Creation of interfaces in composite/hybrid nanostructured materials using supercritical fluids

2.1.2.1 Inorganic-inorganic interfaces

Intense research work was focused on metal (Pt, Pd, Cu, Ru, etc.) NP-based nanostructuring. Single metal or metal mixtures can be deposited onto various supports, e.g. on ceramic foams [59, 60], carbon black or aerogels, nanotubes, graphene [61–68], or oxide such as γ-alumina, silica/silica aerogels, Nafion 112 films [69], etc.; for each material, a specific precursor and an adapted procedure is used. Prior to deposition, an investigation of the phase behavior and the precursor solubility is usually made to determine the optimum temperature and pressure range for SFCD. Small Pt NPs (around 3 nm) with narrow size distribution were deposited onto ceramic foams coated with a layer of tin dioxide as supports (Figure 6A) (no interaction between Pt and the support) showing higher catalytic activity compared to conventional deposition of Pt by means of aqueous impregnation. By a simultaneous deposition, Pt and Cu were deposited onto ceria-coated ceramic foams [60]. It was found that the CeO₂ coating consisted of small NPs – <20 nm – showing an improved mobility of lattice oxygen and thus the appearance of oxygen vacancy known to stabilize late transition metal NPs by forming a strong metal-support bonding. The outcome is that the ceria layer was penetrated by the dissolved copper complexes, leading to close contact between copper and ceria [60], thus explaining the very good catalytic activity. On the other hand, Pt precursor was fully reduced to metal Pt, and it was supposed that it acts as either a catalyst to reduce the Cu(tmhd)₂ or as a dopant for the CuO/CeO₂ catalyst. Other works were reported for Pt NPs (Figure 6B, D, E, I, J–L) deposited onto C-type substrates (e.g. carbon aerogels, carbon black, nanotubes, or graphene) [61–66] or oxide and Nafion [69]. With these examples, it can be noticed that even considering the same metal, different reduction methods and different substrates will critically influence the average size, distribution, and morphology of the Pt NPs onto/within the carbon, oxide framework, or Nafion film. Different Pt sizes were obtained when different reduction methods were employed [70]. The simultaneous deposition of Pt and Cu precursors onto C aerogels yielded an alloy of Pt-Cu (Figure 6H) and depending on the two metal ratios, the size was varying accordingly – larger when the Cu content was higher [61]. Depositing Ru NPs onto carbon aerogels (Figure 6C), very small (~4 nm) homogeneously distributed NPs were prepared, the mean size being dependent on the reduction temperature [62]. When Pt was combined with Pd, similar observations were reported for metals deposited onto carbon black [63, 64]. Pd alone (Figure 6F) yielded very irregular NP distribution onto support with a very broad size distribution (3–100 nm), whereas Pt NPs (Figure 6E) were homogeneously distributed with a size ranging from 2 to 6 nm. Adding Pt to Pd increases the homogeneity and decreases the NP size. Depositing Pt on graphene (Figure 6D), small (~2 nm) monodispersed NPs are obtained, with the loading percentage being controlled by varying the precursor/graphene ratio [65]. Thus, a maximum loading of 80 wt.% was achieved, uniformly distributed without any significant changes in the Pt NPs size, and the material showing enhanced catalytic activity compared to Pt deposited on carbon nanotubes (CNTs) or carbon black. Combining two precursors (salt type) of Pt and Ru, very small (~3 nm) Pt-Ru alloy NPs (Figure 6G) were uniformly distributed on graphene, with Ru atoms partially dissolved in the Pt fcc lattice [66]. Depositing Pt on the surface of CNTs (Figure 6I) [67], the NPs adopted a roughly hemispherical morphology of 2 nm. By studying the adsorption isotherms, it was found that the precursor’s affinity is higher for CNTs than for sc-CO₂, allowing controlling precisely the metal loading onto the support.

Figure 6:

Different metal NPs deposited onto ceramic foams, carbon type, oxide type, or Nafion film supports with thermodynamically controlled SFCD [61–66, 69]. In all the presented cases, pure metal phases were prepared with no presence of interface compound. Reprinted with permission from Bozbag SE, Unal U, Kurykin MA, Ayala CJ, Aindow M, Erkey C, J. Phys. Chem. C 2013, 117, 6777–6787 [61], Copyright 2013 American Chemical Society; from Zhang Y, Kang D, Aindow M, Erkey C, J. Phys. Chem. B 2005, 109, 2617–2624 [62], Copyright 2005 American Chemical Society; from Cangül B, Zhang LC, Aindow M, Erkey C, J. Supercrit. Fluids 2009, 50, 82–90 [63], Copyright 2009 Elsevier; from Bayrakceken A, Cangul B, Zhan LC, Aindow M, Erkey C, Int. J. Hydrogen Energy 2010, 35, 11669–11680 [64], Copyright 2010 Elsevier; from Zhao J, Yu H, Liu Z, Ji M, Zhang L, Sun G, J.Phys. Chem. C 2014, 118, 1182–1190 [65], Copyright 2014 American Chemical Society; from Zhao J, Zhang L, Xue H, Wang Z, Hu H, RSC Adv. 2012, 2, 9651–9659 [66], Copyright 2012 Royal Society of Chemistry; and from Zhang Y, Kang D, Saquing C, Aindow M, Erkey C, Ind. Eng. Chem. Res. 2005, 44, 4161–4164 [69], Copyright 2005 American Chemical Society.

Nanostructured materials with controlled morphology were obtained by depositing Pd (Figure 7A, C) [56] and Ag (Figure 7B, D) [68] metals within mesoporous silica. It was observed that at low metal loading, small NPs were formed (Figure 7A, B), while an increase of the metal loading or impregnation time led to nanowires (NWs) formation (Figure 7C, D). NPs were confined within the pore of the support and therefore their growth was limited in two dimensions, turning into small NWs. More unusual nanostructuration, for instance core-shell type nanocables (Figure 7E, F), where the core is an oxide and the shell is a metal or vice versa, can also be prepared [71].

