Study of the morphology of copper hydroxynitrate nanoplatelets obtained by controlled double jet precipitation and urea hydrolysis
Introduction
Nanocomposite materials cover a very large variety of organic–inorganic hybrids. A nanocomposite is characterized by a high degree of dispersion of inorganic filler, to a nanoscopic level, in a polymer matrix. This can be done either by mixing them together at a temperature above the melting of the polymer, or by using an aqueous suspension of the inorganic filler in the case of emulsion polymerization. Besides, many studies have been recently devoted to the development and characterization of new nanocomposites exhibiting anisotropic morphologies [1], [2], [3], [4], [5], [6], [7]. Specific mechanical and physical properties are expected when the filler particles are not spherical but elongated in one dimension (fibers) or two dimensions (platelets) [8], [9]. The interest of composites containing nanoplatelets is mainly due to the improvement of their mechanical reinforcement properties as well as “barrier properties” that are supposed to play a significant role in fire resistance of such composites [10]. In this regard, efforts to control the morphology and agglomeration behavior of inorganic copper hydroxynitrate nanoplatlets are of interest.
Moreover, while inorganic cation exchangers, including layered silicates or transition metal oxysalts, are well represented in the literature, their anion exchangers counterparts have received fewer attention [11]. As an example, copper (II) phenylphosphonate can be successfully obtained by intercalation of the phosphonate ion, using the layered copper hydroxynitrate as the host material [12]. The main current application of copper hydroxynitrate deals with its use as an oxidizer in non-toxic, low-ash solid propellant and ignition compositions, especially for inflation of vehicle airbags [13] that gave rise to several patents. On a morphology control point of view, the study of inorganic chemistry of aqueous copper basic salts is of paramount importance in the scope of coprecipitation methods for synthesizing precursors of copper-containing mixed oxide ceramics, in particular, the superconducting phases [14], [15].
We present in this paper some recent results about the influence of the precipitation conditions on the morphology of Cu-layered double hydroxide (LDH)-type nanoplatelets obtained by controlled double jet precipitation (CDJP) [16], [17], [18], [19], [20] and by hydrolysis of urea [21], [22]. The dispersability of the powders was also studied in aqueous suspension.
The Cu2(OH)3NO3 structure has been recently re-examined by Guillou et al. [23] and belongs to the P21 space group with z=2; the crystal system is monoclinic with β=94.619° and the cell parameters a=5.6005 Å, b=6.0797 Å, c=6.9317 Å at 25°C. The structure can be viewed as layers of Cu octahedra stacked upon each other. Two types of copper octahedra exist: Cu(1) atoms are coordinated by four OH−groups and two oxygen atoms belonging to NO3− groups. The Cu(2) atoms are coordinated by four hydroxyls, the fifth OH− standing a bit further and an oxygen atom belonging to a NO3− group. The copper octahedra form layers of stoichiometry [Cu2(OH)3]+. Consequently, NO3− ions stand between the positive layers for charge balancing and are linked by hydrogen bonding to the hydroxyl groups belonging to the copper octahedra layers (Fig. 1). Due to its layered structure, Cu2(OH)3NO3 is expected to crystallize in platelet-shaped particles.
While working on the preparation of uniform colloidal copper oxide by CDJP, Lee and Matijevic [24] have observed that the stoichiometry as well as the morphology of the particles obtained by reaction between NaOH and Cu(NO3)2 depend on the [NaOH]/[Cu(NO3)2] molar ratio. For a molar ratio comprised between 1.0 and 1.5, a copper hydroxynitrate Cu2(OH)3NO3 is precipitated as hexagonal platelets. When this molar ratio is increased above 1.5, CuO ellipsoids are formed and CuO needles appear if the ratio is increased further above 2.0. The temperature also plays a role in the morphology and chemical composition of the end-product: hexagonal platelets of copper hydroxynitrate do not form anymore at a temperature higher than 50°C. The main drawback of this technique is the very low concentration of solid in the suspension after complete addition of the reactant solutions—around 1 g/l. In order to improve the yield of this batch synthesis while keeping the desired size and morphology of the particles, we have tried to increase the concentration of the reactant solutions as well as their rate of addition into the reactor. Besides, a new continuous precipitation reactor has been designed in an attempt to bypass the problem of high dilution of the product. Finally, some trials have been made concerning the addition of growth modifiers that are expected to adsorb preferentially on certain crystal planes of the growing particles and therefore inhibit the growth in such direction, yielding more anisotropic particles.
Alternately to the precipitation technique, copper hydroxynitrate can be obtained by reacting copper nitrate and urea [21], [22]. Urea will act as a reservoir for hydroxyl ions in solution, since these are released along the whole process by hydrolysis of urea upon heating. Krathovil et al. [21] have studied the influence of urea concentration and copper nitrate concentration onto the particles morphology obtained at 90°C after 2 h of maturation in solution. They have observed that platelet-like particles are not formed for every copper nitrate concentrations but are obtained in a narrow range of solution composition.
Section snippets
Double jet precipitation
The synthesis of Cu2(OH)3NO3 have been performed following the conditions described by Lee and Matijevic [24]: the reaction takes place between copper nitrate and sodium hydroxide (Eq. (I)):Copper nitrate has to be maintained in excess in order to prevent the formation of copper hydroxide (Cu(OH)2) or copper oxide (CuO): the [OH−]/[Cu2+] molar ratio is thus adjusted to 1. The final pH of the suspension is weakly acid, around pH 5. The “classical conditions”
Double jet precipitation
Several parameters have been studied: type of reactor (samples 1 and 2), flow of reactant solutions (samples 1, 3–7), concentration (samples 1 and 8) and addition of a growth modifier (samples 9 and 10). All synthesis conditions are summarized in Table 1, Table 2.
Conclusions
From the results that have been described above, some guidelines can be proposed concerning the production of copper hydroxynitrate by a precipitation reaction starting from sodium hydroxide and copper nitrate solutions. The flow of injection into the CDJP reactor does not affect the size nor the morphology of the particles, at 25°C as well as at 40°C. Experiments related to the concentration of the reactant solutions have shown that the size of the particles depends strongly on the molal
Acknowledgements
The authors acknowledge Dr. C. Vogels, SOLVAY S.A., for financial support and valuable scientific discussions.
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