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Tatiana I. Lappi, Stéphane Cordier, Yakov M. Gayfulin, Soraya Ababou-Girard, [Ngan T. K. Nguyen](https://orcid.org/0000-0001-8935-1306), Fabien Grasset, [Tetsuo Uchikoshi](https://orcid.org/0000-0003-3847-4781), Nikolay G. Naumov, Adèle Renaud
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[Heterometallic {Re4Mo2Qi8} cluster-based building blocks: towards the rational nanoarchitectonics of optimized photoelectrodes for solar cells and water splitting](https://mdr.nims.go.jp/datasets/7d0f2d96-7cdd-4e2c-9c2a-27163f3066f8)
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Heterometallic {Re4Mo2Qi8} cluster-based building blocks: towards the rational nanoarchitectonics of optimized photoelectrodes for solar cells and water splitting6974 | J. Mater. Chem. C, 2024, 12, 6974–6984 This journal is © The Royal Society of Chemistry 2024Cite this: J. Mater. Chem. C,2024, 12, 6974Heterometallic {Re4Mo2Qi8} cluster-basedbuilding blocks: towards the rationalnanoarchitectonics of optimized photoelectrodesfor solar cells and water splitting†Tatiana I. Lappi,ab Stéphane Cordier,*a Yakov M. Gayfulin, b Soraya Ababou-Girard, c Ngan T. K. Nguyen, de Fabien Grasset,ad Tetsuo Uchikoshi, dfNikolay G. Naumov b and Adèle Renaud *aMetal atom clusters are well-defined nanoscale objects containing a precise number of metal atoms andligands. Face-capped cluster units of the type [{M6Li8}L0a6] (M = Mo, Re, L = S, Se or I, L0 = Cl, Br, I, CNor H2O) exhibit unique optical and electronic properties that make them relevant building blocks for therational design of nanomaterials using nanoarchitectonic concepts. Photoelectrodes based on Mo6 andRe6 clusters with various compositions obtained by deposition of uniform layers of those building blocksonto semiconducting surfaces were recently reported. Remarkably, high quality interfaces were formednot only between building blocks but also between the building blocks and the semiconductingsurfaces. On the one hand, layers based on active {Mo6Ii8} cluster cores exhibit an ambipolar behaviorlike carbon nanotube, graphene and transition metal chalcogenides. On the other hand, mixing the twotypes of {Re6Si8} and {Re6Sei8}-based building blocks enables the creation of micro-(p–n) junctions withenhanced photogenerated current intensity. Herein, we report new advances in the design of photo-electrodes using heterometallic Re4Mo2 cluster-based building blocks. The association of Mo and Re in{Re4Mo2Qi8} cluster cores (Q = S and Se) leads to electronic properties and absorption properties signifi-cantly different from those of homometallic {Mo6Ii8} and {Re6Qi8}. Indeed, beyond different molecularorbital diagrams, the {Re4Mo2Qi8} cluster-based units exhibit 22 valence electrons per cluster (VEC)whereas the VEC value for {Mo6Ii8} and {Re6Qi8} cluster units is 24. The mixing of rhenium and molybde-num within the same heterometallic cluster enables not only the optical and transport properties of theactive layers to be optimized but it also enables the position of the energy levels to be tuned. Thisappears very appealing for band alignment engineering in order to design optimized photoelectrodes forsolar energy conversion. We show herein that the energy levels of the photoelectrodes built on{Re4Mo2Q8} cluster-based layers immobilized on FTO surfaces are compatible with the photoelectro-chemical water splitting.1. IntroductionTransition metal atom clusters are well-defined nanoscaleobjects containing a very precise low number of metal atomsand ligands.1 Soluble metal-atom cluster-based compoundswith the general formula Ax[{M6Li8}L0a6] (A = alkali, M = Mo,Re; L = I, S, Se; L0 = Cl, I, CN) are prepared by solid-statechemistry. Their syntheses and the solubilization processes arefully detailed in the literature.2–7 In recent years, the nanoarch-itectonic concept was applied to the design of photoelectrodesbuilt up from [{M6Li8}L0a6] face-capped octahedral cluster nano-building blocks (M = Mo, Re; L = I, S, Se; L0 = Cl, I).3,5,6According to Ariga,8 ‘‘the concept of nanoarchitectonics issupposed to involve the architecting of functional materialsa Univ Rennes, CNRS, ISCR-UMR, 6226, F-35000 Rennes, France.E-mail: Stephane.cordier@univ-rennes.fr, adele.renaud@univ-rennes.frb Nikolaev Institute of Inorganic Chemistry, Siberian Branch of Russian Academy ofSciences, 3 Acad. Lavrentiev pr., 630090 Novosibirsk, Russiac Univ Rennes, CNRS, IPR (Institut de Physique de Rennes) – UMR 6251, F-35000,Rennes, Franced CNRS-Saint-Gobain-NIMS, IRL 3629, Laboratory for Innovative Key Materials andStructures (LINK), National Institute for Materials Science, 305-0044 Tsukuba,Japane International Center for Young Scientists, Global Networking Division, NationalInstitute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japanf Research Center for Electronic and Optical Materials, National Institute forMaterials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3tc04635dReceived 15th December 2023,Accepted 16th April 2024DOI: 10.1039/d3tc04635drsc.li/materials-cJournal ofMaterials Chemistry CPAPEROpen Access Article. Published on 18 April 2024. Downloaded on 5/20/2024 5:51:20 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article OnlineView Journal | View Issuehttps://orcid.org/0000-0002-6378-0409https://orcid.org/0000-0001-6381-7710https://orcid.org/0000-0001-8935-1306https://orcid.org/0000-0003-3847-4781https://orcid.org/0000-0002-7531-6291https://orcid.org/0000-0002-7427-0148http://crossmark.crossref.org/dialog/?doi=10.1039/d3tc04635d&domain=pdf&date_stamp=2024-04-30https://doi.org/10.1039/d3tc04635dhttps://doi.org/10.1039/d3tc04635dhttps://rsc.li/materials-chttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d3tc04635dhttps://pubs.rsc.org/en/journals/journal/TChttps://pubs.rsc.org/en/journals/journal/TC?issueid=TC012019This journal is © The Royal Society of Chemistry 2024 J. Mater. Chem. C, 2024, 12, 6974–6984 | 6975using nanoscale units based on the principles of nanotechnol-ogy’’. Face-capped [{M6Li8}L0a6] (M = Mo, Re, L = S, Se or I, L0 =Cl, Br, I, CN or H2O) building blocks exhibit several singula-rities compared to other nanosized objects: (i) they are based onan atom-precise composition, (ii) they do not contain toxicmetallic elements as it is the case in the well-known CdQ orPbQ QDs (Q = S or Se), (iii) they are not submitted to surfacedefects (i.e. loss of metal atoms or ligands contrary to semi-conducting oxide nanoparticles for which a passivation pheno-menon in air results in a reconstructed oxygen-terminatedsurface for instance9) that could alter their properties, (iv) theirproperties can be efficiently tuned by chemical engineering bythe judicious choice of the metal, capping ligands and terminalligands, (v) the high solubilization of cluster-based solid-stateprecursors and the immobilization process onto semiconduct-ing surfaces do not significantly modify the chemical formulaof the building blocks that bears comparable properties tothose found in bulk compounds. Those singularities constitutesignificant fundamental differences with