My research interest includes synthetic inorganic and metal-organic chemistry and materials science. Rational design of homo- and heterometallic molecular precursors for the development of new, tuneable nanomaterials via MOD, CVD or Sol-Gel methods form the core objective of my research interest. The synthesis of molecular precursors is usually done in an inert atmosphere using glove box or Schlenk tube & vacuum line techniques. Several physico-chemical techniques such as FT-IR, NMR (multinuclei), UV-visible, DTA-TGA, single crystal X-ray diffraction are used for the characterization of the newly synthesized metal complexes, whereas materials obtained from these precursors are characterized by XRD, TG-DTA, electron microscopy (SEM, TEM), BET surface area, atomic force microscopy, XPS, etc.

My other interest relates to judiciously engineered inorganic-organic hybrid materials exhibiting interesting and fascinating properties.

Starting point: Mole­cu­lar pre­cur­sors

In the bot­tom-up ap­proach of the synthesis of nanomaterials, the mol­e­c­u­lar pre­cur­sors are used as build­ing-block units with pre­cisely con­trolled and tun­able chem­i­cal com­po­si­tion to cre­ate nano­ma­te­ri­als with the de­sired prop­er­ties. These molecules are transformed into nano­ma­te­ri­als by the soft-chemical methods such as sol-gel (in the solution phase) or chemical vapor deposition (in the gas phase) while retaining the in­her­ent struc­tural units of the pre­cur­sor mol­e­cule. Thus, these well-de­fi­ned pre­cur­sors con­trol the evo­lu­ti­on of ma­te­ri­als from sin­gle or po­ly-ato­mic units to ex­ten­ded so­lid-sta­te struc­tu­res. The main advantages of this approach are low processing tem­pe­ra­tu­res (due to the pre-ex­ist­ing che­mi­cal­ bonds among the constituent atoms), low or­ga­nic con­ta­mi­na­ti­on (due to clean li­gand eli­mi­na­ti­on me­cha­nis­ms) and higher quality of the materials (due to the homogeneity at molecular level).

1. Smart precursors for smart nanomaterials

(in collaboration with Prof. Stéphane Daniele (IRCELYON) and Dr. Erwann Jeanneau (single crystal X-ray structures, Centre de Diffractométrie))

(a) Nanostructured metal oxides and their nanocomposites

The versatility of metal alkoxides as convenient precursors to advanced materials via sol-gel process is widely acclaimed. Heterometallic species are a means to provide homogeneity at molecular level. I have developed new, very convenient and easy-to-scale up routes to synthesize novel homo- and heterometallic alkoxides as ‘single-source precursors’ to get nanostructured metal oxides and their nanocomposites for several applications. These include:

(i) Novel heteroleptic heterobimetallic alkoxide complexes as facile single source precursors for Nb⁵⁺/Ta⁵⁺ doped TiO₂-SnO₂ nanoparticles for thermoelectricity: Thermoelectric materials, which convert thermal energy into electric energy, have attracted a renewed interest over the past decade, thanks to the discovery of some new thermoelectric materials with high values of the thermoelectric figure-of-merit (zT). The TiO₂-SnO₂ system is very attractive as a possible candidate for oxide thermoelectric materials because not only a variety of stable nanostructures with low thermal conductivity can be reproduced in this system but it is also free from toxic or precious elements. However, the band gaps of rutile TiO₂ (~3.0 eV) and SnO₂ (~3.6 eV) are too large to meet the requirements for good thermoelectrics. In addition, SnO₂-based systems are difficult to be sintered to high density due to the predominance of non densifying transport mechanisms. As a consequence, the classical approach of mixing equimolar TiO₂ and SnO₂ (nano)powders gives rise to porous ceramic with low mechanical strength, which would be problematic for the thermoelectric properties. While the electrical conductivity can be improved by doping this system with cations such as Nb⁵⁺/Ta⁵⁺ or by heat treatment in reducing atmosphere, an improved densification can be achieved by intimate mixing of constituent elements at atomic level. I, therefore, synthesized novel molecular precursors for above materials by using a new, very convenient route of mixing a metal halide and metal alkoxide in a hydrocarbon solvent. The resulting complexes of the type [MSnCl₄(OR)_x(ROH)_y] (M = Ti, Nb, Ta; x = 4, 5; y = 0-2; R = Et, Prⁱ), which have simple addition molecular compositions and structures, are the first example of its kind (Scheme 1). The Ti-Sn heterometallics were then employed as sol-gel precursors to get Ti_0.5Sn_0.5O₂ nanoparticles, without forming the undesired pure SnO₂ grains. While the use of single source precursors ensured homogeneous doping of the Nb⁵⁺ /Ta⁵⁺ ion in the TiO₂-SnO₂ matrix, the conventional sintering led to a less than desired density of the TiO₂-SnO₂ matrix (62%). To improve the density, we then used Spark Plasma Sintering (SPS) technique, which allowed the formation of highly dense and nanostructured spinodal form of TiO₂-SnO₂. Impacts of the pressure, heating rate and soaking time of SPS process onto the densification and the nanostructuration of the pellets have been studied.

