For luminescent solar concentrators (LSCs), it is important to enhance the fluorescence quantum yield (FQY) and photostability. Our measurements have demonstrated that the addition of silver nanoparticles to dye solution causes broadening of absorption bands, so the spectral range of sunlight absorbed by LSC has increased. Silver nanoparticles (NPs) were characterized by X-ray diffraction (XRD) and UV-Vis absorption spectra. UV-Vis spectrum showed a single peak at 442 nm due to the surface plasmon resonance (SPR). The position of SPR peak exhibited a red shift after the sample was exposed to UV irradiation (unfiltered light). The optical band gap values have a reduction from 2.46 to 2.37 eV after irradiation for 960 minutes. Such reduction in optical band gap may be due to change in particle size calculated using Mie theory. The photostability of organic dyes used was improved after adding silver nanoparticles. The area under fluorescence spectra of dyes with silver NPs increased by 41–31% when compared with identical dye concentrations without silver nanoparticles as a result of interaction of the species with silver NPs.
Luminescent solar concentrators (LSCs) were introduced for the first time in 1976 by Weber and Lambe [
One of the major factors that affect the efficiency of a LSC module is the fluorescence quantum yield (FQY) of the luminescent species used in its design. It has been well established that the fluorescence of dye molecules can be intensified by their interaction with silver plasmons [
When silver nanoparticles (NPs) are added to the dye solution, the dye molecules will be adsorbed on islands films of the metallic NPs. Also, when the surface plasmon resonance (SPR) of the metallic NPs coincides with the dye absorption band, it will modify the intensity of the electromagnetic field around the molecules which will increase the emitted fluorescence intensity [
Mansour examined copolymer films of styrene (ST) with methyl methacrylate (MMA) of different percentage. Differential scanning calorimetry showed a single glass transition at 50/50 ST/MMA. Also, the FTIR spectra of copolymer 50/50 ST/MMA after exposure to UV radiation for 24 h were similar to those before exposure to UV radiation; this indicates that the copolymerization of styrene with MMA modifies the photodegradation behavior of polystyrene [
Chahal et al. studied the effect of ultraviolet irradiation on the optical and structural properties of PVP-Ag nanocomposite. The optical band gap values reduced from 4.90 eV in pure PVP to 4.11 eV for PVP-Ag nanocomposite prior to irradiation. This value is further reduced to 3.55 eV after UV irradiation for 180 minutes [
Jaleh et al. studied the effect of UV radiation on the optical properties, crystallinity, surface energy, and degradation of polystyrene (PS) and PS-TiO2 nanocomposite. It was found that the optical band gap values reduced from 4.54 eV in pure PS to 4.45 eV for PS-TiO2 nanocomposite prior to irradiation. This value was further reduced to 3.46 after UV irradiation for 45 h [
In the present work, we aimed to study behavior of silver NPs before and after UV irradiation and enhance the fluorescence of selected organic dyes used in LSC by adding silver nanoparticles into the dye solutions. The photostability and energy gaps of organic dyes before and after adding silver NPs and after UV irradiation have been also investigated.
The organic dyes used in this research were obtained from Radiant Dye Laser Accessories GmBH; silver NPs were obtained from Merck. The solvent used is Triton X-100 supplied by Merck. Triton X-100 is favorable because of its highly polar properties which permits dye being studied to dissolve completely in it. Plate samples of thickness ~0.02 cm and dye concentration of 10−4 ML−1 were prepared by casting method. The dye was homogeneously diffused in the polymer before casting. The samples were irradiated by a 300 W Xenon arc lamp which has a similar spectrum to the sun for 960 minutes with absorbing UV filter and without filter. Xenon arc lamps have a relatively smooth emission curve in the UV to visible spectra, with characteristic wavelengths emitted from 750 to 1000 nm. Absorption spectra for Coumarin 6, Fluorescein, and Rhodamine 6G in Triton X-100 without and with silver NPs were measured from 190 to 900 nm using UV/Visible absorption spectrometer Perkin-Elmer Lambda 4B. The spectra were recorded for the samples in the form of rectangular discs of area of 3 × 1 cm2 and thickness of 0.02 cm. Fluorescence spectra were detected by a Shimadzu RF-5301 PC spectrofluorimeter (Kyoto, Japan) equipped with a 150 W Xenon lamp and using 1.0 cm quartz cells. Details of internal microstructural features are examined by transmission electron microscope (TEM).