Figure 7:

Mesoporous silica filled with NPs of Pd (A) and Ag (B) at lower precursor concentration; increasing the metal loading, nanowires of Pd (C) and Ag (D) are formed within the pores. Reprinted with permission from Aschenbrenner O, Kemper S, Dahmen N, Schaber K, Dinjus E, J. Supercrit. Fluids 2007, 41, 179–186 [53], Copyright 2007 Elsevier; and from Zhao J, Yu H, Liu Z, Ji M, Zhang L, Sun G, J.Phys. Chem. C 2014, 118, 1182–1190 [65], Copyright 2014 American Chemical Society, respectively. (E, F) For Co@Fe₃O₄ or Fe₃O₄@Co nanocables. Reprinted with permission from Daly B, Arnold DC, Kulkani JS, Kazakova O, Shaw MT, Nikitenko S, Erts D, Morris MA, Holmes JD, Small 2006, 2, 1299–1307 [71], Copyright 2006 J. Wiley.

2.1.2.2 Inorganic-organic interface

Türk et al. studied the influence of organic ligand being part of the metal precursor over the metal NPs’ deposition onto porous alumina support [72]. Cyclooctadiene-stabilized Pt complexes containing different perfluoroalkane chains, [Pt(cod)Me-(C_n F_2n+1)], namely n/i-C₃F₇, n-C₄F₉, n-C₆F₁₃, n-C₈F₁₇, where n=normal and I=branched chains, were considered. The type of organic ligand, its size, and configuration could change the solubility of the Pt complexes in sc-CO₂, thus influencing the Pt NP size, size distribution, and metal loading. Surprisingly, the Pt NP size (<3 nm) and size distribution were not influenced by the substitution of the CH₃ end group by the C_n F_2n+1 end groups; however, their effect was observed on the Pt loading, the highest (67 wt.%) being for [Pt(cod)Me₂] – the reference. By varying the precursor’s concentration, the formation of NPs agrees with classical nucleation theory, i.e. smaller NPs for lower precursor concentration. Pt(COD)Me₂ dissolved in sc-CO₂ and impregnated on β-cyclodextrin followed by reduction with H₂ [73] yielded Pt NPs of different sizes (labeled as <20, <100, and >100 nm). Their shapes depend on the experimental conditions, such as the precursor and support mixing conditions and the precursor reduction strategy (Figure 8A–C).

Figure 8:

Nanocomposites where the metal NPs morphology is influenced by different experimental conditions: Pt NPs incorporated in β-cyclodextrine with diameters of (A) <20 nm, (B) <100, and (C) >100 nm depending on the procedure. Reprinted with permission from Pelka J, Gehrke H, Esselen M, Türk M, Crone M, Bräse S, Müller T, Blank H, Send W, Zibat V, Brenner P, Schneider R, Gerthsen D, Marko D, Chem. Res. Toxicol. 2009, 22, 649–659 [73], Copyright 2009 American Chemical Society. Au NPs incorporated in different types of polymers as (D) PTFE, (E) polyamide, and (F) polypropylene. Reprinted with permission from Wong B, Yoda S, Howdle SM, J. Supercrit. Fluids 2007, 42, 282–287 [74], Copyright 2007 American Chemical Society.

Using the same approach, gold NPs were incorporated into various polymers (Figure 8D–F) [74], and it was found that the NP morphology (size, shape, and dispersion) was critically influenced by the type of polymer:

1. Small homogeneously dispersed Au NPs were obtained when the polymer was poly(tetrafluoroethylene) (PTFE) (Figure 8D).

2. Considering polyamide, a gradient of size and NP dispersion was observed, the larger NPs (~19 nm) being close to the surface whilst the smaller ones (~3 nm) dispersed in the center (Figure 8E).

3. In polypropylene, large erratic non-circular particles of 20 nm were prepared (Figure 8F).

An explanation for this could be attributed on the one hand to a different interaction between the polymers and precursors/NPs and, on the other hand, by the diffusion of different sc-CO₂/precursors within the polymer. It was assumed that polyamide and polypropylene might have a lower affinity than PTFE to sc-CO₂ and hence the precursor uptake, the nucleation, and particle growth being significantly affected, as experimentally observed.

To summarize, the use of the SFCD route via thermodynamic control allows achieving nanoengineering of complicated architectures. Starting with metal oxide/metal films deposited on solid substrates – engineered or not – continuing with supported NPs of many types and finishing with embedded NPs in various materials, a satisfactory control over nano-objects morphology can be obtained. Even so, this approach generally requires long and multistep procedures. An alternative approach is the kinetically controlled SFCD.

2.1.3.1 Inorganic-inorganic interfaces

The thermodynamically controlled approach focuses on (i) selecting the right precursor to favor adsorption onto the substrate and (ii) wait for enough time to promote this adsorption. Oppositely, the kinetically controlled approach pays no particular attention to these considerations. The latter concept was developed by our group and was first reported for Cu NPs deposited on silica spheres in supercritical CO₂/alcohol mixture [32]. Another important aspect to be mentioned is that there are no requirements for precursor adsorption onto the substrate, such as the case of copper NP deposition onto silica beads, where the considered hydrophobic copper hexafluoroacetylacetonate precursor cannot be adsorbed onto hydrophilic silica surface. Additionally, the contacting time is generally in the order of 1 h, which is much shorter than the adsorption time in the thermodynamically approach. Thus, kinetically controlled SCFD is based on the homogeneous nucleation and heterogeneous growth (coalescence of nuclei or direct reduction of precursor over the existing) induced by a chemical reaction [32, 37] (Figure 9), being validated by numerical simulation [75]. Reduction of a metal precursor, generally bearing β-ketonate counter-anions, takes place usually in the presence of H₂ with or without alcohols, and additionally, a surfactant could be used [18]. It was found that in the absence of any interaction between the NPs and support, the experimental parameters such as temperature (T), residence time (t_s), and mass ratio of Cu(hfac)₂·H₂O/silica spheres (r_m) influence the deposited NP size in the range 5–17 nm, and also the coverage of the silica surface (40–80%, Figure 10A). We would like to underline that, whatever the percentage of Cu NP surface coverage is, the NPs are monodispersed on the support surface, without any NP agglomeration.