Collaborations:

Dr. Marie-Hélène Berger and Jean-François Hochepied (densification and TEM, MINES-ParisTech); Prof. Gilbert Fantozzi (SPS densification, INSA Lyon); Dr. Sylvie Le Floch and Dr. Stéphane Paihes (TEM and thermoelectric properties, ILM).

Scheme 1

Dalton Trans., 2015, 44, 6848–6862; Inorg. Chem., 2010, 49, 11184-11189.

(ii) New homo- and heterometallic aminoalkoxides of group 13 metals as sol-gel precursors for (mixed) metal oxides as high surface area support for gold catalysts: Mono- and multimetallic oxides of group 13 metals cover a wide range of electronic, optical and catalytic properties. In particular, mixed metal oxides having the spinel-type structure are often advantageous over binary oxides with regard to catalytic performance, life-time and resistance to sintering. The homoleptic [M₄(μ-η³:η¹-mdea)₆]·xCHCl₃ and heteroleptic [M₆Cl₆{(μ-mdea}₆]·xCHCl₃ (M = Ga, In, Fe; mdeaH₂ = N-methyl diethanolamine; x = 0, 2, 4 or 6) complexes were synthesized as sol-gel precursors to group 13 metal oxide nanomaterials, the latter derivative being used as a synthon to obtain first structurally characterized group 13 heterometallic alkoxide [Al₄Ga₂(O)₂(OPrⁱ)₁₀(mdea)₂] by metathesis reaction. Depending upon synthetic procedures employed and crystallization conditions, the homoleptic derivative [M₄{(OC₂H₄)₂NMe}₆] could be crystallized in two isomeric forms: a chair-shaped and a star-shaped. Various sol-gel processing parameters such as solvent, concentration and hydrolysis ratio (h) were studied in order to stabilize nano-sized colloidal suspensions for access to thin films by spin coating. These binary and ternary group 13 metal oxides were subsequently used as high surface area support for the deposition of gold nanoparticles of sub-5 nm size to form effective catalytic systems for the aerobic trans-stilbene epoxidation reactions. The Au/In₂O₃ was the most active catalyst of the series but Au/Ga₂O₃ catalysts were more selective, Au/β-Ga₂O₃ being the best (85-90% selectivity for t-stilbene oxide in the beginning of the reaction, Scheme 2). The selectivity of all these homometallic Au/M₂O₃ (M = Ga, In) catalysts decreased considerably as the reaction progresses while the selectivity of the heterometallic Au/Al₄Ga₂O₉ catalysts remained stable. In addition to influencing mass-transfers, the support affected the selectivity of the gold-catalyzed aerobic epoxidation of stilbene in methylcyclohexane.

Collaboration : Dr. Valérie caps (epoxidation of t-stilbene, IRCELYON)

 Scheme 2

Eur. J. Inorg. Chem., 2013, 500‒510; Dalton Trans., 2010, 39, 7440-7443; Dalton Trans., 2009, 2569-2577.

