysis was performed with JEM2010 (JEOL) transmission electron microscope.
3 Results and Discussion
Fig.1 (a–c) indicates the morphology and histograms of pure, 6% Mg and 6% Ni- doped Ag2S nanoparticles. It is shown from figure that the shape of particles are spherical to ellipsoid were formed. Fig.1 (a–c) shows the histograms of pure, Mg-doped Ag2S and Ni-doped Ag2S nanoparticles; the samples average diameter are 7 nm (pure Ag2S), 16 nm (Mg-doped Ag2S) and 5 nm (Ni-doped Ag2S), respectively.
A UV–vis spectrum analysis study is a powerful techniques for studying the influences of doping on the optical properties of Ag2S nanoparticles 26,27. The absorption spectrum of corresponding undoped and Mg-doped in Ag2S nanoparticles is illustrated in Fig. 2. The UV-vis spectra displayed continuous absorbance increasing from 230 nm to 800 nm. The absorption edge shifted towards higher wavelengths/lower energies with incorporation of Mg content as shown in Fig. 2. It indicates that the Mg ions replace the Ag ions within the Ag2S lattice. The optical band gap energies of different dopants are estimated by Tauc’s relation given as below 28
where A is a constant, h the photon energy and Eg is the energy gap. Direct band gap of the samples are calculated by plotting (?h?)2 versus h? and then extrapolating the straight portion of the curve on the h? axis at ? = 0. The straight lines plots shown in Fig. 3 show that the Mg-doped Ag2S samples have direct energy band gap and the band gap was decreased from 2.42 to 2.30 eV. It is noticed that the energy gap reduced with the increase in the Mg ions (Fig. 3). This red shift is related to increase in the particle size that causes to vary in particle energy levels and finely reduce the band gap. Similar type of decrease was reported on Mg doped CdS 29.
The size dependent of band gap energy of Mg-doped in Ag2S may be obtained using an effective mass approximation as following equation.
Where is the bulk band gap (eV) , ? is Planck’s constant, r is the particle radius, me is the electron effective mass, mh is the hole effective mass, mo is the free electron mass, e is the charge on the electron, ? is the relative permittivity, and ?o is the permittivity of free space. Generally, it is accepted that in Ag2S = 1.0 eV, me = 0.22 m0 and mh = 1.096 m0 are, correspondingly, the electron and hole effective masses 30, ? = 5.95 is the permittivity 31. The band gap values of the particles formed with various concentration of the magnesium and the particle sizes calculated using the eq (2) are given in the Table 1. It is clearly seen that the band gap energy decreased with increasing the particle size due to the quantum size confinement (see fig. 4). These are in good agreement with the values from TEM. The above results indicate that the dimension of the produced Mg-doped Ag2S nanoparticles and their corresponding optical properties could be controlled by the synthesis method.
Fig. 5 displays the room temperature optical absorption spectra of the undoped Ag2S and doped Ni-doped Ag2S nanoparticles. On substitution Ni to Ag2S, the absorption band shifts to blue, indicating an increase in the band gap energy as shown in Fig. 6. The band gap values were 2.42, 2.435, 2.46, 2.49,2.52, 2.535 and 2.56 eV for the Ag2S, 1% Ni-doped Ag2S, 2% Ni-doped Ag2S, 4% Ni-doped Ag2S, 6% Ni-doped Ag2S, 8% Ni-doped Ag2S and 10% Ni-doped Ag2S, respectively. It needs to be cited that the Ni2+ion, with an ionic radius of 0.69 Å, has a smaller ionic radius than Ag+ ion (1.15 Å). As a result, due to prevalence of the above referred to phenomena, the particle size of the Ni-doped Ag2S nanoparticles can be reduced and reasons small blue shifts is noticed. The similar observation of band gap variation of Ag2S with Ni is reported by Salem et al. 32. The increase in the band edge suggests that Ni has been substituted inside the Ag2S lattice.