Figure 9:

Sketch of the kinetically SFCD controlled surface nanostructuration process. Reprinted with permission from Marre S, Cansell F, Aymonier C, Nanotechnology 2006, 17, 4594–4599 [32], Copyright 2006 IOP Science.

Figure 10:

Examples of inorganic (A–C) or hybrid (D, E) nanostructures. (A) Silica surface decorated with monodispersed Cu NPs, with the surface coverage increasing when the mass ratio (r_m) between Cu precursor and silica is increasing. Reprinted with permission from Marre S, Cansell F, Aymonier C, Nanotechnology 2006, 17, 4594–4599 [32], Copyright 2006 IOP Science. (B, C) Silica surface decorated with large spherical agglomerates (~200 nm) made of small (~8 nm) Pd NPs. (D, E) Silica surface decorated with larger more uniform spherical agglomerates (~300 nm) made of small (~8 nm) Pd NPs in the presence of an amine-type surfactant. Reprinted with permission from Pascu O, Cacciuttolo B, Marre S, Pucheault M, Aymonier C, J. Supercrit. Fluids 2015 [18], Copyright 2015 Elsevier.

Employing similar conditions with another metal, for instance Pd, different deposition behaviors were found [18]. Small metal Pd NPs were formed but they were deposited in large agglomerates of spherical shape (Figure 10B–E). This behavior could be explained by the Pd’s autocatalytic behavior, creating active nucleation. By adding an amine-type surfactant as a weaker reducing agent, the Pd NP size is not changing considerably as it should be expected in a more reducing media; however, it affects the morphology and dispersion on the silica surface, creating more homogeneous nucleation sites. Changing the silica support with more active ones, such as CeO₂, TiO₂, and Fe₂O₃, a similar Pd NP size (around 9 nm) was obtained, preserving at the same time the agglomeration trend [18].

When Pd NPs were deposited onto magnesium-scandium alloy (Mg_0.65Sc_0.35), 10-nm Pd NPs were formed and the same deposition trend was observed [76]. The platelet-like morphology of this support hampers an easy observation of the Pd/ Mg_0.65Sc_0.35 interface, while an earlier work of our group reported the absence of an interphase with palladium on Mg support (Figure 11A–D), but with the presence of an interphase with copper (i.e. MgCu₂) and nickel (i.e. Mg₂Ni) [19] (Figure 11E–H). It was found that all three metals, Pd, Cu and Ni, were deposited onto the Mg surface either as aggregates or single NPs, in agreement with other previous observations, concordant with the bimodal kinetic surface nanostructuration model. It is worth noticing that, contrary to the surface modification of spherical particle supports (200–500 nm) with monodispersed metal NPs, the supports showing lower specific surface area (Mg crystals, Mg-Sc platelets, or silica-irregular shapes) and so less available surface for the deposition will promote the coalescence mechanism of the deposited supported NPs. More interestingly, thanks to the SFCD approach, an interphase between metal NPs and their support can be formed, opening interesting perspectives for the preparation of nanostructured materials with enhanced properties. Specifically, in the case of hydrogen storage, we have demonstrated that this interface is at the origin of a significant cyclability improvement of the materials [77].

Figure 11:

Examples of inorganic nanostructures with the absence (A–D) or presence (E–H) of an interphase compound. (A, B) Magnesium-scandium alloy decorated with agglomerated Pd NPs. Reprinted with permission from Couillaud S, Kirikova M, Zaïdi W, Bonnet JP, Marre S, Aymonier C, Zhang J, Cuevas F, Latroche M, Aymard L, Bobet JL, J. Alloys Compd. 2013, 574, 6–12 [76], Copyright 2013 Elsevier. (C, D) Magnesium surface decorated with agglomerated Pd NPs. Reprinted with permission from Aymonier C, Denis A, Roig Y, Iturbe M, Sellier E, Marre S, Cansell F, Bobet JL, J. Supercrit. Fluids 2010, 53, 102–107 [19], Copyright 2010 Elsevier. (E, F) Magnesium surface decorated with aggregates/single Cu NPs with the formation of an interphase compound of MgCu₂ [19]. (G, H) Magnesium surface decorated with aggregates/single Ni NPs with the formation of an interphase compound of Mg₂Ni [19].

2.1.3.2 Inorganic-organic interfaces

A recent work reported the deposition of Pd NPs [from Pd(hfac)₂ precursor] onto cellulose NCs (CNXLs) in sc-CO₂ in the absence of H₂, only using the cellulose support as reducing agent (due to OH surface groups) [78]. It was found by X-ray photoelectron spectroscopy (XPS) measurements that Pd²⁺ appears at the interface between metal Pd and CNXLs, and a rather strong bond keeps the Pd NPs attached to the surface of CNXLs. However, when the NPs exceed the critical diameter of around 13 nm, NPs start detaching from the surface, possibly because the cellulose cannot longer sustain the NPs mechanically above this size.

