Shape control of silver selenide nanoparticles using green capping molecules

2.1 Chemicals

Selenium powder (99.5%, 100 mesh), sodium borohydride (NaBH₄) (98.5%), silver nitrate (AgNO₃) (99%), ammonium hydroxide (NH₄OH) (99.9%) and D-glucose reagent grade were purchased from Merck (Darmstadt, Germany). Acetone (99.8%), ascorbic acid and chitosan reagent grades were purchased from Sigma Aldrich (Aston Manor, Johannesburg, South Africa). All chemicals were used as purchased, without any further purification.

2.2 Synthesis of Ag₂Se nanoparticles: effect of capping agents

The methodology used for the synthesis of Ag₂Se nanoparticles in the aqueous phase was adopted from Oluwafemi and Revaprasadu [2] with some modification. To prepare the selenium precursor solution, selenium powder (0.1 M) was added to deionized water (20 ml) in a three-necked flask at room temperature. Sodium borohydride was then added to this reaction mixture and the flask was immediately purged with nitrogen gas. The reaction mixture was then stirred for 2 h at room temperature. A solution of 0.1 M silver nitrate was added to 1.0% of each capping agent (glucose, ascorbic acid, chitosan, green tea) in a one-necked round-bottom flask with constant stirring at room temperature. The pH of the solution was adjusted to 11 using ammonium hydroxide. This was followed by addition of the colorless selenide ion solution. The reaction mixture was further stirred for 20 h at room temperature to complete the reaction. The resultant solution was then centrifuged and extracted with methanol to obtain a precipitate of Ag₂Se nanoparticles. The precipitate was washed several times and dried at room temperature. The effect of the capping agent was investigated using different capping agents green tea, glucose, ascorbic acid and chitosan.

2.3 Characterizations

A UV-Vis spectrophotometer (Perkin Elmer Lamda 25) was used to carry out optical analysis at room temperature at 200–1100 nm wavelength range. Fourier transform infrared (FTIR) spectral studies were carried out using a Perkin Elmer spectrum 400 FTIR-NIR spectrometer equipped with a universal ATR sampling accessory. The nanoparticle structure was studied using a JEOL JEM-2100 transmission electron microscope operating at 200 kV. The transmission electron microscopy (TEM) was coupled with an energy dispersive X-ray detector which was used to determine the elemental composition of the synthesized nanoparticles. The samples were prepared by placing an aliquot solution of the water soluble nanocrystalline material onto an amorphous carbon substrate supported on a copper grid and then the solvent was allowed to dry at room temperature. The X-ray diffraction (XRD) patterns were recorded using a Bruker D2 diffractometer at 40 kV and 50 mA and a secondary graphite monochromatic Co Kα radiation (l=1.7902 A˚). The measurements were taken at high angle 2θ in a range of 5°–90° with a scan speed of 0.01° 2θ s⁻¹. A PerkinElmer thermogravimetric analyzer (TGA 4000) was used for thermal analysis. The samples were heated under nitrogen and the heating range was between 25°C and 900°C at a heating rate of 5°/min.

3.1 Optical and structural analysis

The effect of capping agent (green tea, glucose, ascorbic acid and chitosan) on the optical properties of silver selenide nanoparticles (Ag₂Se NPs) is shown in Figure 1. Ag₂Se-NPs showed a peak at 361 nm for green tea, for glucose at 360 nm, for ascorbic acid at 360 nm, and for chitosan at 359 nm. The UV results obtained for glucose, chitosan and green tea are in agreement with the corresponding TEM results obtained which showed that the particles have an average size of 8–31 nm. However, the results obtained for ascorbic acid are not in agreement, since the corresponding TEM result showed that the particles have an average size of 96 nm. The peaks were all blue shifted in relation to the bulk band gap of Ag₂Se-NPs (1033 nm), which indicated the formation of nanoparticles.

Figure 1:

Absorption spectra of Ag₂Se nanoparticles capped with (A) green tea, (B) glucose, (C) chitosan and (D) ascorbic acid.

