Synthetic strategies for lanthanide oxysulfide nanoparticles
Several strategies can be employed to yield oxysulfides nanoparticles. Historically, bulk oxysulfides were formed by partial sulfidation of oxides, oxidation of sulfides or reduction of sulfates (Figure 4). However, solid-gas or solid-solid reactions at high temperatures inevitably lead to sintering and large particles. This should be avoided to control the growth of nanoparticles. Moreover, avoiding sulfates is challenging: their formation is a competitive reaction.
Four major strategies are employed to yield Ln2O2S (bulk and nanoparticles). The two first methods are the sulfidation of an oxygenated phase such as an oxide or a hydroxide (Figure 4, pathway A) and the oxidation of sulfides (Figure 4, pathway B). In the latter case, the term “oxidation” names a substitution between sulfur and oxygen and does not imply oxido-reduction processes. This process is challenging: the partial oxygenation of sulfides is hard to control because sulfates are easily formed. To the best of our knowledge, only bulk materials were synthesized this way.
The reduction of sulfates and oxysulfates is also possible (Figure 4, pathway C). It is generally excluded for the formation of nanoparticles as it demands high temperatures (≥ 800 °C). Finally, another way to achieve the synthesis of metal oxysulfides is the co-insertion of oxygen and sulfur anions. Decompositions of organic precursors containing oxygen or sulfur are especially helpful for this method (Figure 4, pathway D). For syntheses in which oxygen rate has to be finely controlled, inert atmosphere assured by N2 or argon is mandatory.
IN,S dual doped carbon supported Ce2O2S
Recently, an original catalyst for oxygen reduction reaction (ORR) was obtained by using the thermal decomposition of a vegetal, which provides the carbon support for the inorganic catalyst.57 Cerium nitrate (Ce(NO3)3) was dissolved in water along with thiourea and then pomelo skins were added to the solution in order to form a gel. After drying, the gel was annealed at 900 – 950 °C for 2 hours to get Ce2O2S supported on carbon doped by nitrogen and sulfur. When the reaction temperature was set to 850 °C or 1000 °C, the reaction led to the formation of CeO2. The TEM observation of the catalyst shows 50–100 nm crystals of Ce2O2S disseminated on the surface of the samples (Figure 6C). The porous structure, inherited from the pomelo precursor and the oxygen vacancies evidenced by the authors make this material suitable for the ORR.
Emulsion liquid membrane system (ELM)
Emulsion Liquid Membrane System (ELM) employs a water-in-oil-in-water (W/O/W) double emulsion. Originally, ELM was applied to separate metals. Here, the double emulsion is used for the formation of doped yttrium and gadolinium oxalates. These intermediates are converted to oxysulfides, Y2O2S:Yb,Er and Gd2O2S:Eu, by a solid-state reaction with sulfur vapor.63,64 Typically, a first emulsion is obtained by mechanical agitation of an organic phase containing kerosene with bis(1,1,3,3-tetramethylbutyl)phosphinic acid (DTMBPA) (or 2- methyl-2-ethylheptanoïc acid, VA-10) as extractant and sorbitan sesquioleate as surfactant and an aqueous phase containing oxalic acid. This emulsion is then added to the external water phase which contains the metal ions (chloride or nitrates) and the double emulsion is produced by mechanical stirring. The oxalate compounds are thus produced at ambient temperature, and the system is demulsified using ethylene glycol. Oxysulfides nanoparticles of 50 – 100 nm are then obtained by annealing the powders at 600 – 1000 °C in sulfur vapor generated at 200 °C by elemental sulfur and carried by a N2 flow (Figure 6D).
Synthesis in molten sodium chloride
The synthesis in molten salts is an emerging technique which consists in the use of one or several salts as solvents for an inorganic reaction. An eutectic mixture can even be used to benefit from a lower melting point. Molten salts are typically suitable for reaction temperatures between 300 °C and 1000 °C, which enable the formation of nanoparticles while avoiding their sintering.65,66 After cooling, the particles are obtained in a matrix composed by the salts that are washed with water or alcohols.