nanoparticles of semi-conducting sulfides, selenides or oxides.10,11 Indeed, for thelatter, a very fine control of the synthesis pathways to controlthe size of the nanoparticles is mandatory to obtain reprodu-cible chemical and physical properties of nanoparticles with aminimum of surface defects.11 Because such problems are notfaced with [{M6Li8}L0a6], they appear as suitable building blocksto design photoelectrodes with high quality interfaces, not onlybetween building blocks but also between the building blocksand the semiconducting surfaces.The properties of bulk compounds and those of layers onceimmobilized onto surfaces strongly depend on the chemicalcomposition of the [{M6Li8}L0a6] building blocks. For instance,on the one hand, molybdenum cluster iodides based on the{Mo6Ii8} cluster core belong to the family of ambipolar materialslike carbon nanotube, graphene and transition metal chalco-genides.6 In ambipolar materials, once hole/electron pairs arecreated, the holes and electrons are transported simultaneouslyin opposite directions with comparable kinetics.12,13 Their life-times exhibit similar values. Thus, the charge transport is notlimited by a minority charge carrier type as it is the case inunipolar semiconductors. The charge transport in p- or n-typesemiconductors is restrained by electrons and holes, respectively.Moreover, the molecular electronic structure of the cluster’sbuilding blocks is not significantly disturbed by solid-state stack-ing effects or once deposited on surfaces. Electronic structures ofmolecular cluster-based compounds and layers are thus related tothat of intrinsic inorganic semiconductors. On the other hand, thenature of Q (S or Se) drives the photoelectrochemical properties ofphotoelectrodes based on {Re6Qi8} cluster-cores.3 The {Re6Si8}sulfide-based photoelectrodes exhibit an ambipolar behaviourwith a slight domination of electron on the charge transport,whereas the charge transport in {Re6Sei8} selenide-based photo-electrodes is clearly dominated by the hole transport. Last but notleast, photoelectrodes exhibiting tuned electronic properties wereobtained by designing active cluster-based layers wherein the ratiobetween the two {Re6Si8} and {Re6Sei8}-based building blocks wasfinely controlled. The electric field created by the micro-(p–n)junctions formed by the interactions between the {Re6Si8}- and{Re6Sei8}-based clusters was assumed to limit the recombination ofcharge carriers and consequently this leads to improvement in thephotocurrent generated by such photoelectrodes. A ratio of 1 : 1 for{Re6Si8} : {Re6Sei8} maximizes the number of local heterojunctionswithin the layers and allows optimization of the photophysicalproperties.3Herein, we report new advances in the design of photoelec-trodes based on octahedral M6 clusters. Following the investi-gations on Mo6 cluster iodides and Re6 cluster chalcogenides,we explored the photoconductive properties of heterometalliccluster building blocks based on the well-defined {Re4Mo2Qi8}cluster core (Q = S or Se). The mixing of Mo and Re enables us toobtain {Re4Mo2Qi8} cluster-core based building blocks withelectronic properties and absorption properties different fromthose of the homometallic {Mo6Ii8} and {Re6Qi8} (S and Se).Indeed, beyond different molecular orbital diagrams, the clus-ter units based on {Re4Mo2Qi8} exhibit 22 valence electrons percluster whereas those based on {Mo6Ii8} and {Re6Qi8} (S and Se)exhibit 24 valence electrons per cluster. After solid-state synth-eses and excision in solution of the cluster building blocks, theywere precipitated with tetrabutylammonium cations to formthe ((C4H9)4N)4[{Re4Mo2Si8}(CN)a6] and ((C4H9)4N)4[{Re4Mo2-Sei8}(CN)a6] starting precursors. After solubilization of the latterin acetonitrile, the cluster building blocks were deposited ontoFTO surfaces using an electrophoretic deposition (EPD)process.3,5–7,14 The energy diagrams of the photoelectrodesmade of {Re4Mo2Qi8} cluster-core based building blocks depos-ited on FTO are different and lead to different physical andphotochemical properties than those made of homometallic{Mo6Ii8} and {Re6Qi8}-based clusters (S and Se) deposited ontosurfaces. It was shown that after deposition, the active layersexhibit the following formula: (Bu4N)2(H3O)2[{Re4Mo2Qi8}-(CN)a6]�xH2O (Q = S, Se)-based films (Bu4N = (n-C4H9)4N). Theirphysical properties were then investigated through (photo)-electrochemical characterizations. An ambipolar behavior wasevidenced for both S and Se chalcogenides. However, in accor-dance with the previous results obtained on Re6-based films,3the charge transfer is dominated by electrons in the sulfidelayer and, reversely, by holes in the selenide one. Moreover,thanks to the mixing of Re and Mo, the positions of energylevels of {Re4Mo2Q8}-based layers are compatible with waterphoto-oxidation and photo-reduction.2. Experimental part2.1. Materials and methodsAll the reagents were purchased from commercial sources andused without further purification.2.2. Synthesis of precursors (Bu4N)4[{Re4Mo2Si8}(CN)a6] (10)and (Bu4N)4[{Re4Mo2Sei8}(CN)a6] (20)1 and 2 were obtained according to a previous well-knowndescribed method.15,16 Briefly, 1 g of K5[Re3.2Mo2.8S8(CN)6] (1)or K5[Re3.6Mo2.4Se8(CN)6] (2) was dissolved in 20 mL of waterPaper Journal of Materials Chemistry COpen Access Article. Published on 18 April 2024. Downloaded on 5/20/2024 5:51:20 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d3tc04635d6976 | J. Mater. Chem. C, 2024, 12, 6974–6984 This journal is © The Royal Society of Chemistry 2024and a solution of Bu4NCl (1 g, 3.6 mmol) in 20 mL of H2O waspoured into the cluster solution. The precipitation of 2 (Q = Se)happens faster than 1, just after one hour of stirring. On theother hand, for the formation of 1 (Q = S), it was necessary toincrease the pH until the value of 11 by adding dropwise anaqueous KOH solution (0.1 M). The reaction mixture was slowlyevaporated at room temperature for one week until the greenprecipitate of 10 appeared. The precipitates 10 and 20 werefinally filtrated, washed with water and air-dried.2.3. Preparation of the cluster-based electrodes {Re4Mo2Qi8}(Q = S, Se)2.3.1. {Re4Mo2Qi8}-based electrodes (Q = S or Se) beforeannealing. Cluster-based films were prepared using the electro-phoretic deposition (EPD) method already used for the deposi-tion of Mo6 and Re6 clusters-based compounds (Fig. 1).3,5,6,14Tetrabutylammonium salts of cluster complexes 10 and 20 wereweighted and dissolved in acetonitrile to obtain a concen-tration of 0.01 M. The solutions of 10 and 20 were used assources of {Re4Mo2Qi8} (Q = S or Se) cluster core for the EPDprocess as described in the following. Fluorine tin oxide (FTO,TEC7 NGS Pilkington) was chosen as the transparent conduc-tive substrate. An FTO slide was cut in the rectangular format1.5 � 1 cm2 and cleaned according to a 3-step washing processin an ultra-sonication bath: in (1) soapy water, (2) acidifieddeionized water (1 v% HCl concentrated grade) and (3) ethanol.Stainless steel was used as the cathode and FTO substratesas the anode were placed face-to-face at a