Using other supercritical solvents than sc-CO₂ (e.g. EtOH, hexane) [78–80], interesting materials can be prepared by controlling the morphology (size, shape) of the NPs with the reaction temperature, pressure, time, precursor concentration, and presence of surfactants. Employing either an organic surfactant, as reducing and/or stabilizer, or seeds of another material, anisotropic NPs such as nanorods and NWs can be prepared. Using oleylamine (OAm) as both a reducing and a capping agent for controlling the crystallite size and phase, spherical or rod-like LiMnPO₄ NPs were synthesized in batch mode in ethanol at supercritical conditions (T=250–400°C, p=38 MPa, 4–10 min) [79]. Although ethanol is a reducing agent, in the absence of OAm, both phases of LiMnPO₄ and Li₃PO₄ (as impurity) were formed, while in the presence of OAm pure LiMnPO₄ was obtained. This result suggested that OAm – as a surfactant – provides a suitable reducing atmosphere for controlling the oxidation of Mn²⁺ to Mn³⁺. Moreover, it has been found that OAm was acting also as a capping agent, decreasing the LiMnPO₄ NP size down to 7 nm and at the same time directing an anisotropic growth (Figure 12A) via a typical surfactant-assisted oriented attachment growth mechanism (Figure 12B, C).

Figure 12:

Surfactant-assisted oriented attachment growth mechanism for LiMnPO₄ NPs obtained from supercritical ethanol/oleylamine system in batch mode. Reprinted with permission from Rangappa D, Sone K, Zhou Y, Kud T, Honma I, J. Mater. Chem. 2011, 21, 15813–15818 [79], Copyright 2011 Royal Society of Chemistry.

NWs are an interesting class of nanostructured materials, with 1D quantum confinement and able to connect components in a nanointegrated system. An interesting approach to prepare NWs using SCFs is the SCF-liquid-solid NW growth [80, 81]. GaAs NWs (Figure 13) were prepared in sc-hexane (T=500°C, p=37 MPa) in the presence of 7-nm dodecanthiol-stabilized Au NC seeds [80]. An important issue to be taken into consideration with this kind of approach is the precursor degradation kinetics. Indeed, the precursor type chosen has to provide the need for NW formation limiting the particles’ growth (Figure 13A). An important point is that NWs with a diameter >15 nm presents twinning faults, the defects being confined to a single plane (111) (Figure 13D, E), while no defects were observed in smaller-diameter NWs (Figure 13C). Between the Au seed and the GaAS NWs, no interface compound is observed (Figure 13A) but XPS analysis (Figure 13B) showed the presence of Si [from the precursor, As(SiMe₃)₃] as a doping non-metal element in the NWs during the supercritical process. Similar doping/incorporation with a non-metal atom coming from the precursor/reaction media was observed by our group, on Pd NPs with C or H incorporated during synthesis in supercritical acetone [20].

Figure 13:

Seed-oriented NW growth via the SCF-liquid-solid route. (A) Thiol-capped Au seed nanocrystal for growing GaAs NWs; (B) XPS data of NWs, showing the presence of Si (from precursor) as doping atom in NW; (C) single crystal GaAs NW with diameter <15 nm; (D) multitwinned GaAs NW with diameter >15 nm; (E, F) high resolution of twinning faults. Reprinted with permission from Davidson III FM, Schrinker AD, Wiacek RJ, Korgel BA, Adv. Mater. 2004, 16, 646–649 [80], Copyright 2004 J. Wiley.

An analogous SCF-liquid-solid process was used to prepare high yields of single-crystalline Si and core-shell Ge/SiO_x NWs directly nucleated from the stainless-steel or titanium reactor wall and grown in metal reactor cells under high-pressure SCF conditions (sc-CO₂) [81]. This wall nucleation was confirmed by the presence of Fe and Ti at the base of the NWs, with NWs growth proceeding through extrusion from the base region. Furthermore, the NW formation occurred only by mixing diphenyl-silane/germane precursors with a coordinating solvent, trioctylphosphine (TOP), suggesting that the addition of hydrocarbon ligands to sc-CO₂ is acting both as capping ligand for the already formed NCs and steric stabilizer to the emerging nuclei during the reaction [81].

Other types of hybrid nanostructured materials prepared via kinetically controlled SFCD are metal NPs stabilized within organic matrices such as polymers [82] or ionic liquids [83–85]. The preparation method relies on the reduction of a metal precursor in a CO₂ solution containing an insoluble polymer. Reduction of the metal with H₂ leads to small NCs, with good polydispersity, stabilized by the polymer. This hybrid material is recovered as a dry powder, ready to use and free of any organic solvents with the possibility of being resuspended in an appropriate solvent. It is worth mentioning that through this approach, there is no need to have the chemicals (hyperbranched polymer and metal precursor) soluble in sc-CO₂. The following formation mechanism was proposed (Figure 14A): (i) sc-CO₂ diffuses into the powder (step 1) and swells the polymer, which becomes liquid at the bottom of the reactor (step 2). (ii) The metal precursor is dissolved by the liquid polymer and due to the presence of H₂, coming together with CO₂ molecules, the subsequent reduction to metal takes place (step 2) followed by the NPs formation and stabilization by the hyperbranched polymers (step 3). During depressurization, the ligands (from the precursor) together with the CO₂ molecules are released from the powder, yielding dry nanostructured materials (NPs stabilized by hyperbranched polymers), ready to be used. By changing the polymer to onium salts (powder at room temperature), the same principle of hybrid nanostructuration can be considered, due to the significant melting point depression of onium salts in the presence of compressed CO₂, thus forming ionic liquids. This was demonstrated by the preparation of NCs of noble metals (Pd, Pt, Ir, Rh, and Ru) stabilized by quaternary ammonium salts (with different melting temperatures and structures), which were used as efficient nanocatalysts [83–85]. It was found that the metal NCs’ physico-chemical properties, such as size, size distribution, organization, and surface chemistry, could be controlled by varying the type/structure of stabilizer (Figure 14B), metal type, or experimental conditions.