3.2 Photoluminescence studies

The photoluminescence properties of Ag₂Se nanoparticles were also studied using various excitation wavelengths (240–550 nm). Figure 2 shows the photoluminescence spectra of Ag₂Se nanoparticles at room temperature. The figure shows two kinds of emission peaks with an increase in excitation wavelength. The first kind of peaks is sharp and showed an increase in emission wavelength with an increase in excitation wavelength. This is called moving emission (ME). In addition there are two rows of this kind of emission where one row changes the emission from 240 nm to 330 nm and the other alters it from 390 nm to 550 nm. The intensity of ME is also changing with an increase in excitation wavelength and reached its highest intensity at 270 nm. The second kind of peaks is broad and keeps its position as the excitation wavelength increases. This kind is called standing emission (SE). When the applied excitation was 450–550 nm, the SE peak disappeared. The behavior of ME and SE corresponds to the increase in excitation wavelength which indicated that they are dependent on the excitation wavelength [14].

Figure 2:

Emission spectra of Ag₂Se nanoparticles capped with (A) green tea, (B) glucose, (C) ascorbic acid and (D) chitosan.

FTIR analysis was conducted in order to determine the molecular interactions between the capping agents and the silver selenide nanoparticles (Ag₂Se-NPs). The spectrum of pure glucose showed an O-H stretch at 3407 cm⁻¹, CH₂OH side chain at 1250 cm⁻¹, C-O, C-C stretch at 1146 cm⁻¹, and C-O-C stretch at 989 cm⁻¹. After the reaction of Ag₂Se-NPs the O-H stretch shifted to 3565 cm⁻¹, CH₂OH side chain at 1219 cm⁻¹, C-O, C-C stretch to 1147 cm⁻¹, and C-O-C stretch to 988 cm⁻¹[2]. The spectrum of pure green tea showed an O-H stretch at 3562 cm⁻¹and 3737 cm⁻¹, a C=C stretch at 1520 cm⁻¹, a C-O-C stretch at 1236 cm⁻¹ and a C-O stretch at 1058 cm⁻¹. These peaks showed a shift after the reaction with Ag₂Se-NPs to 3745 cm⁻¹, 1535 cm⁻¹, 1228 cm⁻¹ and 1003 cm⁻¹, respectively. The presence of these functional groups on green tea capped Ag₂Se-NPs confirmed a successful capping of the nanoparticles by green tea [15]. The spectrum of pure ascorbic acid showed an O-H stretch at 2719 cm⁻¹, and a stretching vibration at 1655 cm⁻¹, which is due to a C=C bond. It also showed a peak at 1322 cm⁻¹ which is due enol hydroxyl. After the reaction with nanoparticles, the peak due to an O-H stretch shifted to 2665 cm⁻¹. The peak at 1655 cm⁻¹ disappeared and a peak at 1782 cm⁻¹ was observed which is due to an ester carbonyl group. The peak at 1322 cm⁻¹ shifted to 1362 cm⁻¹. These findings confirmed the presence of a poly hydroxyl structure of ascorbic acid on the surface of Ag₂Se-NPs [16]. The spectrum of chitosan capped silver selenide nanoparticles showed transmissions at 3361 cm⁻¹ and 3295 cm⁻¹ assigned to the overlap of O-H and N-H stretching vibrations. The band at 2871 cm⁻¹ corresponds to C-H stretching, at 1644 cm⁻¹ and 1590 cm⁻¹ to -NH₂ bending, at 1416 cm⁻¹, 1374 cm⁻¹ and 1315 cm⁻¹ to C-H bending and at 1023 cm⁻¹ to -C-O skeletal stretching. This trend was also observed in the chitosan capped Ag₂Se-NPs spectrum (Figure 3). A shift in bands was noticed (from 3361 cm⁻¹ to 3341 cm⁻¹, 3295 cm⁻¹ to 3284 cm⁻¹, 1644 cm⁻¹ to 1639 cm⁻¹, 1590 cm⁻¹ to 1589 cm⁻¹, 1416 cm⁻¹ to 1419 cm⁻¹, 1374 cm⁻¹ to 1375 cm⁻¹, 1315 cm⁻¹ to 1320 cm⁻¹ and 1023 cm⁻¹ to 1013 cm⁻¹) [7]. Also, an increase in the sharpness of the peaks was observed in the chitosan capped nanoparticles. These observations indicated the interaction between the Ag₂Se-NPs surface and the chitosan amino and hydroxyl groups. This then suggests that NPs were capped by the chitosan.