distance of 1 cm,held together by carbon tape and connected to a KeithleyModel 2450 Series SourceMeter. The two-electrode setup wasimmersed into the cluster-based solution and the depositionoccurred for an applied voltage ranging from 30 to 40 V for 30 s.The resulting cluster-based films were then removed from thesolution and fully air-dried at room temperature.2.3.2. Annealed {Re4Mo2Qi8}-based electrodes (Q = S or Se).Immediately after the electrophoretic deposition, the{Re4Mo2Qi8}-based electrodes (Q = S, Se) prepared as describedin the above section were annealed at 200 1C for 1 h. To test theeffect of a PMMA protection layer, a coating was deposited onthe surfaces using a solution of PMMA (M = 350 000 g mol�1) inacetonitrile (5 g L�1). The protection coating was appliedimmediately after annealing by dipping and air-drying.2.4. Characterization techniquesThe scanning microscopy images and energy dispersivespectroscopy (EDS) analyses were performed on a JEOL JSM7100 F microscope operating at 10 kV with an energy dispersivemicroscope.The powder X-ray diffraction patterns were collected at roomtemperature in a 2y angle range of 51 to 801 with a scan speed of41 min�1 with a Bruker D8 ADVANCE two-circle diffractometer(y–2y Bragg–Brentano mode) using Cu Ka radiation (l = 1.54056 Å)equipped with a Ge(111) monochromator and Lynx Eye detector.The UV-vis transmission spectra were recorded in a wave-length range of 200 to 2500 nm on a V-770 JASCO spectro-photometer. Powder reflectance spectra were obtained on thesame spectrophotometer using an integrated reflectance sphereaccessory. The optical band gaps (Eg) were determined via theabsorption/back scattering ratio (K/S) calculated from the rawreflectance data using the F(R) = K/S = (1 � R)2/(2R) Kubelka�Munk transform.X-ray photoelectron spectroscopy (XPS) measurements wereperformed with an Mg Ka (hn) 1254 eV X-ray source using aVSW HA100 photoelectron spectrometer with a hemisphericalphotoelectron analyzer, working at an energy pass of 20 eV forthe survey and resolved spectra. The experimental resolutionwas 1.0 eV. Binding energies were referenced to the C 1s signalfixed at 285.0 eV. Data were treated after a Shirley backgroundsubtraction using a mixed Gaussian–Lorentzian product.The Raman scattering spectra were recorded using a Lab-Ram High Resolution spectrometer coupled with a confocalmicroscope (Horiba Jobin Yvon), 600 g mm�1 and 100� objective.A 785 nm laser diode was used for scattering excitation. Ramanspectra were recorded at room temperature with 500 s expositionand two accumulations for powder. Spectra were recorded atdifferent points of accumulation in order to check the homogene-ity of the powder and the films.The (photo)electrochemical measurements (current–voltagecurves, chronoamperometry and electrochemical impedancespectroscopy) were performed using an Autolab PGSTAT204(Metrohm AG) equipped with a FRA32M electrochemical impe-dance spectroscopy module with an illumination source MI-LED illuminator (Edmund Optics) and a standardized AM1.5(1000 W m�2) sunlight simulator (MinisolTM LED LSH-7320,MKS Newport). For all measurements, a three-electrode electro-chemical cell with Pt auxiliary and Ag/AgCl reference electrodeswas used. Various electrolytes were used depending on the typeof electrodes and electrochemical measurements. Therefore, anacetonitrile-based electrolyte using an iodide/triiodide redoxmediator prepared from KI (6 mmol L�1) and I2 (0.3 mmol L�1)was used for the unheated electrodes characterization. The I3�/I� redox couple (0.54 V vs. ENH) was used to observe photo-redox activity of the photoelectrodes, namely, photo-reduction(p-type) or photo-oxidation (n-type) according to the type ofsemiconductor. The annealed films were measured in a water-based solution using KCl (0.5 M) as the supporting electrolyte.Fig. 1 Schematic representation of the electrophoretic deposition pro-cess of {Re4Mo2Qi8} (Q = S or Se) cluster core-based films from solutionsof (Bu4N)4[{Re4Mo2Qi8}(CN)a6] (Q = S, Se).Journal of Materials Chemistry C PaperOpen Access Article. Published on 18 April 2024. Downloaded on 5/20/2024 5:51:20 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d3tc04635dThis journal is © The Royal Society of Chemistry 2024 J. Mater. Chem. C, 2024, 12, 6974–6984 | 6977In aqueous solutions, the use of a supporting electrolyte with aredox couple is not needed because the oxidation and reductionof water can occur in the studied potential window. Thephotoredox activity on water of the annealed electrodes wastested in a 0.5 M phosphate buffer aqueous solution (pH 7).Linear sweep voltammetry was recorded at a scan rate of10 mV s�1 and chronoamperometries were recorded at 0.2 Vand 1.4 V vs. RHE for the investigation of reduction andoxidation processes, respectively. The charge carrier lifetimeswere determined by fitting the OCP decay curves with exponen-tial decays (first or second order).The impedance spectra and the Mott–Schottky (MS) analyseswere obtained in a frequency range from 100 Hz to 100 kHz.The interface semiconductor/electrolyte capacitance (C) wasdetermined using a simplified Randles equivalent circuit byneglecting the Warburg component at high frequency. The Cvalues for each applied potential were calculated from theconstant phase element (CPE, ZCPE = 1/Q(jo)a, 0 o a o 1),reflecting the non-ideality of the interface capacitance usingC�2 = (1/Rs + 1/Rtc)(a�1)/aQ1/a. Flat band potentials (Vfb) weredetermined by extrapolation at C�2 = 0 from the Mott–Schottkyplot (CSC�2 vs. V), which utilizes the Mott–Schottky equation(eqn (1)).1CSC2¼ � 2ee0eA2NV � Vfb �kTe� �(1)CSC corresponds to the capacitance in the space charge regionof the semiconductor, A is the interfacial surface area betweenthe semiconductor electrode and the electrolyte, k is theBoltzmann constant, T is the temperature, e is the electroncharge, e0 is the vacuum permittivity, and e is the relativepermittivity of the semiconductor. CSC�2 was approximated toC�2 due to the large capacitance of the Helmholtz layer, at thesemiconductor surface in the electrolyte, in comparison to CSC.A positive slope is obtained for an n-type semiconductor anda negative slope for a p-type semiconductor. Fermi levels wereapproximated [Efb (eV) = �4.5 � e�Vfb (reversible hydrogenelectrode, RHE)]. In water solvent, the potential versus RHEwas obtained by taking into account the pH dependency,Vfb(RHE) = Vfb(Ag/AgCl) + VAg/AgCl(RHE) + 0.059pH withVAg/AgCl(RHE) = 0.199 V vs. RHE.17 In acetonitrile, the pH wasestimated using the proton activity.183. Results and discussion3.1. Preparation of photoelectrodesWithin the [{Re4Mo2Qi8}(CN)a6]4� cluster unit, the hetero-metallic Re4Mo2 octahedral cluster is bounded to eight face-capping chalcogen ligands (Qi) to form the {Re4Mo2Qi8} clustercore. The latter is bonded through the carbon atoms to sixterminal cyanide groups, denoted (CN)a. (Bu4N)4[{Re4Mo2-Qi8}(CN)a6] are prepared following a multistep procedure invol-ving both solid-state chemistry and solution chemistry. Briefly,the crude heterometallic K6[{Re6�xMoxQi8}(CN)a5] (Q = S, Se)cluster compound is