Lingpeng Yanab,
Yamin Haoab,
Xiaoting Fengab,
Yongzhen Yang*ab,
Xuguang Liu*abc,
Yongkang Chenad and
Bingshe Xuab
aKey Laboratory of Interface Science and Engineering in Advanced Materials, Ministry of Education, Taiyuan University of Technology, Taiyuan 030024, China. E-mail: yyztyut@126.com; liuxuguang@tyut.edu.cn
bResearch Center of Advanced Materials Science and Technology, Taiyuan University of Technology, Taiyuan 030024, China
cCollege of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, China
dUniversity of Hertfordshire, School of Engineering and Technology, Hatfield, Herts. AL10 9AB, UK
First published on 7th September 2015
Sandwich-like Ag–C–Ag nanoparticles (Ag–C–Ag NPs) were synthesized under mild hydrothermal conditions in a one-step method. With this approach, Ag was not only encapsulated in the centre of an individual carbon nanosphere, but was also uniformly dispersed within the carbon matrix up to the sphere's shell. Then, poly(3-hexylthiophene):Ag–C–Ag NPs (P3HT:Ag–C–Ag NPs) composite films were prepared by a spin coating method with a chlorobenzene solution of Ag–C–Ag NPs and P3HT. Both morphology and microstructure of Ag–C–Ag NPs were investigated by field emission scanning electron microscopy and high resolution transmission electron microscopy. The possible formation mechanism was proposed. The results have indicated that the Ag–C–Ag NPs present many functional groups and their energy levels match with those of P3HT. It has been observed that an introduction of Ag–C–Ag NPs to P3HT can induce broad and high-absorbing spectra as well as great photoluminescence quenching of P3HT. It is evident that sandwich-like Ag–C–Ag NPs have a great potential to be a new acceptor material in photovoltaic devices.
Metal materials have also been reported to increase light absorption in active layers of solar cells (SCs) through their plasma effects.9–11 At the nano-scale, metallic nanoparticles can be penetrated by electromagnetic waves, which induce a separation of charges, and consequently a coherent oscillation (surface plasmon). Surface plasmons are essentially light waves that are trapped on the surface because of their interaction with the free electrons of the metal.12 The primary consequences of excitation due to localized surface plasmon resonances are selective photo absorption, scattering, and local electromagnetic field enhancements.13 Silver (Ag) exhibits the highest electrical and thermal conductivities among all types of metal and has the good property of oxidation resistance. Its optical trapping potential is also most effective owing to its high scattering efficiency in the visible range.14 However, silver nanoparticles are prone to coalesce as a result of van der Waals forces and high surface energies unless they are protected.2 Carbon nanospheres have been proposed to provide that role in protection.15 As a part of the carbon family, carbon nanospheres, which have stable chemical and thermal properties, as well as low preparation costs, are promising candidates to be used as metal supports.
Sun and Li16 prepared Ag/C core/shell spheres using water as an environmentally benign solvent. Carbon nanospheres loaded with silver nanoparticles were also obtained by two-step methods17,18. Though Wang et al.17 investigated a new synthesis route for Ag/C core/shell nanospheres with many small Ag particles on the surface of the shell structure, the synthesis steps were complex and not so eco-friendly.
In this study, sandwich-like Ag–C–Ag nanoparticles (Ag–C–Ag NPs), with Ag encapsulated in the middle of a carbon nanosphere distributed in the carbon shell and loaded on the surface of the carbon sphere, were synthesized by a one-step method under typical hydrothermal conditions. Their application as an acceptor for polymer solar cells with P3HT as the donor becomes attractive in this study. This composite structure is expected to yield an enhanced and broader absorption range of the solar spectrum that could induce a remarkable increase in charge generation. Ultraviolet-visible (UV-Vis) and photoluminescence (PL) spectroscopies were used to confirm the optical behaviour of the obtained products.
Glass substrates were pre-cleaned repeatedly in an ultrasonic bath and sequentially cleaned by detergent, deionized water, ethanol and acetone. P3HT:Ag–C–Ag NPs solution was spin-coated onto a cleaned glass substrate at a spin rate of 1000 rpm for 1 minute and dried at room temperature.