Molten sodium chloride (melting point: 801 °C) was chosen for the one-pot synthesis of Y2O2S:Eu. Y(NO3)3, Eu(NO3)3 and NaOH were mixed and stirred before the addition of NaCl, S8 and a surfactant.67 After grinding, the mixture was heated to 850 °C in a CO atmosphere for 4 hours, and then cooled and washed.
Aqueous reactions under autogenic pressure
This section is dedicated synthesis in aqueous solution under pressure, in autoclave. We already mentioned the low boiling point of water as a strong limitation if we consider the temperatures commonly required for crystalline nanoparticles synthesis. Synthesis under pressure might be a way to overcome this limitation. Unfortunately, like the precipitation reactions at atmospheric pressure, the reported syntheses in hydrothermal conditions mainly focus on producing an intermediate solid that requires sulfidation in a second step (Table 3). Nevertheless, these syntheses expanded the range of available morphologies for the final oxysulfide nanoparticles. In the late 2000’s, Thirumalai et al. reported the hydrothermal synthesis of Gd2O2S:Eu (Table 3, entries 1 and 2).99,100 Starting with an amorphous precipitate (obtained by adjusting the pH of an aqueous solution of Gd(NO3)3 with NaOH), they obtained Gd(OH)3 nanoscaled materials (hexagonal nanocrystals, nanotubes, nanobelts,…) after the hydrothermal treatment. The influence of the pH of precipitation and the temperature and duration of the hydrothermal define the morphology of the material. After impregnation of the solid with Eu3+ ions in an aqueous solution, sulfidation was performed using a CS2 atmosphere generated by reaction of sulfur and carbon. The morphology of Gd(OH)3 was retained in the final Gd2O2S:Eu nanopowder, with only slight size decreases. The nanomaterials are well-crystallized and the morphology was finely adjustable varying the reaction conditions. Unfortunately, an undescribed sulfidation process was performed before the annealing step. It is probably similar to the one mentioned before for the composite hydroxide method conducted by the same group.68 Moroever, an original study on the photo-induced impedance was presented. Interestingly, the morphology was retained also with other lanthanides, as similar results were obtained by Thirumalai et al. on Y2O2S:Eu (Table 3, entry 3). The oxysulfides nanoparticles obtained by hydrothermal syntheses were also extensively studied by Li, Ai, Liu et al. who obtained Y2O2S:Eu,Mg,Ti nanoparticles (Table 3, entries 4, 5 and 6).90,102,103 This combination of doping ions is typical for persistent luminescence. Aqueous ammonia NH3·H2O was used as a base for precipitation of hydroxides. The authors then inserted the dopants by solid-solid reaction in the annealing step with Eu2O3, Mg(OH)2·4Mg(CO3)·6H2O and TiO2. Moreover, they noticed that using CS2 formed in situ, rather than solid S8, was crucial to keep the morphology. With S8, the Y(OH)3 nanotubes turned into hexagonal nanoparticles after the annealing step.
Decomposition of sulfur-containing single-source precursors
The decomposition of lanthanide complexes bearing ligands with sulfur in the presence of dioxygen can lead to oxysulfide nanoparticles. It was shown for the first time in 2006 in a communication by Zhao et al. who developed the synthesis of thin monodisperse hexagonal nanoplates of Eu2O2S, Sm2O2S and Gd2O2S.112 In a mixture of organic solvents and surfactants typical for colloidal synthesis (1-octadecene, oleic acid and oleylamine), [Eu(phen)(ddtc)3] (phen = 1,10-phenanthroline, ddtc = diethyldithiocarbamate; Figure 15) was decomposed under air at 290 °C in 45 minutes, forming anistropic nanocrystals (15 x 1.7 nm2, Figure 16). For the first time, the observation of self-assembled oxysulfide nanoplates to nanowires was made (Figure 16A, B and C). The nanoplates were piled one above each other, because of the hydrophobic interaction between the surface surfactant chains of oleic acid (oleylamine-metal bonds are weaker than oleic acid-metal bonds).