Figure 3:

Fourier transformer infrared (FTIR) spectra of (A) pure chitosan and (B) chitosan capped silver selenide nanoparticles.

3.3 Morphological analysis

TEM was used to investigate the effect of capping agents on the size and shape of Ag₂Se nanoparticles. TEM micrographs and size histograms of Ag₂Se nanoparticles capped with green tea, glucose, ascorbic acid and chitosan are shown in Figures 4A–D. All the capping agents investigated gave mixed shapes of nanoparticles. Chitosan gave the smallest size, while ascorbic acid gave the biggest size of nanoparticles compared to the other capping agents studied. The reason could be that chitosan has the ability to chelate metals and can form various chemical bonds with metal components, thus enhancing the stability of the nanoparticles [7]. Green tea capped Ag₂Se nanoparticles gave spherical and rod shapes with an average size 30 nm. Glucose capped nanoparticles gave spherical and cubic shapes with an average size of 31 nm. Ascorbic acid capped nanoparticles gave spherical shape with average size of 96 nm. Chitosan capped nanoparticles gave spherical and rod-like shaped nanoparticles with an average size of 8 nm. Chitosan capped copper nanoparticles have been synthesized and spherical nanoparticles with a mean size of 35–75 nm were obtained [7]. Jafari et al. [1] prepared Ag₂Se nanoparticles using KSN as a complexing agent and hydrazine as a reducing agent, and spherical nanoparticles with less than 50 nm diameter were obtained. Oluwafemi and Revaprasadu [13] synthesized ascorbic acid capped CdSe nanoparticles and obtained spherical nanoparticles. Energy dispersive X-ray detection also confirmed the presence of Ag and Se in a 2:1 ratio in the synthesized nanoparticles, which corresponds with the formula Ag₂Se.

Figure 4:

The transmission electron microscopy (TEM) micrographs and size histograms of Ag₂Se nanoparticles capped with 1.0% (w/v) of (A) green tea, (B) glucose, (C) ascorbic acid and (D) chitosan.

Figure 5 shows the X-ray diffraction patterns of the synthesized Ag₂Se nanoparticles which can be indexed to the ß-phase Ag₂Se (JCPDS card no. 201041), with the presence of (002), (013), (004) and (014) peaks charactering the orthorhombic phase. All capping agents gave the same diffraction patterns, which indicate that there is no effect observed on the phase of nanoparticles that was due to capping agent. The (111) plane indicates the presence of silver in the face centered cubic phase. The sharpness of peaks indicates the high degree of crystallinity of the nanoparticles.

Figure 5:

The X-ray diffraction (XRD) patterns of Ag₂Se nanoparticles capped with 1.0 % (w/v) of (A) green tea, (B) glucose, (C) ascorbic acid and (D) chitosan.

Thermogravimetric analysis (TGA) was used to study the decomposition temperature and stability of the nanoparticles. A TGA thermograph of silver selenide nanoparticles with chitosan capping agent showed two degradation steps, while green tea, glucose and ascorbic acid showed one step as shown in Figure 6A. The degradation around 100°C for chitosan capped nanoparticles was due to water desorption from the nanoparticles. Green tea, glucose, ascorbic acid and chitosan capped nanoparticles showed degradation with weight loss at 409°C (16%), 450°C (23%), 445°C (24%) and 284°C (67%), respectively. These degradation steps were due to the degradation of the capping agent leaving the silver selenide nanoparticles behind. The TGA thermogram showed no further weight loss, which indicated a high thermal stability of the synthesized pure silver selenide nanoparticles. The TGA showed the stability of the capping agents in the following order: green tea > glucose > ascorbic acid > chitosan. The degradation temperature of the capped Ag₂Se nanoparticles was higher than that of the capping agents alone, while the weight loss was lower for the capped nanoparticles than the capping alone, as shown in Figure 6B. This difference was influenced by nanoparticles residue which indicates that the nanoparticles without the capping agent can remain intact at higher temperatures.

Figure 6:

(A) Thermogravimetric analysis (TGA) graph of Ag₂Se nanoparticles capped with green tea (a), glucose (b), ascorbic acid (c) and chitosan (d). (B) glucose capped nanoparticles and pure glucose.