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[Thi Kim Ngan Nguyen](https://orcid.org/0000-0001-8935-1306), Fabien Grasset, Noée Dumait, Stéphane Cordier, David Berthebaud, [Naoki Ohashi](https://orcid.org/0000-0002-4011-0031), [Tetsuo Uchikoshi](https://orcid.org/0000-0003-3847-4781)

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[Tunable photo-induced electronic property of octahedral metal clusters](https://mdr.nims.go.jp/datasets/b5efcb23-ea6b-483e-a228-3e41d8daf2a2)

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Tunable photo-induced electronic property of octahedral metal clustersMaterials Letters: X 11 (2021) 100079Contents lists available at ScienceDirectMaterials Letters: Xjournal homepage: www.elsevier .com/locate /mlbluxTunable photo-induced electronic property of octahedral metal clustershttps://doi.org/10.1016/j.mlblux.2021.1000792590-1508/� 2021 The Authors. Published by Elsevier B.V.This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).⇑ Corresponding author. Research Center for Functional Materials, NationalInstitute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047,JapanE-mail addresses: NGUYEN.Thikimngan@nims.go.j (T.K.N. Nguyen), UCHIKOSHI.Tetsuo@nims.go.jp (T. Uchikoshi).Thi Kim Ngan Nguyen a,b, Fabien Grasset a,b, Noée Dumait c, Stéphane Cordier c,David Berthebaud b, Naoki Ohashi a,b, Tetsuo Uchikoshi a,b,⇑aResearch Center for Functional Materials, National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, JapanbCNRS–Saint-Gobain–NIMS, IRL 3629, Laboratory for Innovative Key Materials and Structures (LINK), National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki305-0044, JapancUniv. Rennes-CNRS-Institut des Sciences Chimiques de Rennes, UMR 6226, 35000 Rennes, Francea r t i c l e i n f oArticle history:Received 10 February 2021Received in revised form 12 April 2021Accepted 22 April 2021Available online 25 April 2021Keywords:Metal clusterMolybdenumTantalumOptoelectronic propertyZeta potentiala b s t r a c tThe tunable photo-induced electronic property of the Cs2[{Mo6Bri8}Bra6] and Br2[{Ta6Bri12}(H2O)a6] octahe-dral metal cluster-based compounds was characterized by the zeta potential measurement upon irradiationby ultraviolet–visible (UV–Vis) light. The enhancement of the electronic charge on the surface of the[{Mo6Bri8}Bra6]2- cluster-based anion caused by the irradiation increased the negative zeta potential. Bothcompounds showed the dependence of the zeta potential as a function of the light excitation. This willbe a promising property for sensor applications based on the reversibility of the photo-electrochemicalproperties of the functional cluster.� 2021 The Authors. Published by Elsevier B.V. This is an open access articleunder the CCBY license (http://creativecommons.org/licenses/by/4.0/).1. IntroductionMultifunctional materials based on the integration of anorganic–inorganic atomic integrated system that demonstrate thereversibility of redox transformation, photochemistry, or electro-chemistry are attracting attention for energy-saving applicationssuch as photovoltaics [1] and light-emitting diodes (LEDs) [2]. Asis known, the metal atom cluster (MC) family, which is built eitheron an edge-bridged {M6(l2-X)i12} (M = Ta, Nb, W) or face-capped{M6(l3-X)i8} (M = Mo, Re) metallic cluster core (Xi = I, Br, Cl) coordi-nated with apical ligands (La = I, Br, Cl, H2O, OH) with a nanometersize (under 2 nm) exhibiting photochemical and photophysical [3],luminescent [4], and redox properties [5], have been extensivelystudied. Generally, the valence electron counts (VEC) are equal to24 electrons or 16 electrons on the electron shell per the [{M6(l3-X)i8}La6]2- or [{M6(l2-X)i12}La6]4- cluster unit, respectively, and it willbecome a strong