Interestingly, EuS (EuII) nanocrystals were obtained with the same synthesis but under inert atmosphere with oleylamine alone (which played the role of reducing agent).114 A more detailed study on the pyrolysis of the [Ln(phen)(ddtc)3] precursor and the nanoparticles properties was also reported. A noticeable work using the same strategy was conducted by Tan et al. in 2016.58 In comparison with europium, the decomposition of [La(phen)(ddtc)3] and [Pr(phen)(ddtc)3] only yielded LaS and PrS. From oxidation of the sulfides, oxysulfates nanoparticles of La2O2SO4 and Pr2O2SO4 were obtained. The nanoparticles of Eu2O2S, La2O2SO4 and Pr2O2SO4 were then tested for the water-gas-shift reaction.
Syntheses with high boiling-point organic solvents at atmospheric pressure
Altough colloidal synthesis in organic solvents have been used for years in the synthesis of metal and metal oxide nanoparticles, the first report for metal oxysulfides was published by Ding et al. in 2011.115 Lanthanide acetylacetonate Ln(acac)3 (1 equiv.), elemental sulfur (1 equiv.) and sodium acetylacetonate (1 equiv.) were added in an OM/OA/ODE mixture and heated for 45 min. at 310 °C under inert atmosphere after degassing under vacuum at 120 °C. Size-monodisperse hexagonal nanoplates of Ln2O2S were obtained. They were thin (a few monolayers) and 5 – 40 nm wide depending on the lanthanide. The composition of the powder showed a lack of sulfur (Na0.4La1.6O2S0.6), which was attributed to terminal [Ln2O2]2+ layers. The crucial advantage of this method is its versatility: La2O2S, Pr2O2S, Nd2O2S, Sm2O2S, Eu2O2S, Gd2O2S, Tb2O2S were prepared. The sodium ions, added in stoichiometric amounts, were proposed to help the crystallization and favor the oxysulfide formation. The hypothesis of the authors is that the close ionic radii of sodium (r(NaI(VII)) = 1.26 Å) and larger lanthanide ions (r(LaIII(VII)) = 1.24 Å to r(TbIII(VII)) = 1.12 Å) enable cation exchanges in the solid and favors the oxysulfide crystallization. Lithium ions were tested and were efficient for Y2O2S synthesis. In 2013, a more complete study (experimental study and calculations based on density functional theory) on the alkaline additives on the formation and morphology of the obtained nanocrystals also showed the possible use of potassium to synthesize oxysulfide nanoparticles (La2O2S, Eu2O2S, Gd2O2S, and Yb2O2S).116 In 2017, Lei et al. investigated the roles of yttrium and sodium in the formation and growth of Gd2O2S, by using them separately or combined. They also demonstrated that a large excess of sulfur allows forming gadolinium oxysulfide nanoplates without adding sodium ions.
Table of contents :
Table of contents
Part I: Lanthanide oxysulfide nanoparticles
Chapter I – Lanthanide oxysulfide syntheses: from bulk to nanoparticles
Chapter II – Synthesizing and storing Ln2O2S nanoparticles: from Gd2O2S to Ce2O2S
Chapter III – Unveiling the structure of Ln2O2S nanoplates in high boiling point organic solvents with an alkaline source: Gd2O2S as a case study
Chapter IV – Benefits of bimetallic composition: the tunable optical and magnetic properties of Gd2(1-y)Ce2yO2S nanoparticles
Part II: d-Block transition metal oxysulfide nanoparticles
Chapter V – The synthesis of bulk and nanoscaled transition metal oxysulfides: a burgeoning challenge
Chapter VI – Attempts at transposing Ln2O2S synthesis to nanoscaled transition metal oxysulfides
Chapter VII – Towards nickel oxysulfide nanoparticles
Conclusion and perspectives