Fig. 2(a) presents a typical FESEM image of Ag–C–Ag NPs, which exhibit regular spherical shapes with diameters in the range from 300 to 520 nm. Energy dispersive spectroscopy (EDS) indicates that the sandwich-like structure consists of C, O and Ag, as shown in the top right corner of Fig. 2(a). To further investigate the detailed structure of the products, TEM analyses were conducted. First, it can be distinctly seen from Fig. 2(b) that Ag particles (arrow A) are encapsulated in the carbon shell, and the diameter of the core and the thickness of the shell are about 80 and 200 nm, respectively. Second, Fig. 2(b) also shows that the Ag nanoparticles (about 20 nm in size) that are dispersed in the carbonaceous matrix can be divided into two parts with different distributions. The first part is on the surface of the carbon shell with the crystal spacing of 0.23 nm that corresponds to Ag (111), as marked by the square in Fig. 2(b) and shown in an inset at the bottom left corner of Fig. 2(c). As shown in an inset at the top left corner of Fig. 2(c), the electron diffraction pattern indicates the single-crystalline nature of the Ag nanoparticles. It can also be observed that a thin layer of carbon (less than 2 nm) surrounds the Ag nanocrystals. The second part is dispersed inside the carbon matrix (arrow B in Fig. 2(b)). Third, a small amount of hollow spheres without an Ag nanoparticle core or with a split Ag nanoparticle core was found, as marked with the circles in Fig. 2(b) and (d). More importantly, compared with the carbon spheres with a full Ag nanoparticle core, these hollow spheres without an Ag nanoparticle core or with a split Ag nanoparticle core have more Ag nanoparticles dispersed in the carbon matrix.
Fig. 3 presents the XRD profiles of Ag–C–Ag NPs. It can be seen that there is a broad peak in the range of 15–30° that is attributed to the (002) reflection of a carbon layer in Ag–C–Ag NPs, indicating the existence of amorphous carbon. It is found that strong diffraction peaks assigned to Ag nanocrystals at 8.15°, 44.30°, 64.45° and 77.30° are associated with the (111), (200) (220) and (311) reflections of Ag nanocrystals, respectively, illustrating that the Ag nanocrystals are stacked in commonly known face-centred cubic structure. Otherwise, the strongest diffraction peak of Ag nanocrystals is the (111) reflection, indicating that the preferable growth face of Ag nanocrystals is in (111) plane.
To determine the ratio of Ag nanocrystals in Ag–C–Ag NPs and the thermal stability of Ag–C–Ag NPs, a TG test in air was carried out. As can be seen in Fig. 4, Ag–C–Ag NPs exhibit only 5.50% of weight loss when the temperature is increased up to 275 °C. When the temperature exceeded 285 °C, the Ag–C–Ag NPs began to lose weight rapidly and reached a steady state above 380 °C. During the period of fast weight loss, carbon plies were oxidized to carbon dioxide, and eventually the oxidization and deoxidization of Ag nanocrystals was in equilibrium. The final product was Ag nanocrystals because Ag2O is unstable at high temperatures. Thus, the ratio of Ag nanocrystals in Ag–C–Ag NPs is about 5.30%.
FTIR measurements were carried out to investigate the functional groups of products after the hydrothermal treatment. As shown in Fig. 5, the bands at 1701 and 1625 cm−1 were attributed to CO and C
C vibrations, respectively, indicating the aromatization of glucose during the hydrothermal treatment. It can be seen that the as-synthesized Ag–C–Ag NPs exhibited a strong band of –OH (O–H stretching vibrations) at 3429 cm−1, along with the weak peaks of C–OH at 1385 and 1302 cm−1, which means that the as-synthesized Ag–C–Ag NPs are rich in hydroxyl groups. The presence of a functional group makes Ag–C–Ag NPs disperse better in aqueous medium and common organic solvents, which can greatly widen the range of their applications.