(b) Metal fluoride nanomaterials

Novel Na-Y(Ln) heterometal-organic complexes as first single source precursors for up-converting NaYF₄:Yb³⁺, Er³⁺/Tm³⁺ nanocrystals and thin films: The up-converting nano-fluorescent materials that convert near infrared (NIR) to visible light have attracted tremendous attention recently. We have developed first heterometal-organic single source precursors for NaY(Ln)F₄ up-converting nanomaterials. These novel Na-Ln heterobimetallic trifluoroacetate derivatives (Ln = Y, Gd, Er, Tm and Yb) are advantageous in terms of being anhydrous and having lower decomposition temperature in comparison to the homometallic Ln(TFA)₃(H₂O)₃, the only used precursor for chemical routes so far. In addition, they also contain chelating glyme ligands, which act as capping reagents during decomposition to control the NaY(Ln)F₄ particle size and render them monodisperse in organic solvents. Their decomposition in 1-octadecene gave NaY(Gd)F₄ nanocrystals with controlled size and desirable dispersibility in organic media. Interestingly, the isostructural [NaY(TFA)₄(diglyme)] and [NaGd(TFA)₄(diglyme)] showed different decomposition patterns. While the latter decomposed at 285°C to give directly the hexagonal NaGdF₄ NCs, a phase many folds more efficient than the cubic phase for the upconversion process, the yttrium analogue afforded only a mixture of the cubic and hexagonal phases of NaYF₄, in a relative ratio that depended on the reaction temperature. We attributed this difference in the decomposition pattern to the different ionic radii of the two metal cations Y³⁺ (r = 0.89 Å) and Gd³⁺ (r = 0.94 Å), which would influence the free energy of the NaLnF₄ system. Surface modification of these NCs by ligand exchange reactions with Poly Acrylic Acid (PAA) and polyethyleneglycol diacid 600 was carried out to render them water soluble. The multicolour upconversion fluoroscence was  successfully realized in the Yb³⁺/Er³⁺ (green/red) and Yb³⁺/Tm³⁺ (blue/violet) co-doped NaYF₄ NCs and thin films, demonstrating that these are promising UC nanophosphors of immense practical interest. We have also developed designed precursors for scintillating CeF₃ and upconverting Ln³⁺-doped GdF₃ nanoparticles.

Collaborations: Dr. Marie-France Joubert and Dr. Gilles Ledoux (Upconversion properties, ILM), Prof. Christophe Dujardin (Scintillating properties, ILM)

Near-infrared driven photocatalysis of the nanocomposites TiO₂: NaGdF₄:Yb³⁺,Tm³⁺: TiO₂ is one of the most promising photocatalysts. However, a band gap of 3.2 eV limits the application of TiO₂ only in the UV region (<398 nm), leaving a large amount of the solar energy being unused. Although several attempts have been made to extend the absorption range of TiO₂ into the visible region, not much work has been devoted to extend the absorption range of TiO₂ into the IR region of sunlight, which accounts for almost half of the energy of the sunlight radiation (as against only about 5% of the UV radiation). We have used the upconverting NaGdF₄:Yb³⁺,Tm³⁺ NCs to prepare core-shell TiO₂@NaGdF₄:Yb³⁺,Tm³⁺ photocatalyst where the core NaGdF₄:Yb³⁺,Tm³⁺ acts as a medium to convert NIR to UV-visible light via multiphoton upconversion processes. The prepared photocatalysts showed sufficient degradation of the methylene blue (MB) dye, both in laser and light conditions, thus validating the concept of transferring the NIR into the visible light to activate the anatase TiO₂ for the photocatalysis.

NaYF₄ as non-oxide support for gold catalysts for oxidation reactions: The cubic and hexagonal phases of NaYF₄ were also used as supports for gold nanoparticles. These two phases of NaYF₄, which have different surface properties and BET surface area, behaved differently during the colloidal deposition of PVA-protected gold nanoparticles using HAuCl₄∙ 3H₂O as precursor. From the TEM images, average gold nanoparticles size of 6.5 and 4.9 nm were calculated for cubic and hexagonal phases, respectively. These supported gold nanoparticles exhibited catalytic activity in the CO oxidation as well as aerobic epoxidation of t-stilbene in methylcyclohexane. The hexagonal phase which stabilizes smaller gold nanoparticles, was a better catalyst, which showed the conversion rate and the selectivity even better than the reference Au/TiO₂ catalyst from the World Gold Council (Scheme 3).

Collaboration: Dr. Jean-Luc Rousset and Dr. Franck Morphin (CO oxidation, IRCELYON)

Scheme 3

Chem. Asian J., 2014, 9, 2415‒2421; Dalton Trans., 2013, 42, 12633‒12643; Dalton Trans., 2012, 41, 1490‒1502; Chem. Commun. 2010, 46, 3756-3758.