obtained by high-temperature reactionbetween rhenium and molybdenum sulfides (or selenides) inthe presence of potassium cyanide.14,15 K6[{Re6�xMox-Qi8}(CN)a5] is a solid solution and its crystal structure is basedon [{Re6�xMoxQ8}(CN)a4(CN)a–a2/2] polymeric chains of cluster-cores that exhibit different Re : Mo ratios: {Re4Mo2Qi8},{Re3Mo3Qi8} and {Re2Mo4Qi8}. Recently, methods for separatingthe {Re6�nMonQi8} cores with integer values of n have beendeveloped. Among them, {Re2Mo4Q8} cluster core-based com-pounds could be isolated as salts of tetrabutylammoniumcyanides.15 Briefly, the polymer K6[{Re6�xMoxQi8}(CN)a5] is dis-solved in water in the presence of KCN, and further evaporationof the solution leads to the crystallization of the discrete saltK5[Re6�xMoxQi8(CN)a6] (Q = S, Se). Dissolution of discrete saltsin water and subsequent addition of an excess of Bu4NCl leadsto the one-electron oxidation of K5[{Re4Mo2Qi8}(CN)a6] byair oxygen to form the corresponding salt (Bu4N)4[{Re4Mo2-Qi8}(CN)6], while the salts containing {Re3Mo3Qi8} and{Re2Mo4Qi8} cluster cores remain in solution. The colors ofthe (Bu4N)4[{Re4Mo2Qi8}(CN)6] powders are green and purplefor Q = S and Se, respectively. The absorption spectra arecharacterized by a shift at lower energies of the absorptiononset of the selenide compared to the sulfide. This corre-sponds, as expected, to an increase in the bandgap by increas-ing the size and decreasing the electronegativity of thechalcogen atom. Thus, the optical bandgaps determined usingthe Kubelka-Munk transforms from the diffuse reflectance dataare 2.31 eV and 2.05 eV for Q = S and Se respectively (Fig. 2a).Using a similar procedure developed and reported for Mo6and Re6 homometallic cluster compounds,3,5,6 heterometallic{Re4Mo2Qi8} (Q = S, Se) cluster core-based units were depositedusing the EPD method from solutions of 10 or 20 in acetonitrile(0.01 M) (Fig. 1) by applying a voltage from 30 to 40 V for 30 s.For both compounds, the depositions are obtained on theFTO-based anode side due to the negative charge of the[{Re4Mo2Qi8}(CN)a6]4� cluster unit. After optimization ofthe protocol, all the characterizations and photoelectrochem-ical investigations were done on films deposited under anapplied voltage of 40 V during 30 s.3.2. Optical and chemical characterization of thephotoelectrodesAs represented in Fig. 2, the shapes of the absorption spectra ofthe coatings are less structured than those of the startingpowders. This finding is emphasized for the selenide com-pound. The evolution of the shape of the spectra is accompa-nied by a decrease in the band gaps, from 2.31 eV to 2.03 eV andfrom 2.05 eV to 1.85 eV for the sulfide and the selenide-basedfilms, respectively. As for powders, the bandgaps of selenidecoatings are smaller than those of sulfides. As a consequence,the colors of the coatings appear noticeably darker than thoseof the starting powders (Fig. 2 and Fig. S1, ESI†). As discussedbelow, the evolution of the absorption properties between thedeposited films and those of the precursor powders is aconsequence of possible changes in composition and/orchanges in the strength of interactions between the clustercore-based units before deposition within 10 and 20 and afterdeposition within the deposited layers. For example, the largerPaper Journal of Materials Chemistry COpen Access Article. Published on 18 April 2024. Downloaded on 5/20/2024 5:51:20 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d3tc04635d6978 | J. Mater. Chem. C, 2024, 12, 6974–6984 This journal is © The Royal Society of Chemistry 2024absorption bands after deposition translate to a modification ofphysical properties from molecular to solid-state. In otherwords, the electronic structure initially built from molecularorbitals in 10 and 20 tends to form narrow bands with moredelocalized states because of stronger electronic interactionsbetween building blocks once they are immobilized onto thesurface. Beyond the effect of the deposition, the replacement of2 Re by 2 Mo in the cluster M6 seems to slightly influence theabsorption properties of the films. Indeed, the values of opticalbandgaps of the heterometallic clusters are slightly lower thanthose previously reported on the homometallic Re6 chalcogen-ides (2.38 eV and 2.16 eV for {Re6Si8} and {Re6Sei8} cluster core-based films, respectively)3 and are higher than that of the Mo6cluster iodide (around 1.9 eV).6This can be attributed to the evolution of energy diagramsinduced notably by the mixing of two different metals withdifferent electronegativities and the decrease of the VEC valuefrom 24 to 22 and to a less extent a loss of symmetry comparedto the homometallic Re6.Fig. 3 depicts the evolution of the coloration, the morpho-logy and the thickness of the sulfide-based films according tothe applied voltage. From 30 V to 40 V, the thickness of thefilms increases from 3 to 5 mm and consequently they lookdarker. Surprisingly, looking at the SEM images, the crystal-linity of the layers seems also affected by the increase of thevoltage. These observations were then confirmed by the X-raydiffraction analyses. Looking at the X-ray diffraction powderpatterns gathered in Fig. 4a and b, it turns out that the X-raypatterns of powders are different from those of deposited films.Moreover, the diffraction peaks at low angles (i.e. around 9.4,9.9, 11.5 and 12.51) grow when the applied voltage increases(Fig. S2, ESI†). The deposited surfaces obtained for a voltage of40 V appear as uniform layers made of 1 mm-size like-octahedralcrystals (Fig. 3 and 5a). As far as we know, the growth of Mo6and Re6-based crystals by EPD without a sintering step wasnever reported. Conversely, it has been previously observed thatthe deposition of Mo6 and Re6 clusters leads to amorphousfilms.3,5,6 The same conclusions can be drawn concerningselenide-based layers. The thickness and the intensity of thecolor rise by increasing the applied voltage (Fig. S1, ESI†).Crystallized dark brown films until 7 microns thick areobtained for an applied voltage of 40 V for 30 s (Fig. S1, ESI†and Fig. 5b). The microstructure is however finer. Submicroniccrystalline phases as in the case of sulfides are not observed.Spherical nanoparticles of about 100 nm in diameter areformed. The diffraction peaks are thus broader than thoseFig. 