Based upon an analysis of the Zeta potential on the products, the surfaces of the Ag–C–Ag NPs have been negatively charged in water (pH = 7, ξ = −25 eV). This negative charge originated from the ionization of –COOH and –OH groups on the surface of Ag–C–Ag NPs.20 A small amount of hollow or split nanoparticles, as marked by the circles in Fig. 2(b) and (d), may be induced by the electrostatic suction between a Ag core with positive electricity and a carbon matrix.19 Under the action of electrostatic attraction, Ag nanoparticles were diffused from the core into the carbonaceous matrix, resulting in the splitting of the Ag core and the presence of trace amounts of hollow or split Ag nanoparticles, as shown in Fig. 6(e) and (f). Thus, it can be reasonably suggested that some of the Ag nanoparticles dispersed in the carbon matrix and on the surface of carbon shell originated from the outward diffusion of Ag nanoparticles through electrostatic forces. This is also verified by the fact that carbon spheres with a hollow or split Ag nanoparticle core have more Ag nanoparticles dispersed in the carbon matrix than the carbon spheres with full Ag cores, as shown in the circles of Fig. 2(b) and (d). Therefore, it is suggested that the formation of Ag nanoparticles dispersed in the carbon matrix and on the surface of the carbon shell may result from two reasons: a reduction of Ag+ and outward diffusion of the Ag nanoparticles under electrostatic forces.
HOMO = −(Eox0 + 4.74) (eV), | (1) |
LUMO = −(Ered0 + 4.74) (eV), | (2) |
It can be seen in Fig. 7 that Eox0 and Ered0 of Ag@C NPs are 0.95 V and −0.06 V, respectively. Therefore, the calculated HOMO and LUMO energy levels are −5.69 and −4.68 eV, respectively. Eg, the difference between HOMO and LUMO energy levels, is calculated to be 1.01 eV. Similarly, the HOMO level, LUMO level and Eg of Ag–C–Ag NPs are calculated to be −5.55, −4.40 and 1.15 eV, respectively. Free holes and electrons cannot be created directly in composite films when light is absorbed because excitons have a large binding energy of 0.3 eV;22 thus, the energy gaps of electron donors and electron acceptors ΔE1 and ΔE3 should be greater than 0.3 eV, where ΔE1 = DLUMO − ALUMO and ΔE3 = DHOMO − AHOMO; D refers to electron donors and A refers to electron acceptors. It has been reported that the HOMO and LUMO of P3HT are −5.0 and −3.0 eV,19 respectively. As a result, ΔE1 and ΔE3 of the active layer based on electron-donating P3HT and electron-accepting Ag@C NPs are 0.69 and 1.68 eV, respectively, and those of the active layer based on electron-donating P3HT and electron-accepting Ag–C–Ag NPs are 0.55 and 1.40 eV, respectively. Both are greater than the exciton binding energy. Therefore, it is suggested that Ag@C NPs and Ag–C–Ag NPs could be suitable to be used as the acceptor materials of polymer solar cells. For donor–acceptor bulk heterojunction solar cells, the Voc value is mainly determined by the difference between the acceptor's LUMO level and the donor's HOMO level,23 which can be expressed as follows:
Voc = e−1 × (|HOMOdonor| − |LUMOacceptor| − 0.3 eV) | (3) |
In the active layers, the HOMO level for P3HT is −5.0 eV, the LUMO levels of Ag@C NPs and Ag–C–Ag NPs are −4.68 and −4.40 eV, respectively. Thus, the Voc values of P3HT:Ag@C and P3HT:Ag–C–Ag NPs are determined to be 0.32 and 0.60 V, respectively, according to eqn (3). It can be suggested from the Voc values of P3HT:Ag@C and P3HT:Ag–C–Ag NPs that Ag–C–Ag NPs are more suitable as active materials for photovoltaic cells.
Fig. 9 shows the morphology of a typical P3HT:Ag–C–Ag NPs (2:
1; w/w) composite film. As can be seen from the TEM image of the composite film, the sandwich-like Ag–C–Ag NPs can also been observed clearly, which is consistent with the TEM image of Ag–C–Ag NPs, and they have a good dispersion in P3HT, attributed to the good solubility of Ag–C–Ag NPs in chlorobenzene. This good dispersion contributes to the formation of an interpenetrating network structure in the composite film, which is conducive to the efficient transfer of photo-induced charge in the composite film. This result is in good agreement with the PL characterization results shown in Fig. 8.
