Optical properties of Mg and Ni- doped Ag2S colloidal nanoparticles
Jamil K. Salem1, Talaat M. Hammad2*, Aowda M. Shallah2
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 Mg and Ni-doped Ag2S nanoparticles. The UV–vis and Pl spectra of the undoped and doped Ag2S nanoparticles were investigated. The absorption spectra of the Mg -doped samples are red shifted from 2.42 to 2.20 eV, but 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 incorporation of Ni inside Ag2S lattice. The Pl intensity of Mg- doped Ag2S nanoparticles increased as Mg concentration was increased. However the Pl intensity of Ni-doped Ag2S decreased with increasing the Ni concentration.
Keywords: doped- Ag2S, optical, photoluminescence
* Corresponding author. Tel.: +9722876672.
E-mail address: [email protected] (T.M.hammad).
The properties of semiconductor nanostructured materials depend on their chemical composition, shape and size 1–6. Semiconductor nanostructured revealed a good electronic, magnetic, optical and photochemical properties and greatly differing from those observed in the corresponding 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 extensively investigated due to its many potential applications in optical and electronic devices 10–14. Ag2S has a direct band gap (0.9–1.05 eV), with large absorption, good optical limiting and great 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 metal ions as optically active luminescence and magnetic centers create new opportunities for luminescent 23 due to the formation of the extra electronic levels within the band gaps and the amendment of the band structure. There are very few reports on 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 morphological study showed that the products were spherical shape in with diameter size of 30 nm and the Pl consequence confirmed that the vary 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 content material on the photocatalytic pastime of Ag2S 25.
However, to our information Mg and Ni-doped Ag2S nanoparticles synthesized by a wet chemical methodology and their optical properties were presented for the first time in this study.
2 Experimental1. 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 done with JEM2010 (JEOL) transmission electron microscope.
3 Results and Discussion
It is necessary to get the particle size and the information about the nanostructures via direct measurement such as TEM, which may reveal the dimension and the morphology of the particles. Fig.1 (a–c) indicates the morphology and histograms of un-doped, 6% Mg and 6% Ni- doped Ag2S nanoparticles. As shown in figure the shape of particles are spherical to ellipsoid were formed. Fig.1 (a–c) shows the size distribution of undoped, Mg-doped Ag2S and Ni-doped Ag2S nanoparticles; the samples average diameter are 7 nm (undoped Ag2S), 16 nm (Mg-doped Ag2S) and 5 nm (Ni-doped Ag2S), respectively.
A UV–vis spectroscopy study is a powerful method for investigating the effects of impurity doping on the optical properties of Ag2S nanostructures, because doped Ag2S nanostructures are expected to have different optical properties in comparison with undoped Ag2S. As the particle size decreases, the absorption edge shifts to shorter wavelength, due to the band gap increase of the smaller particles 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 the constant and Eg is the band gap energy of the material and the exponent n depends on the type of transition. For direct allowed transition n= 1/2, for indirect allowed transition n= 2, for direct forbidden n= 3/2 and for indirect forbidden n= 3. 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 imply 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 seen that the energy gap reduced with the increase in the Mg ions (Fig. 3). This red shift is attributed 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 variation of particle size with the optical absorption energy may be primarily based on 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 estimated 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 several 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, used as a dopant ion 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 in the UV–vis absorption bands. 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 formed with various concentration of the nickel and the particle sizes estimated 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 due to the quantum size confinement. The particle size of Ni-doped Ag2S was estimated from Brus equation, which matches TEM result.
The photoluminescence (PL) emission is one of the most consequential physical properties in Ag2S nanoparticles and depends upon synthesis conditions, shape, size and energetic position of the surface states 33-35. The PL of the undoped Ag2S and Mg-doped Ag2S nanoparticles is investigated at room temperature to more investigate the optical properties. Fig. 8 displays the emission spectra of undoped and Mg-doped samples (excitation at 300 nm). The spectrum exhibits a broad emission peaks at about 611 for undoped 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 strong PL peaks may correspond to crystalline defects induced during the growth. Visible emissions are referred as deep-level emission and are due to the recombination of electrons deeply trapped in silver interstitials and oxygen vacancies, with photo-generated holes 36. It is clear that the Pl
intensity increased when the dopants of Mg increased. 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 37.
A similar 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 undoped 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 acts as a trapping site, which captures photogenerated electrons from Ag2S conduction band. The presence Ni on Ag2S surface acts as trapping sites to capture photogenerated electrons from the conduction band, separating the photogenerated electron–hole pairs. This effect contributes to a shift in the absorption edge toward shorter wavelengths, which indicates an increase in the band gap energy. Additionally, this blue shift in emission role may be attributed to the fee transfer among the Ni generated band and the conduction band of Ag2S as a semiconductor.
The normalized PL spectra of Ag2S doped with magnesium concentrations of 0%, 1%, 2%, 4%, 6%, 8% and 10% are shown in Figure 10. Clearly, the observed emission band is slightly red-shifted with addition of Mg. From TEM observations it is observed that the particle size is enhanced at higher dopant percentages which substantiate the red shift at these concentrations of the Mg. This red shift of the emission peak as the result of the quantum confinement effect of the nanocrystals. The normalized PL spectra of Ag2S doped with nickel of concentrations of 0%, 1%, 2%, 4%, 6%, 8% and 10% are shown in Figure 11. Clearly, the observed emission band is obviously blue-shifted with addition of Ni. Although it is still remains that blue shift of the emission peak as a result of the quantum confinement effect of the nanocrystals, it is believed that blue shift relates to small particle size, narrow size distribution, and/or surface defects.
4 Conclusions Ag2S and metal (Mg and Ni) doped Ag2S have been successively synthesized by a wet chemical method. The TEM results show that the products were spherical shape with size of about 5–16 nm for all Mg and Ni-doped Ag2S nanoparticles. A red shift in the band gap has been observed 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 incrementing concentration of Mg incorporated in the nanoparticles, the Mg emission intensity increases, while 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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