(c) Metal selenide nanoparticles and nanocomposites

Selenoether complexes as facile precursors for metal selenide nanoparticles and thin films: Enhanced photocatalytic activity of TiO₂-Ag₂Se nanocomposites under UV light The coordination chemistry of several metal slats including halides of copper, gallium and indium with the simple dielkyl selenoether was investigated with the aim to use the resulting complexes as precursors for selenium-containing chalcopyrite semiconducting materials. These easy-to-synthesize anhydrous precursors show good solubility in organic solvents and are decomposed easily to metal selenide nanoparticles or uniform thin films on substrates at low temperatures with no evidence for significant carbon contamination. We have also developed a facile and direct synthetic route for metal selenide nanoparticles at room temperature. The synthesis of Ag₂Se NPs in the presence of commercial TiO₂ (P25) allowed the preparation of Ag₂Se-TiO₂ nanocomposites with different Ag/Ti ratios, which exhibited excellent photocatalytic activity for the degradation of formic acid under UV light irradiation, the best results being for 0.13%Ag₂Se-TiO₂ nanocomposite which degraded 50 ppm of formic acid in 30 min.

Collaboration: Dr. Chantal Guillard (photocatalysis, IRCELYON).

2. Hybrid materials based on solvated lanthanide/barium cations and iodometallate anions showing interesting solid and solution phase transformations:   The fact that the soft and bigger iodide anion in lanthanide/barium iodides undergoes auto-ionization in the presence of many coordinating solvents, and hence can be exploited for the construction of interesting iodometalate fabrics if a suitable iodide abstractor is used, formed the basis of this synthetic approach. The selection of CuI and AgI was inspired by the possibility to substitute MO₄ building blocks in zeolite with MX₄ (M = Cu, Ag; X = Cl, Br, I) tetrahedra, where the cupro- and argentophilic M(I)···M(I) interaction is expected to play a role to sustain the framework structures and, thus, provide a foundation for the new developments in zeolite type functional materials (halozeotype materials). Several of the hybrids synthesized here undergo interesting solid- and solution phase transformations on introducing a systematic substitution of the coordinating ligands in metal-organic cation and/or via condensation of iodometallate anions.

Template properties for constructing rare heterotrimetallic hybrids: We have recently shown that these hybrid iodometallates can serve, in general, as a template for constructing unique metal-organic incorporated heterometallic iodometallates containing three different metals. Not only that, they also have structure-directing effect. As a general trend, the basic structure of metal-organic containing halometallates is retained when a tetrahedral MI₄ or a trigonal co-planar MI₃ unit is replaced by another M'I₄/M'I₃ unit. On the contrary, replacing a MI₄/MI₃ unit with octahedral M'I₆ unit leads to a breakdown in the structure-directing properties of these iodometallates due to compulsory molecular rearrangement required.

Luminescent and semiconducting properties: Many of these hybrids have interesting luminescent properties and show vivid and intense fluorescence when exposed to UV light. In particular, the compound [Ba(triglyme)₂(acetone)₂][Cu₄I₆] shows a quantum yield of 69%, which makes it one of the most highly luminescent copper iodide clusters known so far. In most cases, these emissions could be attributed to the Cluster-Centered (CC) emission deriving from mixed iodide-to-metal charge transfer (XMCT) and ds/dp metal centered (MC) excited states. In some other cases, the luminescence is originated from the metallic centre of the metal-organic cation. For example, the hybrids with solvated terbium(III) cation show interesting luminescent properties where high energy transitions in the excitation spectra are quenched due to the presence of iodometallate moiety. These fascinating complexes were further evaluated as potential candidates for semiconducting (e.g., diffuse-reflectance spectra of iodoplumbate and –bismuthate derivatives show optical band gap in the range of 1.86-2.54 eV) and magnetic properties.

Collaborations: Dr. Erwann Jeanneau (single crystal X-ray structures, Centre de Diffractométrie); Dr. Gilles Ledoux (Luminescent properties, ILM); Prof. Dominique Luneau (Magnetic properties, LMI)

Scheme 4

Inorg. Chem., 2014, 53, 11721-11731; CrystEngComm, 2012, 14, 3894‒3901; Eur. J. Inorg. Chem., 2012, 2749-2758; Dalton Trans., 2009, 4954‒4961; Inorg. Chem., 2008, 47, 9333‒9343; Dalton Trans., 2008, 6296‒6304; CrystEngComm, 2008, 10, 814‒816; Dalton Trans., 2008, 620‒630; Dalton Trans., 2007, 410‒413.