Composites films of PVA-silver NPs were prepared and then UV-irradiated for different times. These composites films were dissolved in distilled water to record the TEM images, as shown in Figures
Transmission electron microscope (TEM) with particle size of PVA/silver nanocomposites (a) before exposure to UV radiation and (b) after exposure to UV radiation for 960 minutes.
Figures
Particle size calculated from Mie theory.
UV exposure time (min) | SPR peak (nm) | FWHM (nm) | Particle size (nm) | Uncertainty |
---|---|---|---|---|
0 | 442 | 83 | 4.20 | ±0.40 |
3.90 | ||||
3.50 | ||||
4.50 | ||||
3.70 | ||||
|
||||
240 | 469 | 96 | 4.10 | ±0.52 |
3.10 | ||||
3.40 | ||||
3.90 | ||||
2.90 | ||||
|
||||
600 | 503 | 119 | 3.40 | ±0.39 |
2.80 | ||||
3.15 | ||||
3.60 | ||||
2.70 | ||||
|
||||
960 | 523 | 132 | 3.20 | ±0.26 |
2.70 | ||||
3.07 | ||||
3.10 | ||||
2.60 |
Absorption spectra of silver NPs in Triton X-100 before and after exposure to UV radiations (unfiltered light).
The absorption band is red shifted and HWHM increases from 83 to 132 nm with increase in exposure time to UV radiations due to the reduction in the particle size of silver NPs as calculated in Table
The optical band gaps,
Plots of (a)
The values of optical band gap so determined are listed in Table
The values of optical band gap and Urbach's energy for silver NPs in Triton X-100 before and after exposure to UV radiation for different times.
UV exposure time (min) | Optical band gap |
Uncertainty | Urbach’s energy |
Uncertainty |
---|---|---|---|---|
0 | 3.00 | ±0.25 | 0.053 | ±0.006 |
2.40 | 0.050 | |||
2.43 | 0.045 | |||
2.46 | 0.043 | |||
2.50 | 0.060 | |||
|
||||
240 | 2.90 | ±0.21 | 0.059 | ±0.006 |
2.39 | 0.055 | |||
2.43 | 0.047 | |||
2.49 | 0.049 | |||
2.50 | 0.062 | |||
|
||||
600 | 2.70 | ±0.13 | 0.061 | ±0.0054 |
2.38 | 0.060 | |||
2.40 | 0.054 | |||
2.47 | 0.050 | |||
2.49 | 0.063 | |||
|
||||
960 | 2.55 | ±0.08 | 0.063 | ±0.0054 |
2.38 | 0.067 | |||
2.37 | 0.061 | |||
2.40 | 0.053 | |||
2.47 | 0.065 |
Urbach’s energy corresponds to the width of the tail of the localized states within the optical band gap. It is linked to the absorption coefficient in the lower energy region of fundamental edge and can be described by the relation [
Figures
Absorption spectra of (a) Coumarin 6, (b) Fluorescein, and (c) Rhodamine 6G in Triton X-100 with/without silver before and after UV irradiation.
Figure
Figures
Plots of
Plots of
The values of optical band gap Urbach’s energy are listed in Table
The values of optical band gap and Urbach’s energy for Coumarin 6, Fluorescein, and Rhodamine 6G in Triton X-100 with and without silver NPs before and after exposure to UV irradiation.