2 Kubelka Munk transforms of (a) (Bu4N)4[{Re4Mo2Q8}(CN)6] (Q = S,Se) precursor powders and (b) sulfide- and (c) selenide-based filmsdeposited at 40 V for 30 s. For each type of film, the three spectracorrespond to the spectra recorded on (i) unheated film, (ii) annealed(T = 200 1C) film and (ii) annealed (T = 200 1C) film covered by PMMA.Fig. 3 Photographs and SEM images of the top views and the cross sections of the {Re4Mo2Si8} cluster-based films deposited at various voltages (from30 to 40 V) for 30 s.Journal of Materials Chemistry C PaperOpen Access Article. Published on 18 April 2024. Downloaded on 5/20/2024 5:51:20 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d3tc04635dThis journal is © The Royal Society of Chemistry 2024 J. Mater. Chem. C, 2024, 12, 6974–6984 | 6979observed for the sulfide-based layers (Fig. S2, ESI† and Fig. 4b).However, their positions are close and their relative intensitiesare similar which suggests closely related crystal packing ofthe cluster units within the layers whatever the nature of thechalcogen. The X-ray diffraction patterns of layers could not beindexed using crystal structures referenced in the databases.The vibration spectra and the composition of depositedfilms were investigated using Raman spectroscopy, EDS andXPS analyses. According to all these complementary techni-ques, the {Re4Mo2Qi8} cluster cores are preserved during thedeposition process. The Raman spectra of the starting powders10 and 20 are globally similar and comparable to those reportedin the literature.19 The bands are in the same position (Fig. 4cand d, bands for the sulfides: around 224 and 332 cm�1 (A1gM6), around 410 cm�1 (A1g M–C) and 431 cm�1 (A1g S8) andbands for the selenides: around 237 cm�1 (A1g M6), aroundFig. 4 XRD diagrams of (a) powder ((C4H9)4N)4[{Re4Mo2Si8}(CN)a6] (10) and corresponding deposited {Re4Mo2Si8} cluster-based films obtained at 40 Vfor 30 s before and after annealing (T = 200 1C), and (b) powder ((C4H9)4N)4[{Re4Mo2Sei8}(CN)a6] (20) and corresponding deposited {Re4Mo2Sei8} cluster-based films obtained at 40 V for 30 s before and after annealing. Raman spectra of (c) powder 10 and the corresponding deposited films before and afterannealing, and (d) powder 2 0 and the corresponding deposited films before and after annealing. The stars and triangles correspond to the FTO substrateand cluster-based film peaks, respectively.Fig. 5 Photographs and SEM images of the top views and the cross sections of (a) unheated and (c) annealed at 200 1C for 1 h {Re4Mo2S8} cluster-basedfilms and (b) unheated and (d) annealed at 200 1C for 1 h {Re4Mo2Se8} cluster-based films. All were deposited at 40 V for 30 s.Paper Journal of Materials Chemistry COpen Access Article. Published on 18 April 2024. Downloaded on 5/20/2024 5:51:20 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d3tc04635d6980 | J. Mater. Chem. C, 2024, 12, 6974–6984 This journal is © The Royal Society of Chemistry 2024253 cm�1 (A1g M–C) and 285 cm�1 (A1g Se8)). The Ramanspectra of the films are similar to those of 10 and 20 but somepeaks are however broader. This broadening of Raman peaksfor the films can be explained by (i) the loss of organization ofthe cluster units within the layers compared to the morecrystallized precursor powder or/and (ii) the loss of symmetryof the cluster units due to the partial substitution of terminalligands as discussed below. Moreover, one can note that theRaman peaks are broader for the selenide layer, which is lesscrystallized than the sulfured phase. Comparing the selenideand sulfide films, the XRD peaks are indeed broader too andthe particle size is smaller. Although the determination of thechemical composition of Mo, Re and Q by EDS is complex dueto the spectra overlap between the Mo L and S K rays, the Re/Moand (Re + Mo)/Q ratios estimated using this technique do notsignificantly change for both cluster complexes in 10 and 20 andin the films after the deposition (Table S1, ESI†). This findingtends to confirm the conservation of the chemical compositionof the cluster cores within the layers. The Re/Mo ratios are1.6(2) and 1.7(2) respectively for the sulfides and the selenides(theoretical ratio for {Re4Mo2Qi8} = 2). The XPS results areconsistent with the Raman and EDS analyses. The peakscorresponding to the Mo and Re contributions (Fig. S3 andS4, ESI†) are similar in positions and relative intensities beforeand after the deposition for each cluster chalcogenide. No shiftdue to a possible oxidation process is observed and the metalratio Re/Mo is preserved (Fig. S3 for the sulfide and Fig. S4 forthe selenide, ESI†). The ratios remain relatively constant andclose to the theoretical ones of 2 for each chalcogenide (i.e.around 1.9 and 2.3 for the sulfide-based powder and film,respectively, and around 1.6 to 2.3 for the selenide-basedpowder and film). This confirms that the chemical compositionand the oxidation states of the {Re4Mo2Qi8} cluster cores are notmodified during the deposition process. Conversely, thedecrease and the shift at lower energy of the peaks corres-ponding to the carbon contribution C 1s (Fig. S3a and S4a,ESI†) and to the nitrogen contribution N 1s (Fig. S3c and S4c,ESI†) in the spectra of the films compared to those of powders10 and 20 indicate the loss of some (C4H9)4N+ counter-cationsand/or the apical CN� ligands. If it is difficult for the C 1s peakto be deconvoluted/de-correlated and assigned, two N 1s con-tributions are observed around 398 and 403 eV. Each contribu-tion found in the XPS spectra of the powders 10 and 20 can beattributed, on the one hand, to the nitrogen of CN ligands and,on the other hand, to the nitrogen of the tetrabutylammoniumcations. Because of the lower content of tetrabutylammonium(i.e. 4 per unit) compared to the cyanide ligands (i.e. 6 per unit)in the powders 10 and 20, the lowest-intensity peak at higherenergy (403 eV) can be assigned to the tetrabutylammoniumcounter-cations and the peak located at lower energy to thecyanide ligands. Comparing the XPS spectra of the powders andthose of the films, the concomitant halving of the C 1s and N 1scontributions and the shift of the peak located at 403 eVsuggest a loss of two counter-cations. This loss is favored bythe deposition process itself. Indeed, due to their oppositecharges, the counter-cations and the cluster-based anionsmigrate in solution in opposite directions. However, the lossof two counter cations leaves open the questions of their chargecompensation. In the previous studies performed on Mo6halides and Re6 chalcohalides,3,5,6 the loss of organic cationswas compensated by the substitution of apical halogen ligands(i.e., iodine for Mo6 and chlorine for Re6) by water molecules.This substitution induced a decrease in the charge of thecluster unit without changing its oxidation state. In the presentstudy, the compensation of the loss of tetrabutylammoniumcation involves another charge compensation mechanism sincethe N 1s contribution at 398 eV is very similar in position and inintensity in the spectra of powders and in those of layers. Themost plausible scenario is the replacement of two tetrabuty-lammonium cations by two H3O+. In terms of ligand fieldtheory, the substitution of halogen apical ligands by watermolecules observed for [{Mo6Ii8}Ia6]2� and [{Re6Qi8}Cla6] inour previous studies can be explained because the H2O ligandis a stronger ligand field than I� and Cl�. This is not the case inthe present study. The CN� groups located in apical positionsare stronger ligand fields than water molecules. We assumethus that the loss of two tetrabutylammonium counter-cationsamong the four ones that counterbalanced the anionic chargeof the [{Re4Mo2Qi8}(CN)a6]4� cluster units in 10 and 20 iscompensated by their replacement by oxonium groups. Aspreviously reported, oxonium groups are generated duringthe EPD process.3,5,6,14 Consequently, the proposed chemicalcomposition of the {Re4Mo6Qi8} cluster-core-based films is(Bu4N)2(H3O)2[{Re4Mo2Q8}(CN)6]�xH2O (Q = S, Se).3.3. Electronic and photoconductivity propertiesThe electronic and photoconductivity properties of the{Re4Mo6Qi8} cluster-core-based films were then investigatedby photoelectrochemical characterizations. Due to the instabil-ity of the layers in aqueous media, the first photoelectro-chemical characterizations were carried out in acetonitrile.Therefore, a solution of KI (6 mmol L�1) and I2 (0.3 mmolL�1) in acetonitrile was used as electrolyte and a classical three-electrode setup, composed of a counter-electrode in platinum, areference in Ag/AgCl and a cluster-based layer depositedon FTO as the working electrode. Even when the redox coupleI�/I3� was inserted in the electrolyte as a redox mediator tofavor the photoredox process, the low conductivity of the filmslimited the intensity of the generated photocurrent. However,although low, the detected photo-responses were sufficient toevaluate the electronic and photoconductivity behaviors ofthe cluster-based films. The current–potential curves underchopped illumination (Fig. 6a) underlined both photo-oxidationand photo-reduction processes on either side of a photocurrentreversal potential close to the open circuit potential (OCP), namelyat 0.1 V and 0.06 V vs. Ag/AgCl for the sulfide and the selenide,respectively. Both photocurrent values are in the same order ofmagnitude that tends to evidence ambipolar behavior for bothfilms. This is particularly true for the sulfide-based photoelectrode.The cathodic photocurrent is indeed slightly higher than theanodic one for the selenide. Ambipolar behaviors were alreadyobserved on Mo6 and Re6 cluster-based films and were explainedJournal of Materials Chemistry C PaperOpen Access Article. Published on 18 April 2024. Downloaded on 5/20/2024 5:51:20 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d3tc04635dThis journal is © The Royal Society of Chemistry 2024 J. Mater. Chem. C, 2024, 12, 6974–6984 | 6981by the location of the Fermi level close to the middle of the bandgap. Similarly, as previously described, the energy level diagram(Fig. 6c) of each chalcogenide layer was established by couplingelectrochemical impedance spectroscopy (EIS, see the Mott–Schottky plots in Fig. 6b), UV-vis-NIR (Fig. 2b and c) and XPSmeasurements (Fig. S5, ESI†).3,16 Thus, the Fermi level wasestimated on an absolute energy scale with respect to the vacuumlevel from the flat band potential value determined by EIS fromMott–Schottky plots (Fig. 6b). The valence band maximum (VBM)was positioned from the difference in energy (DE = EF � EVBM)between the VBM and the Fermi level obtained by XPS and theconduction band minimum (CBM) was determined using theband gap value (Eg = ECBM � EVBM). The origin of the ambipolarproperties of the sulfide film was clearly evidenced by the relativelocation of the Fermi level compared to those of VBM and CBM.It pins up to the middle of the band gap. However, the positiveslope in the Mott–Schottky (MS) diagram in Fig. 6b (green) under-lines the domination of the charge transfer by electrons that isconsistent with the OCP evolution under illumination.3,6,20Indeed, a decrease of the OCP under illumination (Fig. S6a,ESI†) suggests that the minority charge carriers are holes.Conversely, the photoconductivity of the selenide-based layerappears rather dominated by hole transfer. Its Fermi level isclose to the VBM level and the negative slope in the MS plotindicates its p-typeness.3,6,20 The energy diagrams deducedfrom our measurements are thus consistent with the evolutionof the photocurrents versus the potential represented in Fig. 6a(i.e. similar photocurrents for the ambipolar sulfide and highercathodic photocurrent for the selenide). Those results arecomparable to those of {Re6Qi8}-based layers.3 We highlightedan ambipolar behavior with a charge transfer dominated byelectron transfer for the Re6 sulfide and a unipolar p-typebehavior for the Re6 selenide. In the heterometallic case, theunipolarity of the selenide seems to be more questionable.Indeed, a photo-oxidation current in the same range of magni-tude as the photo-reduction current is recorded for potentialshigher than the photocurrent reversal potential of 0.06 V vs. Ag/AgCl. Moreover, the OCP decay underlined an n-type behavior(a decrease of the OCP under illumination is observed inFig. S6b, ESI†) and two slopes can be depicted in the Mott–Schottky plot (Fig. 6b, purple). The presence of surface states ordefects in such water molecules can lead to these two differentbehaviors. Indeed, the surface states control the OCP value thatdetermines the surface charge and thus the depletion oraccumulation condition. In the accumulation condition, thereverse band bending leads to a reverse evolution of the OCPunder irradiation. For the selenide, the OCP value around0.08 V vs. Ag/AgCl is just above the reverse potential of 0.06 Vvs. Ag/AgCl. For a unipolar compound, no significant photo-current of majority charge carrier is observed after band bend-ing inversion. As the Fermi level is near to one of the bands(conduction for an n-type and valence for a p-type), the bandbending inversion causes a strong reduction (n-type) or oxida-tion (p-type) current because the Fermi level enters the valenceband. The electrodes become thus conductive. Finally, theelectronic properties of the Re4Mo2 chalcogenides are globallycomparable to those of their Re6 chalcogenide homologues(i.e. similarities in terms of charge carrier domination).However, the photoconductive properties tend towards ambi-polarity and thus appear halfway between those of the hetero-metallic Re6 chalcogenides and Mo6 iodides.In order to enhance and optimize the interactions betweenthe clusters, the photoelectrodes were annealed at 200 1C for1 h to remove the trapped water molecules. This strategy wasalready implemented on Re6 chalcogenide cluster-based films.3It led to the improvement of the stability under photoelectro-chemical measurement conditions and to the increase of thegenerated photocurrents. UV-vis-NIR, Raman and EDS mea-surements (Fig. 2 and 4) carried out on annealed films tend tohighlight that the chemical composition of the cluster coresdoes not change after annealing. Indeed, after annealing,the {Re4Mo2Qi8}-based films retain their optical properties.A neglected decrease of the optical band gap is observed(a decrease of 0.08 and 0.03 eV for the sulfide and the selenide,respectively). Although the Raman bands (Fig. 4) appearFig. 