Syntheses and Photoelectrochemical Properties of Plasmonic Molybdenum Oxide Nanoparticles Via Ionic Liquid/Metal Sputtering

Introduction

Incident lights interact with noble metal nanoparticles (NPs) to form collective oscillations of free electrons, creating a strong electromagnetic field on their surface. This phenomenon is called localized surface plasmon resonance (LSPR). Recently metal oxide NPs with appreciable concentration of free electrons have been also reported to exhibit LSPR peaks, the peak wavelengths of which were controlled from the visible to near-IR regions.[1] When a plasmonic NP was combined with a semiconductor, the excitation of LSPR could cause the injections of electrons and/or holes from plasmonic NPs to the semiconductor, that is, the plasmon-induced charge separation (PICS).[2] Since the PICS was reported using plasmonic metal oxide NPs,[3] the intense research has been devoted to studying plasmonic metal and metal oxide NPs for the application to photovoltaics, photocatalyts, and biosensors.

Recently we reported the preparation of noble metal NPs by metal sputtering deposition onto room-temperature ionic liquids (RTIL) under a reduced pressure (RTIL/metal sputtering), where NPs of several nanometers in size were stably and uniformly dispersed without additional stabilizing agents.[4] This technique enabled the clean preparation of plasmonic metal NPs, such as Au[4], Ag[5], and AgAu alloy[6]. In this study, we apply the RTIL/metal sputtering to the preparation of molybdenum oxide (MoO_x) NPs. Thus-obtained NPs exhibited the LSPR peak, the excitation of which induced the PICS.

Experimental

The sputter deposition of molybdenum were carried out on the surface of 1-hydroxyethyl-3-methylimidazolium tetrafluoroborate (HyEMI-BF₄) and 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI-BF₄) used as RTILs for 1 h with a discharge current of 30 mA under an argon pressure of 3.0 Pa. The deposited NPs were partially oxidized by the heat treatment at 473 K for 30 min in air, resulting in the formation of MoO_x NPs. The photoelectrochemical properties were measured in a 0.5 M Na₂SO₄ aqueous solution with the ITO electrode immobilized with MoO_x NPs as working electrodes, a Pt counter electrode and a Ag/AgCl reference electrode. The photocurrents were detected with monochromatic light irradiation under the potential application at +0.5 V vs Ag/AgCl.

Results and Discussion

TEM measurements revealed that Mo NPs sputter-deposited in HyEMI-BF₄ had an average diameter of 6.8 nm and then the heat treatment at 473 K increased the particle size to ca. 65 nm. XPS spectra of thus-obtained particles indicated that the thus-obtained NPs were composed of MoO_x containing Mo(V) and Mo(VI) species. The MoO_x NPs prepared in HyEMI-BF₄ showed a LSPR peak at around 840 nm, the peak wavelength of which was close to that of LSPR reported for chemically synthesized MoO_3-x NPs.[1] On the other hand, MoO_x NPs prepared in EMI-BF₄ showed no LSPR peak in the visible and near-IR regions. Anodic photocurrents were observed by the irradiation to MoO_x NP-immobilized ITO electrodes with wavelength shorter than ca. 1000 nm. Regardless of the kinds of MoO_x NPs, the action spectra of photocurrents showed the increase of IPCE at the wavelength below 600 nm due to the photoexcitation of interband transition of MoO₃, being in agreement with their extinction spectra. Furthermore, the MoO_x NPs prepared in HyEMI-BF₄ showed a peak at 840 nm, the peak wavelength of which agreed with that of their LSPR peak. In contrast, MoO_x NPs formed in EMI-BF₄ exhibited no photocurrent by the irradiation of near-IR lights.

Conclusively, we successfully prepared the MoO_x NPs with RTIL/metal sputtering technique followed by the heating treatment. The NPs formed in HyEMI-BF₄ showed the LSPR peak in near-IR region, the photoexcitation of which induced the PICS from the MoO_x NPs to the ITO electrodes.

References

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(2) Y. Tian and T. Tatsuma, J. Am. Chem. Soc., 127, 7632 (2005).

(3) S. H. Lee, H. Nishi, and T. Tatsuma, Nanoscale, 10, 2841 (2018).

(4) T. Torimoto, K. Okazaki, T. Kiyama, K. Hirahara, N. Tanaka, and S. Kuwabata, Appl. Phys. Lett., 89, 243117 (2006).

(5) T. Suzuki, K. Okazaki, T. Kiyama, S. Kuwabata, and T. Torimoto, Electrochemistry, 77, 636 (2009).

(6) K. Okazaki, T. Kiyama, K. Hirahara, N. Tanaka, S. Kuwabata, and T. Torimoto, Chem. Commun., 6, 691 (2008).