Optical properties of Cd and Co- doped Ag2S colloidal solution Jamil K

Optical properties of Cd and Co- doped Ag2S colloidal solution

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 new optical properties of Cd and Co- doped Ag2S colloidal solution. The influence of doping on the optical properties of Ag2S:Cd and Ag2S:Co nanoparticles was investigated. The morphological study indicated that the products were spherical shape with size of about 6–18 nm for all Cd and Co-doped Ag2S nanoparticles. The absorption spectra of all the doped samples are red shifted as compared with of the undoped Ag2S samples. The Pl intensity of Cd- doped Ag2S nanoparticles was decreased while its intensity of Co-doped Ag2S increased with increasing the amount of doping Cd and Co into Ag2S nanoparticles. The particle sizes as calculated from the absorption spectra were in agreement with the results obtained from TEM.

Keywords: metal (Cd,Co), doped- Ag2S, optical, photoluminescence
* Corresponding author. Tel.: +9722876672.
E-mail address: [email protected] (T.M.hammad).

1 Introduction
In recent years, nanometer-sized chalcogenide semiconductors have drawn attention as a component of nanotechnology, mainly due to their physical and chemical properties, heavily dependent on their shape and size. The Ag2S is found amongst the most important chalcogenides
and because of its unique optoelectronic properties. Ag2S has been extensively studied due to its many potential applications in optical and electronic devices such as infrared detectors, photoconductive cells, magnetic field sensors and photoconductors, amongst others 1–5.
Semiconductor nanostructures of wide band gap are used in optoelectronics, blue-ultraviolet light emitter, detectors and devices in which high-switching speed, high-voltage and high-temperature operation are needed and allow them to be more energy efficient, more powerful, faster and smaller 6,7. Different wide band gap semiconductors such as CdS, ZnO, Ag2S and GaN nanostructures were reported as good materials for nonlinear optical and photonic devices
6, 8–11. Ag2S is a direct, thin band gap semiconductor with good optical limiting and great chemical stability properties 12,13. Ag2S has a direct band gap (0.9–1.05 eV), and large absorption coefficient which demonstrates a performance semiconductor material for photovoltaic application 12. Silver sulfide is a useful semiconductor and solid ionic conductor conducting both ions and electrons at room temperature 13-15. Nanostructured silver sulfide was investigated intensively in recent years due to the possible application in optoelectronics, biosensing and catalysis 16–18. Nanostructured silver sulfide Ag2S can be used in infrared detectors 19, in resistance-switches and nonvolatile memory devices 20,21. Because of the important role of silver sulfide, considerable efforts on synthesis and properties characterization of this type of materials have being done and their potential application areas have being developed.
Several methods have been developed for the synthesis of Ag2S nanoparticles such as solvothermal method, hydrothermal route, and single source precursor routes 22. Yu et al. synthesized submicrometer Ag2S particles thru a simple hydrothermal method but it is difficult to control the size and shape of the nanoparticles for the large-scale synthesis of high quality
nanoparticles 23. Qin et al. successfully synthesized Ag2S nanorods by a biomimetic route in the lysozyme solution at physiological temperature and atmospheric pressure 4. In another work, Wang et al. synthesized spherical silver sulphide nanoparticles (Ag2S) at 205 °C under N2 atmosphere by direct reacting silver acetate with n-dodecanethiol 24. Wang’s group developed a strategy for obtaining single-crystalline, monodispersed Ag2S nanoparticles with a size of ~10 nm using a single source precursor route, and the NIR emission at 1058 nm was first observed 25. Furthermore, Wang and other co-workers prepared sub-micrometer Ag2S particles with normal and flattened rhombic dodecahedral morphologies through a facile hydrothermal method 24. Doping of Ag2S is a commonly applied method to treat magnetic, electrical and optical properties of semiconductor compounds, used in making most optoelectronic and electronic devices. Doping silver sulfide has attracted a lot of attention as it is a convenient way to tailor its physical properties. There are very few reports on the optical properties of the doped Ag2S in the literature; E.S. Aazam synthesized Ni-doped Ag2S by using a hydrothermal method and studied the effect of Ni content on the photocatalytic activity of Ag2S 26. Ali Fakhri et al. 27 reported the optical properties Cu-doped Ag2S nanoparticles synthesized by simple chemical co precipitation method. The present work focuses on synthesizing the Ag2S nanoparticles doped with Cd and Co by a wet chemical method and their optical properties were presented for the first time in this study.