6 (a) Current–potential curves under chopped illumination of thesulfide (green) and the selenide (purple)-based electrodes. (b) Mott–Schottky (MS) diagrams of those photoelectrodes and (c) correspondingenergy diagrams obtained by coupling MS measurements to access theFermi level position, XPS measurements to obtain the valence bandmaximum (VBM, Fig. S5, ESI†) and the UV-vis-NIR measurements toposition the conduction band minimum (CBM).Paper Journal of Materials Chemistry COpen Access Article. Published on 18 April 2024. Downloaded on 5/20/2024 5:51:20 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d3tc04635d6982 | J. Mater. Chem. C, 2024, 12, 6974–6984 This journal is © The Royal Society of Chemistry 2024broader for the sulfide than those recorded on the precursorpowders, their frequencies are similar. The Raman peaks of theannealed selenide films are more shaped which highlightsthe preservation of the vibration mode of the cluster cores.The broadening of the peaks can be attributed to an increase inthe disorder and a decrease of the particle size. Indeed, the XRDdiagrams (Fig. 4a and b) and the SEM images (Fig. 5) clearlyevidenced the disappearance of the crystalline architecture. TheXRD peaks at low angles (around 101, Fig. 4a and b) and thefaceted crystals for the sulfide phase are no longer observedafter annealing. This results in a thin microstructure built oncollapsed nano-sized particles for both chalcogenide layers(Fig. 5c and d). As already observed on Re6-based layers, thethickness of the {Re4Mo2Qi8}-based films slightly decreases byannealing, from 5 to 3 mm for the sulfide and from 7 to 3 mm forthe selenide. This suggests a loss of water molecules and adensification of the layers that is consistent with the collapsingobserved in the SEM images. This gives them a higher stability.Thanks to the annealing step, photoelectrochemical measure-ments in aqueous media become now possible. The electro-chemical measurements were thus carried out from the samethree-electrodes set up as previously described but using a KCl(0.5 M) aqueous electrolyte (Fig. 7a and Fig. S7, ESI†). Thecomparison with the signal of the FTO substrate alone depictedin Fig. S7 (ESI†) clearly evacuates the contribution of thesubstrate. For both chalcogenide-based photoelectrodes, aphotocurrent inversion potential is observed (Vinv are around�0.08 V and 0.29 V vs. Ag/AgCl for the sulfide and the selenide-based photoelectrodes) with similar values of photo-oxidationand photo-reduction current density that evidences ambipolarproperties. With positive slopes, MS measurements (Fig. 7b)however highlighted the domination of the charge transfer byelectrons. If this tendency is similar for the sulfide-based layersbefore and after annealing, a reverse electronic behavior isobserved for the selenide ones. In that case, the annealingseems to increase to the electron concentration in the films thatmodifies the nature of minority charge carriers. It turns outthat this modification of the electronic behavior of the selenide-based film after annealing originates from a shift in its Fermilevel at higher energy (Fig. 7c). Indeed, the flat band potentialvalues determined using the MS method (Fig. 7b) are lower forboth annealed films that cause a shift in higher energies of EF(less negative) around 0.5 eV. This evolution of electronic proper-ties affects the relaxation kinetics of photogenerated charge car-riers too. Indeed, the lifetime of the photoexcited electrondetermined by fitting the OCP decay curves in Fig. S6 (ESI†) islonger after annealing which supposes a lower recombination rateand indicates more efficient photoelectrodes. The impact of theannealing appears weaker for the sulfide even if the values ofthe photo-reduction currents seem slightly lower than those of thephoto-oxidation currents. In both cases, the lower values of theslope translate into an increase in the charge carrier density intothe films and, possibly, of their conductivity too. The lower slopefor the sulfide may indicate a higher charge carrier density.The photoconductivity properties of the homometallic rhe-nium cluster-based photoelectrodes were reported in a previouspublication.3 Promising photoredox activities in water werethus highlighted. Herein, a deeper study was carried out onthe heterometallic {Re4Mo2Qi8}-based photoelectrodes. Theorigin of the photocurrent in water may involve the reductionof water along with its oxidation. Indeed, as underlined inFig. 6c, the position of energy levels of {Re4Mo2Qi8}-based layersare in agreement with both water photo-oxidation and waterphoto-reduction. Thus, the photo-reduction and the photo-oxidation currents were studied under standard conditionsusing a phosphate buffer solution (pH 7) and a solar simulator(AM1.5). A neutral pH was chosen in order to help the stabili-zation of the layers and limit the release or corrosion effects.On one hand, the current–potential curves in Fig. 8 and Fig. S8(ESI†) depict the evolution of the cathodic (Fig. 8a and b) andanodic (Fig. S8a and b, ESI†) current densities with and withoutlight irradiation for each chalcogenide-based photoelectrodes.On the other hand, the chonoamperometry curves recorded at0.2 V (Fig. 8c and d) and 1.4 V (Fig. S8c and d, ESI†) vs. RHEhighlight the stability of the reduction and oxidation photo-currents. The applied potentials were chosen in order to limitthe cluster release. The cathodic photocurrent probably corre-sponds to the water reduction leading to the H2 productionassisted by sunlight whereas the anodic photocurrent probablycorresponds to the water oxidation leading to the O2 produc-tion. It turns out that the transient photocurrent densities, inreduction and in oxidation, generated by both types of chalcol-genide photoelectrodes are globally comparable. Although theabsorption properties are more favourable for the selenide(i.e. bandgaps of 1.82 eV and 1.95 eV for the selenide andthe sulphide, respectively), the photo-reduction current valuesare slightly higher from the sulfide-based photoelectrodes.Fig. 7 (a) Current–potential curves under chopped illumination of theannealed (T = 200 1C) sulfide (green) and selenide (purple)-based electro-des. (b) Mott–Schottky (MS) diagrams of such photoelectrodes and(c) comparison of the estimated Fermi level positions of such annealedelectrodes (orange) with the unheated ones (grey).Journal of Materials Chemistry C PaperOpen Access Article. Published on 18 April 2024. Downloaded on 5/20/2024 5:51:20 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d3tc04635dThis journal is © The Royal Society of Chemistry 2024 J. Mater. Chem. C, 2024, 12, 6974–6984 | 6983This can be explained by the higher charge carrier density andprobably the superior conductivity of the sulfide-based films.However, certainly due to the slower relaxation kinetics ofcharge carriers, the global photocurrent generated by theselenide-based photoelectrodes is similar and even higher after1 h of illumination compared with the sulfide ones. A reversetrend is observed on the anodic side, and the photocurrentgenerated by selenide-based photoelectrodes is higher by aboutone order of magnitude. The chronoamperometry at 1.4 V vs.RHE (Fig. S8c and d (ESI†)) however, highlights the instabilityof the photocurrent. After a strong increase (first part of theamperometry curves in Fig. S8c and d, ESI†), the photocurrentrapidly decreases after 20 s of illumination that is concomitantwith a deterioration of the layer. In order to limit the degrada-tion of the photoelectrodes, a coating of a polymer, namelypoly(methylmethacrylate) (PMMA), was deposited on the top-side of the chalcogenide-based layers. The optical propertiesare preserved (Fig. 