and powerful oxidant in the oxidized state[6–7].The luminescence of MC has been remarkably studied for the{M6(l3-X)i8}4+ metallic core cluster that results in a strong lightemission in the red/near-infrared light range which could bequenched by oxygen to generate singlet oxygen [8–10]. In addition,the redox photochemistry of the {M6(l2-X)i12}2+ metallic core can beused to form {M6(l2-X)i12}3+/4+ species in an electric field [11]. Thedetailed studies are progressing to determine a new tunable processbetween the electronic, optical, and chemical properties of the clus-ter by using further techniques. The reversion between the opticaland electrochemistry of the [{Mo6Ii8}Cla6]2- cluster studied by chemi-cal and electrochemical methods has been reported [12–14]. Thedeposition of the negative [{M6(l3-Br)i8}Bra6]2- and positive[{M6(l2-X)i12}(H2O)a6]2+ charged cluster on ITO-coated glass was suc-cessfully prepared by electrophoretic deposition and the film-forming mechanism was reasonably proposed [15–16]. It is knownthat the control of the surface charge potential is a fundamental pro-cessing parameter for the EPD process. However, how to control thesurface charge of the cluster has not yet been investigated. In thisstudy, we first investigated the effect of the light absorbing charac-teristic on the charging property by excited electron transfer in the[{Mo6(l3-Br)i8}Bra6]2- cluster and [{Ta6(l2-Br)i12}(H2O)a6]2+ clusterunits. An electro-osmosis analysis was used to measure the zetapotential on the charged surface of the cluster unit in an organic sol-vent during light irradiation. The interesting behavior of the zetapotential versus the photon energy from light irradiation has beendetermined.http://crossmark.crossref.org/dialog/?doi=10.1016/j.mlblux.2021.100079&domain=pdfhttp://creativecommons.org/licenses/by/4.0/http://creativecommons.org/licenses/by/4.0/https://doi.org/10.1016/j.mlblux.2021.100079http://creativecommons.org/licenses/by/4.0/mailto:NGUYEN.Thikimngan@nims.go.jmailto:UCHIKOSHI.Tetsuo@nims.go.jpmailto:UCHIKOSHI.Tetsuo@nims.go.jphttps://doi.org/10.1016/j.mlblux.2021.100079http://www.sciencedirect.com/science/journal/25901508http://www.elsevier.com/locate/mlbluxThi Kim Ngan Nguyen, F. Grasset, Noée Dumait et al. Materials Letters: X 11 (2021) 1000792. ExperimentalThe study was performed using two kinds of MCs: i) a suspen-sion of the Cs2[{Mo6Bri8}Bra6] compound (CMB) prepared in methylethyl ketone (MEK, 99%) at a concentration of 0.75 g/l; and ii) a sus-pension of the Br2[{Ta6Bri12}(H2O)a6] compound (TBH) dispersed inacetone/distilled water (100/4) at a concentration of 1 g/l. The MCswere synthesized as reported in refs. 11 and 17 without purification[11,17]. Acetone or MEK are the best solvents for the EPD process ofthe Mo6 cluster-based compounds as already reported [15], whileacetone/H2O allows the [Ta6Bri12(H2O)a6]2+ species to remain activein the green-colored TBH suspension and slows down the oxidationof TBH [16]. The cluster structure scheme and optical absorbance ofthe MCs in solution are presented in Fig. 1a and b. The green-coloredTBH suspension displayed a strong absorbing peak at 350 nm and aweaker absorbing peak at 425 nm, while the yellow-colored CMBsuspension showed a broad absorbing band from 320 nm to500 nm. These results agreed with previous studies [15–16].The zeta potential (ZP) and conductivity of the MC suspensionswere measured by using a zeta-potential and particle size analyzerequipped with a green laser (ELSZ-2000 series). A green lightsource was created by a high power semiconductor laser thatwas selected with the purpose to reduce the light-absorbing andemitting effects caused by the MC. The cell temperature was notchanged even after light irradiation, because the heat-capacity ofthe solution is very large, and the heat formed by the light absorp-tion is small enough to increase the temperature of the system. TheFig. 