Figure 14:

Hybrid nanostructuration via the preparation of metal NCs stabilized by organic molecules insoluble in sc-CO₂. (A) Proposed formation mechanism of Pd@polymer dry powders. Reprinted with permission from Moisan S, Martinez V, Weisbecker P, Cansell F, Mecking S, Aymonier C, J. Am. Chem. Soc. 2007, 129, 10602–10606 [82], Copyright 2007 American Chemical Society. (B) Metal NCs stabilized by different quaternary ammonium salts, such as THAB (tetrahexylammonium bromide) (reprinted with permission from Cimpeanu V, Kocevar M, Parvulescu VI, Leitner W, Angew. Chem. Int. Ed. 2009, 48, 1085–1088 [83], Copyright 2009 J. Wiley), TBAB (tetrabutylammonium bromide), CTAB (cetyltrimethylammonium bromide), and CTANTf₂ (cetyltrimethylammonium bis(trifluoromethylsulfonyl)imide) (reprinted with permission from Liautard V, Pascu O, Aymonier C, Pucheault M, Catal. Today [85], 2015 Copyright 2015 Elsevier).

Kinetically controlled SFCD leads to high-quality materials with useful properties. A larger and more sophisticated garden of nanostructures can be achieved with the continuous SCF route, presented in the next section of this review paper.

2.2.1.1 Inorganic-inorganic interface

Using the continuous approach, one-pot synthesis of high-quality nano-objects (well crystallized, narrow size distributed, pure, stoichiometric, etc.) can be achieved in a very short time (from few seconds to few minutes) and with a good reproducibility. Supercritical water is an attractive reaction environment for hydrothermal crystallization of metal oxide particles due to the radical changes of its properties (e.g. density, dielectric constant, etc.) around the critical point, therefore influencing the phase behavior of supercritical water-light gas (O₂, H₂, etc.) mixtures, the reaction equilibrium/rate, leading to the synthesis of new materials or specific particle morphologies. The Adschiri group was pioneer in the continuous supercritical hydrothermal synthesis, preparing a myriad of materials. Briefly, the method developed by Adschiri et al. [86] consists in feeding the reactor with one stream of aqueous metal salt solution (at room temperature), which is allowed to mix with a hot preheated water stream (at a temperature above the reaction temperature), allowing fast heating of the reactive fluids to the desired temperature conditions. This leads to a rapid reaction in the reactor and afterward, the outlet solution is rapidly quenched, particles being collected in the effluent after passing through a filter (to remove the largest particles). Hydrothermal reactions proceed faster at supercritical conditions than in subcritical water because the lower dielectric constant results in an enhancement of the reaction rate and thus smaller particles can be obtained. Similarly, particle morphology can be controlled with temperature and pressure, especially around the critical point, due to the variations of the dielectric constant and the ionic product of water, exemplified with the boehmite (AlOOH) case. It was found that AlOOH particle morphology was determined by selective adsorption of positively charged species, Al(OH)²⁺ or Al(OH)₂⁺, on the negatively charged surface of the AlOOH crystals at chemical equilibrium. By small changes in temperature and pressure, this equilibrium was modified, thus affecting the chemical species distribution and therefore the crystal habits [86]. It is important to recall that the formation of a homogeneous phase for supercritical solution and light gases (e.g. O₂ or H₂) provides a uniform oxidizing or reducing environment promoting the oxidizing/reducing capability of the reaction environment, leading to the control of the material composition. Through this procedure, complex metal oxide particles were prepared, such as barium hexaferrite (BaFe₁₂O₁₉), metal-doped oxide [Al₅(Y+Tb)₃O₁₂, YAG/Tb], and lithium cobalt oxide (LiCoO₂). Barium titanate (BaTiO₃) is yet another example of material worthy to be presented. Intense research has been devoted to prepare this material by various methods [26], and the SCF approach can be considered. Starting from barium hydroxide and titanium dioxide as precursors, fine NPs were prepared by supercritical hydrothermal synthesis (400°C and 30 MPa) [87]. A pH >13 was necessary to obtain a pure BaTiO₃ phase, as this condition is required to dissolve titanium dioxide in water, releasing the titanium ions readily to react with barium ions. Through rapid heating, small particles in the range 10–50 nm or larger (up to 150 nm) were obtained (Figure 16A). By changing the supercritical media (water+ethanol), the precursor type (barium and titanium isopropoxides – dissolved in ethanol), and the type of chemical reaction, namely supercritical sol-gel, in the temperature range 150–380°C at 16 MPa pressure, ultra-fine barium titanate (BaTiO₃) and also barium strontium titanate (Ba_1-x Sr_x TiO₃) [88] powders with a crystallinity as high as 90% have been successfully prepared without barium carbonate contamination [26, 89]. The particles exhibited sizes of ~40 nm with -OH defects, similar to the NPs synthesized by hydrothermal reaction. By optimizing the process [85] (higher temperature, 1 min residence time), the NP size was decreased to 20±6 nm (for BaTiO₃, Figure 16B) and even further by increasing the amount of strontium (Ba_1-x Sr_x TiO₃, 0≤x≤1), e.g. 16±2 nm for Ba_0.6Sr_0.4TiO₃ (Figure 16C). To understand the NP formation mechanism, in situ synchrotron powder diffraction was coupled with ex situ analyses such as Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, XPS, X-ray diffraction, and high-resolution transmission electron microscopy (HR-TEM) [86]. A reduction of -OH surface defects when strontium was added was revealed, this being attributed to the presence of less dissolved ions in solution, because of the Sr-O stronger ionic bond, which was impeding the ions’ dissolution. Owing to the sol-gel mechanism, the surface -OH is responsible for the reaction, but -OH loss will decrease the ability of the precursor to react at the surface, thus limiting the particle growth. Inorganic systems – as core-shell-type particles – were prepared by precipitating Ni shell over Fe₃O₄ nuclei in subcritical water by hydrogen reduction from Ni(CH₃COO)₂ aqueous solution [90]. Magnetite nuclei were first formed by the hydrothermal synthesis from FeSO₄ aqueous solution through a Fe²⁺+“phen” complex (phen=1,10-phenanthroline), the latter compound inhibiting the hydrolysis of Ni²⁺ at higher temperatures, therefore avoiding NiO formation. The thickness of the Ni shell over the Fe₃O₄ nuclei can be increased by increasing the reaction temperature and the Ni/Fe molar ratio in the feed solution. Unfortunately, no further details about the inorganic interface could be found in the report. Another type of solvent to produce inorganic nanostructured materials is supercritical ammonia [91]. By performing thermal decomposition of metal precursors in a supercritical ammonia-methanol mixture in a temperature range 170–290°C at p~16 MPa, nitrides (Cr₂N, Co₂N, Fe₄N, Cu₃N, and Ni₃N), metal (Cu), and oxides (Al₂O₃, TiO₂, and Ga₂O₃) can be prepared. With the exception of pure Cu₃N cubic monocrystal particles of 10 μm, the other nitride particles obtained were organized in shapeless aggregates of a few micrometers constituted of crystalline nanodomains. Their composition varied depending on the metal type from pure nitrides for Ni and Co to mixed oxy-nitrides for Fe and Cr. The preparation of nitrides at relatively low temperatures occurs for the metals when the Gibbs free energy of oxide formation is not too low (e.g.Cr, Co, Cu, Ni, and Fe) whilst oxides are obtained at very low free Gibbs energy (e.g. Ga, Al and Ti).