2b and c) and the stability of the layers wasimproved while limiting the decrease of the photocurrent dueto the higher resistivity of the film brought by the isolatingPMMA layer (Fig. S8c and d, ESI†). No strong corrosion currentsare observed with the PMMA protection (Fig. 8c, d and Fig. S8c,d, ESI†). The anodic (at 1.4 V vs. RHE) and cathodic (at 0.2 V vs.RHE) photocurrents, around 10 mA cm�2 for both chalcogenide-based photoelectrodes, stay stable for more than 8 h (Fig. S9,ESI†) which corresponds to the longest time of measurement.These values are far below the best photogenerated currentvalues reported on QD-based photoelectrodes that are greaterthan a few mA cm�2 to more than 10 mA cm�2.20 However, theonset reduction potential values around 0.3–0.5 V vs. RHE withcurrent densities around 0.2 mA cm�2 at 0 V vs. RHE highlightpromising electro- and photoelectrocatalytic activities. Dueto the increase of the Fermi level with the annealing, the energybarrier between EF and the redox level of the couple H+/H2 isdecreased which favours the water reduction. Thus, (electro)catalyticand photo(electro)catalytic processes seem to contribute to thefaradaic current. Moreover, these photocurrent values wereobtained on thick resistive layers in which the cluster chalco-genides are not strongly bonded, which favours the chargerecombination and the reloading of clusters in solution. Thisis the first proof of concept and the charge transfer processesneed to be improved by designing cluster-based photoelectrodesto fully characterize the photo-water splitting efficiency of suchsystems. The chemisorption of thin layers on nanostructuredsemiconducting oxide-based electrodes like QD-based photoelec-trodes or the design of heterojunctions should improve theperformance of these systems.3,21,22 It is also worth emphasizingthat the photocurrent values obtained under the same condi-tions are increased by almost one order of magnitude comparedto those reported on homometallic Re6 chalcogenide-basedphotoelectrodes in the previous study.3 The values of photocur-rent at 0 V vs. Ag/AgCl were around 0.47 and 1.3 mA cm�2 for{Re6Si8}- and {Re6Sei8}-based photoelectrodes, respectively, andare around 50 and 20 mA cm�2 for {Re4Mo2Si8}- and {Re4Mo2-Sei8}-based photoelectrodes, respectively. Mixing Re and Mometal atom within the same heterometallic octahedral clusterclearly exalts the photoconductive properties.4. ConclusionTo sum up, as new advances in the use of metal atom clustersin the design of photoelectrodes, the photoconductivity proper-ties of heterometallic {Re4Mo2Qi8} (Q = S or Se) chalcogenideswere investigated. Photoelectrodes were prepared by depositionof {Re4Mo2Qi8} cluster-based films onto FTO substrates usingan electrophoretic deposition technique with solutions of thesalts (Bu4N)4[{Re4Mo2Qi8}(CN)a6] (Q = S, Se). The resulting(Bu4N)2(H3O)2[{Re4Mo2Qi8}(CN)a6]�xH2O (Q = S, Se)-based crys-talline films are composed of crystals whose sizes range from ahundred nanometers for the selenide phase to a few micronsfor the sulfide. This is the first time that the growth of M6(M = Mo and Re) cluster-based crystals using an EPD processhas been reported. The photoconductivity properties of such(Bu4N)2(H3O)2[{Re4Mo2Qi8}(CN)a6]�xH2O (Q = S, Se)-based filmswere then investigated through (photo)electrochemical charac-terizations. Ambipolar behavior was evidenced for both S andSe chalcogenides. However, in accordance with the previousresults obtained on Re6-based films, the charge transfer isdominated by electrons in the sulfide film and, reversely, byholes in the selenide film. The stability in water and thephotoconductivity properties are improved by an annealingstep at 200 1C. This annealing step allows expelling the watermolecules trapped inside the layers between the cluster-basedunits. Therefore, the interactions between the clusters insidethe layers are enhanced that improves thus the density ofcharge carriers and the conductivity. For both chalcogenidecluster-based films, the Fermi level migrates towards the higherenergies after annealing. Although still ambipolar, the chargetransfer is dominated by the electrons whatever the chalcogen-ide. The influence of both the chemical compositions and ofFig. 8 Current–potential curves and chronoamperometry at 0.2 V vs.RHE in the dark (dashed lines) and under illumination (solid lines) obtainedfrom the annealed (T = 200 1C) (a) and (c) sulfide (green) and (b) and (d)selenide (purple)-based electrodes. (a) and (b) were obtained underchopped illumination and (c) and (d) under constant illumination.Paper Journal of Materials Chemistry COpen Access Article. Published on 18 April 2024. Downloaded on 5/20/2024 5:51:20 AM. This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence.View Article Onlinehttp://creativecommons.org/licenses/by-nc/3.0/http://creativecommons.org/licenses/by-nc/3.0/https://doi.org/10.1039/d3tc04635d6984 | J. Mater. Chem. C, 2024, 12, 6974–6984 This journal is © The Royal Society of Chemistry 2024the strength of interactions between clusters on their optical andelectronic properties was highlighted. They can be controlled byusing the nanoarchitectonics starting from discrete molecularclusters to form layers that are better organized. Compared toprevious works, the mixing of rhenium and molybdenum atomswithin the same heterometallic clusters enables not only theoptical properties and transport properties within the active layersto be optimized, but it also enables the positions of the energylevels to be tuned. Indeed, the origin of the photocurrents in waterrely on the reduction and the oxidation of water owing to favour-able alignment of the HOMO and LUMO energy levels of {Re4Mo2-Qi8}@FTO-based photoelectrodes with those of reductions ofprotons and oxidation of water. The photo-reduction and photo-oxidation processes were then investigated under standard condi-tions at pH 7. The chalcogenide-based films are more stable undercathodic conditions. However, a PMMA protection allows generat-ing stable photo-reduction and also photo-oxidation currents formore than 8 h. This study opens the door to the development ofnew cluster-based photoelectrodes, namely photocathodes but alsophotoanodes, by coupling clusters with various compositions, forthe photoelectrochemical water splitting.Conflicts of interestThere are no conflicts to declare.AcknowledgementsThe authors thank the CMEBA platform (Francis Gouttefangeasand Loic Joanny) from UMS 2001 ScanMAT CNRS-UniversitéRennes 1 for the FE-SEM images and analyses. Raman’s inves-tigations were performed using facilities available on SIR Plat-forms from UMS 2001 CNRS-Université Rennes 1 (Fig. S2, ESI†).The authors are very grateful to Bertrand Lefeuvre ISCR UMR6226 CNRS-Université Rennes. The authors thank RennesMetropole for its financial support via the AIS funding. TheInternational Associate Laboratory CLUSPOM (2018–2023) isacknowledged for financial support as well as the FrenchEmbassy in Moscow to support a Verandsky grant for T. I.Lappi. Naumov N. G., Lappi T. I. and Gayfulin Y. 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