1. Optical absorption, MC scheme of A) TBH in acetone/water and B) CMB in MEK. C)value of D) TBH, and E) CMB.2scheme of the quartz cell and the measurement system is pre-sented (Fig. 1c). The absorbance and photoluminescence character-izations were studied.3. Results and discussionIn Fig. 1d and 1e, the ZP values were recorded at 24.9 mV and�34.7 mV for TBH and CMB, respectively. These results agreedwith the assumption that they are positively charged [{Ta6(l2-Br)i12}(H2O)a6]2+ and negatively charged [{Mo6(l3-Br)i8}Bra6]2- clusterunits [7,15–16].CMB in MEK excited at the wavelength of 370, 410, and 440 nmby using the Xenon lamp results in a fluorescence light emissionwith a broad peak at 685 nm (Fig. 2a). Based on this result, all sus-pensions were irradiated for 10 min, then stopped before startingthe ZP measurement. In Fig. 2b, the ZP of CMB without irradiationwas �35.9 ± 0.2 mV that was based on 5 measurements. The elec-tric conductivity was about 0.095 mS/cm which was almostunchanged for the different wavelengths. Considering this result,the property of the CMB suspension (pH, concentration) is rela-tively stabilized under the applied voltage. With the irradiationat 540 nm, the value was unchanged due to the weak absorptionof CMB at this wavelength. Upon irradiation at 440, 410, and370 nm, the tendency of the negative value increase correspondsto the higher optical absorbance of the CMB. The amount of theexcited electrons on the charged particle surface predictablyModel of the quartz cell and ZP measurement equipped with light irradiation. The ZPFig. 2. A) Fluorescence (FL) and fluorescence excitation (FLE) spectra of emission light at 685 nm, B) The dependence of the ZP and electric conductivity of CMB in MEK excitedat different wavelengths, and C) ZP versus the irradiation time at 370 nm. The excitation wavelengths of 320, 370, 410, 440, and 540 nm were denoted as the a, b, c, d, and esymbols, respectively.Thi Kim Ngan Nguyen, F. Grasset, Noée Dumait et al. Materials Letters: X 11 (2021) 100079increases due to the enhancement of the exciting electrons causedby the light absorption in the required range. The negative ZP ofCMB when excited at 320 nm reached a maximum of �51.8 ± 0.3mV. This point, which does not follow the regular tendency dueto the absorption without luminescent emission irradiated atFig. 3. A) ZP versus time excited at 370 nm and B) the dependence of the ZP and electr410 nm of the TBH in acetone/water.3320 nm, was confirmed in Fig. 2a. The total photon energy whichexcites electrons will not be lost by the light emission. ZP versustime was continually performed for 55 min using the same CMBsuspension upon irradiation at 370 nm (Fig. 2c). The values wererecorded when the irradiation was stopped. The first ZP value uponic conductivity on the irradiation, C) the sensitivity of ZP during light irradiation atThi Kim Ngan Nguyen, F. Grasset, Noée Dumait et al. Materials Letters: X 11 (2021) 100079irradiation at 370 nm was about �42 mV and reached a peak of�47 mV after 40 min. The continual irradiation could keep theelectrons in the excited state configuration.The ZPs of TBH were continually recorded at about 24.3 ± 0.3mV for 30 min without changing the electric conductivity of0.029 mS/cm (Fig. 3a). This means that the chemical compositionof TBH in a solvent is relatively stable without exchanging the api-cal ligand under an electric field created by a current of �0.6 mA.The absorption spectrum of TBH coupled with the ZP measuredat the selected wavelength is illustrated in Fig. 3b. The conductivityremained at around 0.029 mS/cm