Figure 16:

BaTiO₃ and Ba_1-x Sr_xTiO₃, 0≤x≤1, particles synthesized by two different supercritical chemical routes: (A) hydrothermal reaction (BaTiO₃) (reprinted with permission from Atashfaraza M, Shariatvy-Niassar M, Ohara S, Minami K, Umetsua M, Naka T, Adschri T, Fluid Phase Equilibr. 2007, 257, 233–237 [87], Copyright 2007 Elsevier) and (B) sol-gel route in supercritical mixture (water+EtOH) for BaTiO₃. (C) Ba_0.6 Sr_0.4 TiO₃ NCs obtained through sol-gel routes in H₂O/EtOH mixtures. Reprinted with permission from Philippot G, Jensen KMO, Christensen M, Elissalde C, Maglione M, Iversen BB, Aymonier C, J. Supercrit. Fluids 2014, 87, 111–117 [88], Copyright 2007 Elsevier.

Depending on the final application, naked nano-objects could be of interest; however, it is difficult to have full control over their morphology, stability, and organization. As an alternative, in situ/ex situ functionalization can bring additional opportunities when specific properties are required.

2.2.2.1 In situ/ex situ functionalization via the hydrothermal route

The development of different morphologies and organization of oxides or bimetallic-based hybrid nanostructured materials was achieved by the Adschiri [92–102] and the Türk [103–105] groups via a supercritical hydrothermal approach in the presence of organic stabilizers or clickable anchors. The Adschiri group method is based on the concepts of organic-solution phase and liquid-solid-solution phase synthetic transfer routes, where the organic surfactant plays a key role in the shape-controlled growth and organization/stability of oxide NCs. The NCs formation strategy is illustrated in Figure 17 and is based on the following:

1. Single crystals of <10 nm are formed during the supercritical hydrothermal process (Figure 17A).

2. The organic ligand molecules become miscible with the supercritical water, thanks to the low dielectric constant of water.

3. The NCs’ growth is controlled by proper selection of the organic ligand molecules with selectivity to a specific inorganic crystal surface (Figure 17B).

Figure 17:

Batch supercritical hydrothermal synthesis of oxide-based hybrid nanostructures: (A) nanocrystals synthesis strategy; (B) growth mechanism influenced by organic surfactant; (C) CeO₂ NCs in the absence of surfactant; (D) cubic CeO₂ NCs when the surfactant concentration is low; and (E) hybrid CeO₂ NCs with a high surfactant concentration. Reprinted with permission from Zhang J, Ohara S, Umetsu M, Naka T, Hatakeyama Y, Adschiri T, Adv. Mater. 2007, 19, 203–206 [94], Copyright 2007 J. Wiley.

The use of water, instead of organic solvents, provides a green chemistry route to prepare nanoblocks for advanced materials and devices [94]. In the case of ceria NCs, smaller surfactant concentration (decanoic acid) leads to cubic structures (Figure 18D), while increasing its concentration will form truncated octahedral NCs similar to the naked ones (Figure 17C) but smaller (Figure 17E) [94, 95].

Figure 18:

Principle of the hydrothermal synthesis of ceria NCs (A), in the presence (B) or absence (C) of hexanedioic acid. Reprinted with permission from Takami S, Ohara S, Adschiri T, Wakayama Y, Chikyow T, Dalton Trans. 2008, 5442–5446 [96], Copyright 2008 Royal Society of Chemistry.

Changing the surfactant type from a monoacidic to a diacidic one (hexanedioic acid), ceria NCs auto-assembled in cubic organizations were obtained due to an octa-coordination of the primary octahedral NCs with di-acidic surfactants forming a bridge at the hybrid interface (Figure 18) [96]. The two carboxyl groups of hexanedioic acid are bounded to the Ce atoms on the surface of octahedral CeO₂ NCs, preventing the growth of the primary octahedral NCs. HfO₂ NPs were prepared in similar conditions [98].

Using a surfactant with double functionalities, hydroxyl (-OH) and carboxylic (-COOH) groups, such as 3,4-dihydroxycinnamic acid (DHCA), different oxide morphologies, exemplified with Fe₃O₄ [99, 100] and HfO₂ [98], are achieved. Fe₃O₄ in the form of raspberry-like aggregates (Figure 19B, C), made of small magnetic NPs (~20 nm), were prepared under hydrothermal conditions at 200°C in the presence of DHCA. The NPs within the aggregates were connected by DHCA molecules to form the clusters (Figure 19A–C) and can be easily dispersed in water media, thanks to COOH groups on their surfaces. The clusters size ranging from 50 to 400 nm could be tuned by controlling the residence time without changing the primary particle size [99] while keeping their superparamagnetic properties. Increasing the reaction temperature (in the range of 230–350°C), smaller monodispersed iron oxide NPs (10–18 nm) were obtained (Figure 19D, E) [100]. At similar hydrothermal conditions (T=350°C, p=16 MPa), HfO₂ NPs were prepared. It was found that the precursor solution alkalinity and the amount of DHCA – used as a multifunctional capping agent – were key factors for controlling the size and shape of the NCs, namely flower-like nanostructures (20 nm in diameter), polycrystalline nanoagglomerates (25 nm in diameter), and water-dispersible single NPs (4 nm in diameter) (Figure 19F) [98]. The double-functionality surfactants could act as strong bridges (covalently bonded) connecting the surface of two NPs and organizing them in aggregates with various morphologies.