The band gap values of the Ni-doped Ag2S nanoparticles at various concentration of the nickel and the particle sizes calculated using the eq (2) are given in the Table 2. The variation of band gap energy with the particle size is shown in Fig. 7. It is clearly seen that the band gap energy increased with decreasing the particle size as a result of the quantum size confinement. The particle size of Ni-doped Ag2S was estimated from Brus equation, which matches TEM result.
Fig. 8 displays the Pl spectra of pure and Mg-doped samples (excitation at 300 nm). The spectrum shows the emission peaks at about 611 for pure Ag2S, 612 for 1% Mg-doped Ag2S, 614 nm for 2% Mg-doped Ag2S, 615 nm for 4% Mg-doped Ag2S, 616 nm for 6% Mg-doped Ag2S, 617 nm for 8% Mg-doped Ag2S and 618 nm for 10% Mg-doped Ag2S. The PL peaks may correspond to crystalline defects induced during the growth 33. It is noticed that the intensity of Pl emission increased when the dopants of Mg enhanced. A slight shift is seen in PL spectra towards higher wavelength after doping Mg into Ag2S lattice and intensity of luminescence is also increased, as compared to the undoped sample. An increase within the intensity of the deep trap emission of Mg-doped Ag2S is noticed with increasing the concentration of Mg. The presence of Mg has been reported to enhance the intensity of deep trap emission of bulk Ag2S 34.
A comparable photoluminescence spectrum was observed for the Ni-doped Ag2S nanoparticles as (excitation at 350 nm) seen in Fig. 9. The emission peaks at 717, 716, 715, 714, 713, 712 and 711 nm are observed for pure and 1%, 2%, 4%, 6%, 8%, 10% Ni-doped Ag2S. It’s clear that the Pl intensity is reduced whilst the concentration of Ni increased. Ni represents as a trapping site, which captures photogenerated electrons from Ag2S conduction band. This effect contributes to a shift in the absorption edge toward shorter wavelengths, which indicates an increase in the band gap energy.
The normalized PL spectra of Mg-doped Ag2S are shown in Figure 10. It is observed the emission band is slightly red-shifted with Mg dopants. From TEM observations it is observed that the particle size is enhanced at higher dopant percentages which substantiate the red shift at Mg concentrations. The normalized PL spectra of Ni-doped Ag2S are shown in Figure 11. Clearly, the observed emission band is obviously blue-shifted with additives of Ni.
Mg and Ni-doped Ag2S have been synthesized using a wet chemical method. The TEM images showed the shape of samples is spherical of average particle size of about 6–18 nm for all pure and doped Ag2S nanoparticles. A red shift has been noticed from the optical absorption and PL spectra of magnesium doped Ag2S nanoparticles, while a blue shift is observed for Ni-doped Ag2S nanoparticles. With an increasing concentration of Mg incorporated within the nanoparticles, the Mg emission intensity will increases, whereas emission intensity of Ni decreases.
Fig. 1. TEM images and histograms, a Undoped Ag2S, b 6 % Mg-doped Ag2S and c 6 % Ni-doped Ag2S
Fig. 2. UV–vis spectra of Mg-doped Ag2S nanoparticles
Fig. 3. Optical band gap spectra of Mg-doped Ag2S nanoparticles
Fig. 4.Variations of band gap energy with particle size of Mg-doped Ag2S nanoparticles
Fig. 5. UV–vis spectra of Ni-doped Ag2S nanoparticles
Fig. 6. Optical band gap spectra of Ni-doped Ag2S nanoparticles
Fig. 7.Variations of band gap energy with particle size of Ni-doped Ag2S nanoparticles
Fig. 8. PL spectra Mg-doped Ag2S nanoparticles
Fig. 9. PL spectra of Ni-doped Ag2S nanoparticles
Fig. 10. Normalized PL spectra of Mg-doped Ag2S nanoparticles
Fig. 11. Normalized PL spectra of Ni-doped Ag2S nanoparticles
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