when TBH was irradiated at dif-ferent wavelengths. The average ZP was based on 5 measurements.In the case of TBH, the measurement was performed during theirradiation because TBH has no luminescent emission [11,16].Due to the very low absorption around 540 nm, TBH maintainedthe same ZP as the non-irradiated sample which is reasonable.However, the positive ZP of TBH increases during irradiation at370, 440, and 580 nm, following the extinction coefficient of thecluster. Very interestingly, the ZP is only reduced by about2.7 mV when irradiating at 410 nm. Hence, the ZP was recordedin the same suspension with and without irradiation at 410 nm(Fig. 3c). After two cycles, ZP can repeat the non-irradiated value.This phenomenon was not observed at the other irradiation wave-lengths. Based on this result, the electronic state on the chargedsurface of the [{Ta6(l2-Br)i12}(H2O)a6]2+ cluster during irradiation at410 nm should be further investigated.4. ConclusionsWe report in this study the dependence of the zeta potential asa function of the light excitation wavelength for the [{Mo6Bri8}Bra6]2- anionic cluster unit and the cationic [{Ta6Bri12}(H2O)a6]4+ clus-ter units. With the existence of an electron in a valence band, thecluster can be oxidized during the excitation by light irradiation inthe required range, which was observed by the change in the ZP ofthe charged particle surface in an electric field. Besides this essentialsuggestion, different possibilities relating to the change in theligands of the MCs, geometrical relaxations, and size effect couldbe the origin of the changing zeta potential values. For the strongemitting Cs2{Mo6Bri8}Bra6 cluster, the photo-oxidation property alsodepends on the photoluminescence and the stability of the excitedelectron. Interestingly, for the Br2[{Ta6Bri12}(H2O)a6] octahedral clus-ter, the reversion of the ZP as well as the electronic state on thecharged surface with and without irradiation at 410 nm appears tobe a promising property for sensor applications. These preliminaryresults have highlighted the high potentials to generate an efficientphoto-induced charge transfer based on these low-toxic, cheap,4and stable M6-based functional materials. These results will pavethe way for extending the applicability of such cluster units in thefields of catalysis, optoelectronics and biologicals.Declaration of Competing InterestThe authors declare that they have no known competing finan-cial interests or personal relationships that could have appearedto influence the work reported in this paper.AcknowledgmentThese studies were carried out as a part of the France-JapanInternational Collaboration Framework (IRL3629 LINK). Theauthors wish to thank Mr. D. Lechevalier of Saint-Gobain KK(Tokyo, Japan) for his significant support involved in LINK, andMr. S. Hashida and Mr. K. Tanaka for their significant cooperationin measuring the zeta potential at Otsuka Electronics Co., Ltd.References[1] F. Meinardi, F. Bruni, S. Brovelli, Nat. Rev. Mater. 2 (2017) 1–9.[2] P.S. Kuttipillai, Y. Zhao, C.J. Traverse, R.J. Staples, B.G. Levine, R.R. Lunt, Adv.Mater. 28 (2016) 320–326.[3] P.C. Ford, A. Vogler, Acc. Chem. Res. 26 (1993) 220–226.[4] T.G. Gray, C.M. Rudzinski, E.E. Meyer, R.H. Holm, D.G. Nocera, J. Am. Chem. Soc.125 (2003) 4755–4770.[5] L.F. Szczepura, J.A. Edwards, D.L. Cedeno, J. Clust. Sci. 20 (2009) 105–112.[6] N.G. Naumov, K.A. Brylev, Y.V. Mironov, S. Cordier, V.E. Fedorov, J. Struct.Chem. 55 (2014) 1371–1389.[7] S. Cordier, F. Grasset, R. Boukherroub, N. Saito, H. Haneda, Y. Molard, S.Ravaine, M. Mortier, M. Amela-Cortes, N. Ohashi, J. Inorg, Organomet Polym. 25(2015) 189–204.[8] B. Dierre, K. Costuas, N. Dumait, S. Paofai, M. 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