Figure 19:

Hydrothermal synthesis of Fe₃O₄ (reprinted with permission from Togashi T, Naka T, Asahina S, Sato K, Takami S, Adschiri T, Dalton Trans. 2011, 40, 1073–1078 [99], Copyright 2011 Royal Society of Chemistry) and HfO₂ (reprinted with permission from Sahraneshin A, Asahina S, Togashi T, Singh V, Takami S, Hojo D, Arita T, Minami K, Adschiri T, Cryst. Growth Des. 2012, 12, 5219–5226 [98], Copyright 2012 American Chemical Society) NPs in situ functionalized with 3,4-dihydroxycinnamic acid surfactant. (A) Sketch of the formation mechanism of iron oxide aggregates, (B) TEM image of the aggregates, and (C) HR-SEM image of the raspberry-like aggregates made by small Fe₃O₄ NPs [99]. (D, E) Changing the synthesis parameters (T=350°C), monodispersed Fe₃O₄ NPs were prepared (~18 nm). (F) At similar conditions, 4-nm HfO₂ NCs were obtained [98].

Zinc oxide (ZnO) is yet another example of material synthesized by the hydrothermal route with or without organic stabilizer [101, 102] (Figure 20). Starting from nitrate precursor at supercritical water conditions, ZnO nanorods were prepared in the absence of organic surfactant [101] (Figure 20A). In most cases, the surfaces of the metal oxide NPs formed in hydrothermal conditions are covered by hydroxyl or ether groups, rendering them hydrophilic and thus dispersible in water. The surface hydrophilicity can be changed to hydrophobicity using reagents that can react with surface hydroxyl groups, such as alcohols, aldehydes, carboxylic acids, and amines [101, 102], and at the same time controlling the NP morphology (size, shape, and distribution), as exemplified in Figure 20B, C.

Figure 20:

Hydrothermal synthesis of ZnO without (A) (reprinted with permission from Ohara S, Mousavand T, Sasaki T, Umetsu M, Naka T, Adschiri T, J. Mater. Sci. 2008, 43, 2393–2396 [101], Copyright 2008 Springer) or with different types of organic stabilizers, such as amine (B) or alcohol (C) (reprinted with permission from Mousavand T, Ohara S, Naka T, Umetsu M, Takami S, Adschiri T, J. Mater. Res. 2010, 25, 219–223 [102], Copyright 2010 Cambridge University Press).

Research related to the synthesis and in situ continuous functionalization of iron oxide NPs in supercritical or near-critical water with groups that can undergo “click” reactions was reported in the last 3 years by the Türk group [103–105]. It is well known that iron oxide NPs’ structure and properties strongly depend on their size, their history of formation, and the surrounding medium. On the basis of these considerations, the Türk group has investigated, using a conventional T union setup, the role of mixing ratio, fluid velocity (Figure 21A–C), and type of the precursor (Figure 21D–F) on the size distribution of both primary and agglomerated particles, colloidal stability [103]. Additionally, they performed a Raman spectroscopy study of the system directly in the aqueous dispersion, based on the fact that the properties of the iron oxides can be changed during the preparation steps [103]. It was found that the synthesis from pure aqueous iron(III) nitrate solutions exhibits a bimodal distribution of particles, but increasing the total mass flow rate and mixing ratio between precursor (metal salts) stream and supercritical water stream, the fraction of the larger population was decreased. By changing the iron(III) nitrate precursor with acidified mixtures of iron(II) acetate and iron(III) nitrate, much narrower particle/agglomerate size distributions were obtained. Moreover, the presence of a mixture of α-Fe₂O₃ and γ-Fe₂O₃ and/or ferrihydrite was detected using Raman analysis, showing that the structure and size of the synthesized particles can change upon drying, resulting in inaccurate results.

Figure 21:

Iron oxide synthesized at different conditions (reprinted with permission from Daschner de Tercero M, Röder C, Fehrenbacher U, Teipel U, Türk M, J. Nanopart. Res. 2014, 16, 2350-1–2350-27 [103], Copyright 2014 Springer): mixing ratio, fluid velocity (A) at 3 kg/h (B) and 9 kg/h (C): different chemical compositions (D): mixture at 3 kg/h (E) and 6 kg/h (F).

Further works [104, 105] were based on the in situ synthesis of iron oxide NPs with different types of carboxylic acids, such as hexynoic, hexanoic, and undecynoic acids, in order to undergo a second functionalization via “click chemistry”. Knowing that the precursor and organic capping type influence the NPs morphology, surface reactivity – important for the in situ functionalization – was tested (Fe nitrate and citrate salts and also different mixing setups). It was found that a mixture of maghemite/magnetite and hematite NPs was obtained: larger (>50 nm) rectangular NPs of maghemite with smaller (~6 nm) spherical NPs of magnetite. The presence of surface acids thus allows a further coupling, opening new application opportunities.