Dyes | Before adding silver |
|
|
After adding silver |
|
|
---|---|---|---|---|---|---|
Coumarin 6 |
|
2.56 | 0.066 |
|
2.47 | 0.073 |
|
2.51 | 0.069 |
|
2.47 | 0.075 | |
|
2.49 | 0.071 |
|
2.43 | 0.077 | |
|
||||||
Fluorescein |
|
2.30 | 0.039 |
|
2.27 | 0.051 |
|
2.29 | 0.041 |
|
2.27 | 0.052 | |
|
2.28 | 0.042 |
|
2.26 | 0.054 | |
|
||||||
Rhodamine 6G |
|
2.19 | 0.044 |
|
2.16 | 0.062 |
|
2.18 | 0.045 |
|
2.16 | 0064 | |
|
2.17 | 0.046 |
|
2.15 | 0.067 |
The stability of fluorescent dyes is one of the main factors in LSC development. To examine the stability of Coumarin 6, Fluorescein, and Rhodamine 6G in Triton X-100, the absorbance was measured before and after irradiation with 300 W Xenon arc lamp with absorbing UV filter and without filter for 960 minutes. Silver NPs added to the above samples and measurements are repeated. Figures
The photodegradation of Coumarin 6, Fluorescein, and Rhodamine 6G in Triton X-100 with and without filter before and after adding silver NPs.
The stability of the organic dyes may be due to the interaction with surface plasmon resonance (SPR) in metal particles; there is also interaction result in shorting of the excited-state lifetime thus improving the photostability of the dye [
Rate constants of photodegradation of dyes are estimated according to [
The kinetics of photodegradation
Dyes | Conditions | Without filter | With filter | ||
---|---|---|---|---|---|
|
|
|
|
||
Coumarin 6 | Before adding silver | 8.37 × 10−5 | 8278 | 5.33 × 10−5 | 12984 |
After adding silver | 3.49 × 10−5 | 19847 | 1.04 × 10−6 | 665694 | |
Fluorescein | Before adding silver | 9.80 × 10−5 | 7062 | 4.85 × 10−6 | 14270 |
After adding silver | 5.22 × 10−5 | 13255 | 1.24 × 10−6 | 554690 | |
Rhodamine 6G | Before adding silver | 4.4 × 10−2 | 15.74 | 3.53 × 10−3 | 19.59 |
After adding silver | 4.89 × 10−5 | 14143 | 1.04 × 10−6 | 653773 |
Figure
The area under fluorescence curves for Coumarin 6, Fluorescein, and Rhodamine 6G in Triton X-100 without and with silver and with silver after UV irradiation for 960 minutes.
Dyes | Conditions | Absorbance wavelength (nm) | Emission wavelength (nm) | Stoke shift |
Area under fluorescence curve |
---|---|---|---|---|---|
Coumarin 6 | Without silver | 440 | 538 | 98 | 486.60 |
With silver before exposure | 440 | 540 | 100 | 686.65 | |
With silver after exposure | 455 | 562 | 107 | 585.68 | |
|
|||||
Fluorescein | Without silver | 461 | 542 | 81 | 614.33 |
With silver before exposure | 454 | 542 | 88 | 806.77 | |
With silver after exposure | 472 | 562 | 90 | 684.93 | |
|
|||||
Rhodamine 6G | Without silver | 525 | 557 | 32 | 639.58 |
With silver before exposure | 510 | 557 | 47 | 874.26 | |
With silver after exposure | 526 | 583 | 57 | 738.54 |
Fluorescence spectra of Coumarin 6, Fluorescein, and Rhodamine 6G in Triton X-100 (A) without silver, (B) with silver, and (C) with silver after UV irradiation.
The challenges in LSC development have been highlighted. One promising concept is the addition of silver nanoparticles (NPs). The results are summarized as follows: Silver NPs in Triton X-100 were analyzed by different characterizing methods and it is confirmed that the absorption band exhibited a red shift and broadening after UV irradiation for 960 minutes. The optical energy gap The measurements of FQY of Coumarin 6 and Fluorescein at different concentrations reveal the dependence of FQY on concentrations. The addition of silver (NPs) to dye solution has been investigated as a means of enhancing the fluorescence of our selected dyes. The increase in fluorescence highlights the importance of silver NPs. The photostability of our selected organic dyes in liquid and solid matrix was enhanced by the addition of silver NPs especially with Rhodamine 6G in liquid in which the photostability extremely increased.
Some part of this study is sequentially presented at Solar-TR-2: Solar Electricity Conference and Exhibition Programme, Turkey, 2013.
The authors declare that they have no conflicts of interest.
This work was supported by the German Research Foundation (DFG) and the Technical University of Munich within the Open Access Publishing Funding Programme. Also, the authors would like gratefully to thank Dr. Mohammed Gabr for his cooperation and support.