2 Experimental

Silver sulfate (Ag2S), cadmium sulfate (CdSO4.8H2O), cobalt sulfate (CoSO4.7H2O) 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 Cd and Co-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 CdSO4.8H2O or CoSO4.7H2O 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 were collected using a UV–vis spectrophotometer (Shimadzu, UV-2400) in the wavelength range from 200 to 700 nm. PL spectra were recorded with a spectrofluorometer (JASCO, FP-6500); the extinction wavelength was selected to be 350 nm for Cd-doped Ag2S and 250 nm for Co-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 obtain the particle size and the information about the nanostructures by direct measurement such as TEM, which can reveal the size and the morphology of the particles.
Fig.1 (a–c) indicates the morphology and histograms of un-doped, 6% Cd and 6% Co doped Ag2S nanoparticles. Spherical to ellipsoid shaped particles were formed. The average Size of the individual particle in different dopants is in 6-18 nm.
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 28,29. The absorption spectra of corresponding undoped and Cd doped in Ag2S nanoparticles is illustrated in Fig. 2. The UV-vis spectra displayed continuous absorbance increasing from 230 nm to 800 nm which in agreement with literature.(ref). The optical absorption edge of Ag2S nanoparticles is shifted towards the longer wavelength region with increase the Cd concentration as shown in Fig. 2. The optical band gap energies for different compositions are calculated by Tauc’s relation given as below 30
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 Cd-doped Ag2S samples have direct energy band gap and the band gap was present between 2.62 to 2.27 eV. It is clear that the energy gap decreased with the increase in the Cd ions (Fig. 3). This red shift is attributed to increase in the particle size which causes to change in particle energy levels and finely decrease the band gap and red shift occurrence. Similar type of decrease was reported on
Cd- doped CdS 31. The dependence of particle size on the optical absorption energy can be expressed 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 32, ? = 5.95 is the permittivity 33. 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 Cd-doped Ag2S nanoparticles and their corresponding optical properties could be controlled by the synthesis method.
Fig. 5 shows the room temperature optical absorption spectra of the undoped Ag2S and several Co-doped Ag2S nanoparticles. Co doping shifts the absorption onset to red of Co doping level from 0 to 1 %, indicating a decrease in the band gap from 2. 42 to 2.16 eV (see Fig. 6). The decrease in the band edge is a clear indication for the incorporation of Co inside the Ag2S lattice 29,30. The red shift of band edge for the cobalt doped samples clearly indicates that Co ions are incorporated into the Ag2S lattice 29. The band gap values of the Co-doped Ag2S nanoparticles formed with various concentration of the cobalt 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 decreased with increasing the particle size due to the quantum size confinement. The particle size of Co-doped Ag2S was estimated from Brus equation, which matches TEM result.
The photoluminescence (PL) emission is one of the most important physical properties in Ag2S nanoparticles and depends upon synthesis conditions, shape, size and energetic position of the surface states 34-36. The PL of the undoped Ag2S and Cd-doped Ag2S nanoparticles is studied at room temperature to further investigate the optical properties. Fig. 8 shows the emission spectra of undoped and Cd-doped samples (excitation at 350 nm). The spectrum exhibits a broad emission peaks at about 708 for undoped Ag2S, 710 for 1% Cd-doped Ag2S, 711 nm for 2% Cd-doped Ag2S, 713 nm for 4% Cd-doped Ag2S, 715 nm for 6% Cd-doped Ag2S, 716 nm for 8% Cd-doped Ag2S and 718 nm for 10% Cd-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 37. It is clear that the Pl intensity decreased when the dopants of Cd increased. Cd acts as a trapping site, which captures photogenerated electrons from the conduction band, thus separating the photogenerated electron–hole pairs. It is generally accepted that the incorporation of noble metal nanoparticles into Ag2S 26 enhances the light absorption of the Ag2S nanoparticles in the visible-light region. This effect leads to a shift in the absorption edge toward longer wavelengths, which indicates a decrease in the band gap energy. This effect leads to a shift in the absorption edge toward longer wavelengths, which indicates a decrease in the band gap energy, which is confirmed by UV-vis spectra measurements.
A similar photoluminescence spectrum was observed for the Co-doped Ag2S nanoparticles as (excitation at 250 nm) seen in Fig. 9. The emission peak in the visible region at 513, 515, 516, 517, 518, 519 and 520 nm are observed for undoped and 1%, 2%, 4%, 6%, 8%, 10% Co-doped Ag2S. Fig. 7 also shows that the intensity of these peaks also increases with the doping of Co into Ag2S nanoparticles. A red shift is seen in PL spectra towards higher wavelength after doping Co into Ag2S lattice.
The normalized PL spectra of Ag2S doped with Cadmium and cobalt concentrations of 0%, 1%, 2%, 4%, 6%, 8% and 10% are shown in Figures 10, 11. Clearly, the observed emission band is substantially red-shifted with addition of Cd and Co. From TEM observations it is seen that the particle size is increased at higher dopant percentages which confirm the red shift at these concentrations of the Cd and Co dopant. This red shift of the emission peak is due to the quantum confinement effect of the nanocrystals.
4 Conclusions
Ag2S and metal (Cd and Co) 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 6–15 nm for all Cd and Co-doped Ag2S nanoparticles. A red shift phenomenon was found to increase directly with the concentration of Cd and Co doped on to the Ag2S; this effect has been observed in the UV–vis spectra and Pl spectra of Cd/Ag2S and Co/Ag2S samples. With an increasing concentration of Cd incorporated in the nanoparticles, the Cd emission intensity decreases while intensity of red emission of Co increases. A novel PL phenomenon can be observed from the Ag2S nanoparticles doped with Cd2+ and Co2+ ions. This result indicates the important roles of dopants in controlling the emission color from Ag2S nanoparticles. Therefore, the size calculated using the Brus equation is closer to values obtained from TEM when the size of particles increases due to the quantum size confinement.
Figure Captions:

Fig. 1. TEM images and histograms, a Undoped Ag2S, b 6 % Cd-doped Ag2S and c 6 % Co-doped Ag2S
Fig. 2. UV–vis spectra of Cd-doped Ag2S nanoparticles
Fig. 3. Optical band gap spectra of Cd-doped Ag2S nanoparticles
Fig. 4.Variations of band gap energy with particle size of Cd-doped Ag2S nanoparticles
Fig. 5. UV–vis spectra of Co-doped Ag2S nanoparticles
Fig. 6. Optical band gap spectra of Co-doped Ag2S nanoparticles
Fig. 7.Variations of band gap energy with particle size of Co-doped Ag2S nanoparticles
Fig. 8. PL spectra of Cd-doped Ag2S nanoparticles
Fig. 9. PL spectra of Co-doped Ag2S nanoparticles
Fig. 10. Normalized PL spectra of Cd-doped Ag2S nanoparticles
Fig. 11. Normalized PL spectra of Co-doped Ag2S nanoparticles

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