2.2.2.2 In situ/ex situ functionalization via the alcohol route

Complementary to the hydrothermal route, our group studied alcohol-based approaches to prepare nanostructures. Interestingly, a hybrid interface synergy is created by supercritical alcohols – used as reaction media – on the surface of ceria NPs [21, 22] in the absence of any organic surfactants. Through this approach, seven different alcohols were tested, namely MeOH, EtOH, PrOH, iPrOH, ButOH, PentOH, and HexOH. Their influence over the CeO₂ NC size, morphology, and surface properties in alcothermal conditions (T=300°C, p=24.5 MPa) was investigated. It was found that the CeO₂ size can be precisely tuned from 3 to 7 nm varying the alcohol length. However, the NPs were not dispersed but rather organized in spherical nanostructures of 20–100 nm (Figure 22B, C), exhibiting surfaces modified by alkoxide and carboxylate species, coming from the primary and secondary alcohol reaction with the ceria surface (Figure 22D, E). This study has demonstrated that the CeO₂ surface can be modified by using alcohols in near- or supercritical conditions acting both as solvents and surface stabilizers, thus avoiding the use of other surfactants. This method opened new synthesis pathways, giving the opportunity of using other solvents with an alcohol function, such as their amino derivatives [22].

Figure 22:

Ceria NPs prepared in supercritical alcohol and their derivatives, and ceria surface modification by the alcohol solvent. (A) Ceria NCs prepared in sc-H₂O as reference; (B) ceria prepared in sc-ethanol; and (C) ceria prepared in sc-hexanol. (D) Proposed mechanism of the surface reaction of primary and secondary alcohols with ceria. Reprinted with permission from Slostowski C, Marre S, Babot O, Toupance Th, Aymonier C, Langmuir 2012, 28, 16656–16663 [21], Copyright 2012 American Chemical Society. (E) FTIR and the surface species at the hybrid interface when supercritical alcohol derivatives are used for the ceria synthesis. Reprinted with permission from Slostowski C, Marre S, Babot O, Toupance Th, Aymonier C, Langmuir 2014, 30, 5965–5972 [22], Copyright 2014 American Chemical Society.

Moving from oxide to pure metallic particles, Pd NCs were synthesized in supercritical acetone (T=250°C and p=20 MPa) [20, 39]. The main objective of this study was to control separately the nucleation/growth and the functionalization steps, by tuning the NCs’ surface chemical functionalities and composition. Very reactive naked Pd NCs (up to 3 nm) were synthesized continuously in supercritical acetone, followed by their ex situ functionalization upon depressurization using perfluoro-1-decanethiol or 1-n-butyl-3-methylimidazolium hexafluorophosphate (ionic liquid) placed in the recovery vessel [39]. The presence of organic capped ligand on the surface of the Pd was monitored by coupling dynamic light scattering and nuclear magnetic resonance techniques, and catalytic tests were successfully performed. For a better understanding of the process, Pd NCs were also recovered in a vessel in the absence of any organic stabilizer [20]. This alternative offered the possibility to incorporate non-metal atoms (C or H) at ambient conditions during the subsequent step to nucleation (step II B) and growth (step I) of palladium NCs in SCFs (Figure 23A). It was found that in the absence of any stabilizer, the primary Pd NCs keep growing by coalescence in the recovery vessel with the incorporation of other non-metal atoms (either already dissolved in the environment solution or coming from the ligand part of the precursor or the reaction solvent-acetone), and finally forming aggregates with different morphologies, depending on the considered precursor. It was demonstrated that Pd NCs chemistry (composition and surface energy) is influenced by the organic ligand of the Pd precursor but also by the reaction media. Although a layer of organic material was present at the surface of the Pd NC aggregates, it was possible to perform ligand exchange with thiols, thanks to their physisorbed nature allowing the redispersion of the Pd NCs in solution for further applications [20].

Figure 23:

(A) Sketch of the continuous supercritical process for the synthesis of Pd NCs with separate control of the nucleation/growth and their subsequent surface modification. Reprinted with permission from Pascu O, Moisan S, Marty JD, Aymonier C, J. Phys. Chem. C 2014, 118, 14017–14025 [20], Copyright 2014 American Chemical Society. (B) TEM image of small thiol-capped Pd NCs and (C) naked Pd NCs forming aggregates in the absence of organic stabilizer.

Synthesis of superparamagnetic ferrite NPs (MnFe₂O₄, Fe₃O₄) in supercritical ethanol (sc-EtOH) at a fairly moderate temperature (260°C) is yet another example of in(ex) situ functionalization [41]. The as-obtained NPs present good crystallinity, sizes <8 nm, monodispersity, superparamagnetic behavior at room temperature, and high saturation magnetization. Moreover, depending on the capping strategy, the ferrite NPs present extended (for in situ coated NPs) or short-term (for ex situ coated NPs) colloidal stability. Iron oxide NPs (~6 nm) were synthesized in the presence of an oleic acid/OAm mixture, comparable with the NPs obtained by thermal decomposition (Figure 24). Moreover, the synthesis of MnFe₂O₄ with in situ functionalization afforded very small crystals (~2 nm), while the ex situ produced NPs of around 7 nm, underlying again the different behaviors for different metals in the same synthesis conditions.

Figure 24:

Superparamagentic ferrite NPs synthesized via supercritical ethanol route with in(ex) situ oleic acid (OAc) functionalization: (A) FeFe₂O₄@OAc NPs, (B) MnFe₂O₄@OAc NPs, and (C) MnFe₂O₄ naked NPs. Reprinted with permission from Pascu O, Marre S, Aymonier C, Roïg Y, Nanoscale 2013, 5, 2126–2132 [41], Copyright 2013 Royal Society of Chemistry.

The main limitation of the room temperature ex situ functionalization is the lower colloidal stability of the nanostructures. As the stabilizing agent is mainly physisorbed on the nano-objects’ surface, its sorption/desorption occurs over time. Therefore, due to the higher reactivity of smaller NCs, growth through the Ostwald ripening mechanism was noticed. An optimized surface functionalization approach can be achieved by changing the continuous reactor type setup. The co-flow arrangement, developed by our group [40, 106–109], leading to the one-pot synthesis of multifunctional nanostructures, is a more versatile method detailed in the next paragraph.