Plasmonic Enhancement of Photoenergy Conversion in Visible Light Region using PbS Quantum Dots Coupled with Au NanoparticlesXiaowei Li & Kei Murakoshi (Department of Chemistry, Hokkaido University)

2016 年度 No.1

【研究紹介】

Plasmonic Enhancement of Photoenergy Conversion in Visible Light Region using PbS Quantum Dots Coupled with Au Nanoparticles

Xiaowei Li & Kei Murakoshi (Department of Chemistry, Hokkaido University)

Owing to localized surface plasmonic resonance (LSPR), plasmon-induce photoexcitation can be applied to break a limit of photoenergy conversion via changing photoenergy localization and lifetime of excited states of materials [1]. Further tuning of the target energy of photons would realize using quantum dots (QDs) to sensitize tailored systems, because of the capability to present strong quantum confinement with diameters smaller than the Bohr radius of their exciton, especially lead sulfide quantum dots (PbS QDs) with a relative large Bohr radius of 18 nm [2]. Enhanced photocurrent has been observed by coupling quantized PbS particles with plasmonic Au nanoparticles in near infrared wavelength range [3]. Recently we have succeeded in plasmonically enhanced photocurrent conversion in visible-wavelength region usign the TiO2/Au/TiO2 with ultra-small PbS QDs which are capped with distinct ligands of 3-mercaptopropionic acid (MPA) and oleic acid (OA) (Fig.1 a and b) [4].

The characterization of the TiO2/Au/TiO2 structure via SEM illustrates that gold nanoparticles are well dispersed on the TiO2 surface without dissolution even after deposition of PbS QDs (Fig.1 c to e).

Figure 1. (a) The band energy of MPA- and OA-capped PbS QDs, (b) schematic energy diagram of PbS QDs/TiO2/Au/TiO2 system in electrolyte of 0.05 M Na2S+0.1 M NaOH. SEM images of (c) bare TiO2/Au/TiO2 substrate, and sensitized with (d) MPA-capped PbS QDs and (e) OA-capped PbS QDs, respectively.

Enhanced photo-electrochemical response of PbS QDs was observed for plasmonic solar cell of TiO2/Au/TiO2 in visible wavelength region of 500 to 700 nm compared with bare TiO2 (Fig.2), which suggests the contribution of the plasmonic resonance owing to the enhanced electric field by coupling Au NPs with PbS QDs [5]. In addition, MPA-capped smaller PbS QDs–sensitized plasmonic substrate also produced noteworthy improvement at a shorter wavelength range below 500 nm, in contrast with the unsatisfactory enhancement effect by OA-capped larger PbS QDs.

(c)

(d)

(e)

Figure 2. IPCE comparison of (a) MPA- and (b) OA-capped PbS QDs–sensitized TiO2 and TiO2/Au/TiO2

substrates.

Enhancement factor, calculated as the yielded IPCE ratio of PbS QDs/TiO2/Au/TiO2 substrate to PbS QDs/TiO2 substrate at a certain wavelength, was examined by increasing the loading amount of PbS QDs to exceed a monolayer covering the substrate surface. The maximum EF suggests an effective coupling between PbS QDs and Au NPs as the electromagnetic field owing to LSPR localizes only in the vicinity of 10 nm around Au nanoparticles.

Figure 3. Comparison of enhancement factor changes depending on the loading amount of (a) MPA- and (b) OA-capped PbS QDs measured at -0.1 V in electrolyte of 0.05 M Na2S+0.1 M NaOH.

Other OA-capped PbS QDs with various larger diameters were also studied. Apparent enhancement at a short wavelength less than 500 nm was observed owing to multiple exciton generation of PbS QDs excited by Au NPs. These results demonstrate the validity of plasmon enhancement on photovoltaic response of PbS QDs in a relatively wide wavelength region from ultraviolet, visible to near infrared. Reference [1] H. Nabika et al., J. Phys. Chem. Lett., 2010, 1, 2470.[2] F. W. Wise, Acc. Chem. Res., 2000, 33, 773.[3] T. Kawawaki and T. Tatsuma, Phys. Chem. Chem. Phys., 2013, 15, 20247.[4] X. Li, et al., J. Phys. Chem. C, 2015, 119, 22092.[5] T. Hutter, et al., J. Raman Spectrosc., 2013, 44, 1292.

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