Preparation of monodispersed PbS quantum dots on nanoporous semiconductor thin film by two-phase method
Introduction
Quantum dots (QDs) are semiconductor nanoparticles with particle size below their bohr radius. QDs perform some special properties differing from their bulk materials, and show several potential applications. Therefore, some methods to synthesize QDs have been developing during past several decades. Among these QDs, PbS QDs are currently under intense investigation because of their special characteristics including a large exciton bohr radius as big as 18 nm and a strong quantum size effect. In addition, multiple exciton generation in PbS QDs has been observed [1]. In particular, the synthetic control over PbS QDs diameter enable them to absorb light in different spectrum ranges even extending to near infrared (NIR) region. Because PbS QDs have such a number of advantages, they are reported as good candidates in some areas including photodiodes [2], solar cells [3], [4], photovoltaic photodetectors [5], [6], and luminescent solar concentrators [7]. Recently, QDs have been popularly used as highly-efficient inorganic sensitizers in QDs-sensitized solar cell (QDSSC). Owing to the good electron mobility and appropriate conduction band edge, nanocrystalline thin films of inorganic oxide semiconductor such as TiO2 and ZnO were commonly employed as the electron accepting materials in FTO-supported devices [8]. As a result, the photoanode covered with PbS QDs was fabricated.
Inorganic PbS QDs are usually synthesized on the semiconductor nanocrystalline thin films in two different ways. One is in situ synthesis on nanocrystalline semiconductor thin films directly, namely the successive ionic layer adsorption and reaction (SILAR) process [9], and the other is called as non-in situ synthesis, which needs bi-functional linker molecules to connect the as-prepared QDs with the thin film [10], [11]. As for SILAR method, many adsorbing and rinsing steps have to be repeated for getting sufficient absorber on the thin film. In addition, the growth rate and particle size of as-prepared PbS QDs cannot be easily controlled. On the other hand, to prepare QDs usually need rigorous experiment conditions like high temperature, vacuum and inert environment. During the non-in situ synthesis process, the usage of linker molecules showed negative influence on the charge separation, recombination and transport process. The conversion efficiencies of QDSSCs fabricated using non-in situ synthesis method are lower than those of devices fabricated using SILAR method, as reported in the previous Refs. [5], [8], [9]. Therefore, choosing an appropriate linker molecule is a key step to optimize this kind of QDSSC [12], [13]. Many research groups reported that this is complicated and difficult, because it should be concerned about the length of alkyl chain and the choice of either acid or thiol groups employed as the attachment moieties at the linker ends [11]. Besides, the linker molecules widely used so far are several organic acids, for example, mercaptoacetic acid, ethane dithiol and cysteine [14]. They are more or less toxic and showed a poor long-term stability. Therefore, it is essential to develop new synthesis approach of QDs to avoid the disadvantages of the reported methods as mentioned above.
In this study, an unconventional two-phase method was developed to synthesize PbS QDs on the nanocrystalline SnO2 thin film. In this method, the growth rate of QDs was controlled by the diffusion of S2− ions transferred from oil phase (phase I) within the aqueous phase (phase II). The nanoporous SnO2 thin film adsorbed by Pb2+ ions was firstly put into the phase II. The volume ratio of ethylene glycol and water in phase II was adjusted to change the diffusion rate of S2− ions within the phase II. Thus monodispersed QDs with different particle sizes were prepared. The as-prepared PbS QDs-sensitized SnO2 thin film was then used as a photoanode to fabricate QDs-sensitized solar cell and 1.05% of the photoelectric conversion efficiency was obtained.
Section snippets
Materials
Lead acetate ((CH3COO)2Pb·3H2O), anhydrous sodium sulfide (Na2S), ethylene glycol, oleic acid, cyclohexane, sulfur powder (S), sodium hydroxide (NaOH) and potassium chloride (KCl) were purchased from Alfa Aesar Inc., China. All chemicals were analytic grade and used without any further purification. Tin dioxide (SnO2) nanoparticles with the mean size of about 50 nm were purchased from Alfa Aesar Inc., China. Lead foil (99.9% metal basis) with thickness of 0.5 mm was also purchased from Alfa Aesar
Results and discussion
On the mode of down-top to grow nanoparticles from molecules or ions, rapid nucleation and slow growth are two key processes to prepare monodispersed nanoparticles. Usually, organic solvent with high boiling temperature is used as a medium to grow monodispersed nanoparticles. The growth rate of nanoparticle can be effectively controlled in the organic solvent at high temperature [17]. The commonly used SILAR technique can only realize in situ growth of nanoparticles on the porous thin film.
Conclusion
A facile and low-cost two-phase method was developed to in situ prepare monodispersed PbS QDs on the nanocrystalline SnO2 thin film. S2− ions contained in the sulfide salt was firstly dissolved in the oil phase and nanoporous semiconductor thin film adsorbed by Pb2+ ions was immersed in the aqueous phase. The growth rate of QDs was controlled by the diffusion of S2− ions within the aqueous phase. The particle size and size distribution of the PbS QDs were successfully adjusted by changing the
Acknowledgements
The work was supported by the National Natural Science Foundation of China (Grant Nos. 21273160, 51102011, 21177007 and 51072014) and the Program for Excellent Introduced Talents of Tianjin Normal University in China (5RL116).
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