To investigate the optical properties of P3HT:Ag–C–Ag NPs composite films, the UV-Vis absorption spectra of P3HT:Ag–C–Ag NPs (2:
1; w/w) were obtained. Besides, Ag–C–Ag NPs prepared at 180 °C (Ag–C–Ag NPs-180) were used for a comparison, which contain barely any Ag nanoparticles. Fig. 10 shows the UV-Vis absorption spectra of P3HT, P3HT:Ag–C–Ag NPs (2
:
1; w/w), P3HT:Ag–C–Ag NPs-180 (2
:
1; w/w) and the difference spectra between the composite films and P3HT in the solid state. In the spectrum of pure P3HT, the highest energy peak exists at 526 nm. Two shoulders can also be found at 553 and 599 nm, which represent the vibrational excitations of crystalline P3HT. After pure P3HT is blended with the P3HT:Ag–C–Ag NPs-180, the absorbance of the composite film in the region of 400–700 nm is weakly increased, because the Ag–C–Ag NPs-180 exhibits a weak absorption. However, after the introduction of Ag–C–Ag NPs, the absorption of P3HT:Ag–C–Ag NPs composite films show an obvious enhancement in the range of 300–650 nm, which is likely attributed to the plasma resonances of Ag–C–Ag NPs. In addition, by comparing the difference spectra of P3HT with P3HT:Ag–C–Ag NPs and P3HT:Ag–C–Ag NPs-180, the stronger absorption of P3HT:Ag–C–Ag NPs can also be revealed.
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Fig. 10 The UV-Vis spectra of P3HT, P3HT:Ag–C–Ag NPs (2![]() ![]() ![]() ![]() |
To study the plasmon resonance of Ag–C–Ag NPs, the UV-Vis absorption spectra of Ag–C–Ag NPs (5 mg mL−1, 1000 rpm) and Ag–C–Ag NPs-180 (5 mg mL−1, 1000 rpm) in solid state were characterized, as shown in Fig. 11. In comparison with Ag–C–Ag NPs-180, the Ag–C–Ag NPs show much more intensive light absorption bands in the range of 300–650 nm, which is consistent with the difference spectra of P3HT:Ag–C–Ag NPs and P3HT, suggesting the existence of plasmons in the P3HT:Ag–C–Ag NPs film. Silver nanoparticles can intensely absorb visible light because of the surface plasmon resonance effect;26 thus, the Ag–C–Ag NPs show more intensive light absorption than Ag–C–Ag NPs-180. The spectral enhancement is likely due to two major factors. The first could be a local enhancement of the electromagnetic field on an account of localized surface plasmon resonances.27,28 The other could be plasmon resonance scattering, which becomes dominant when a larger Ag core is used. It appears that the scattering lengthens the optical path several times in the active layer, thereby enhancing the degree of light absorption.29
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Fig. 11 The UV-Vis spectra of Ag–C–Ag NPs (5 mg mL−1, 1000 rpm) and Ag–C–Ag NPs-180 (5 mg mL−1, 1000 rpm). |
(i) Sandwich-like Ag–C–Ag NPs were hydrothermally synthesized by a one-step method and the possible formation mechanism of Ag–C–Ag NPs was proposed. With this method, Ag was not only encapsulated in the centre of a carbon nanosphere, but was also uniformly dispersed within the carbon matrix and on the carbon nanosphere's shell.
(ii) The ratio of Ag nanocrystals in Ag–C–Ag NPs is about 5.30% and the Ag–C–Ag NPs present many functional groups. These groups facilitate further functionalization for various applications.
(iii) With a matched energy level, the Ag–C–Ag NPs and P3HT composite films possess apparent quenching phenomena, indicating an efficiently photo-induced charge transfer in the composite films, and enhanced absorption in the optical range, which is due to the plasmon resonances of Ag nanoparticles in Ag–C–Ag NPs. Therefore, sandwich-like Ag–C–Ag NPs have a great potential to be a new and highly efficient acceptor material for application in polymer solar cells.
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