Evelin
Rauscher
a,
Gábor
Schuszter
a,
Bíborka
Bohner
a,
Ágota
Tóth
a and
Dezső
Horváth
*b
aDepartment of Physical Chemistry and Materials Science, University of Szeged, Rerrich Béla tér 1., Szeged, H-6720, Hungary. E-mail: atoth@chem.u-szeged.hu; Fax: +36-62-546-482; Tel: +36-62-544-614
bDepartment of Applied and Environmental Chemistry, University of Szeged, Rerrich Béla tér 1., Szeged, H-6720, Hungary
First published on 23rd January 2018
We have produced hollow copper-containing precipitate tubes using a flow-injection technique, and characterized their linear and volume growth. It is shown that the ratio of the volume increase rate to that of pumping is constant independent of the chemical composition. It is also found that osmosis significantly contributes to the tube growth, since the inward flux of chemical species dominates during the precipitate pattern formation. The asymmetric hydrodynamic field coupled with the inherent concentration and pH gradients results in different particle morphology on the two sides of the precipitate membrane. While the tubes have a smooth outer surface, the inner walls are covered with nanoflowers for copper phosphate and with nanoballs for copper silicate.
In the tube growth experiments performed in the traditional way (i.e. singular salt crystal placed in the appropriate solution), multiple, irregular hollow tubes evolve.4,5 Therefore, the resource and thus the driving force are consumed at a greater rate hindering the growth of long tubes.6 For better reproducibility, a flow-injection technique has been introduced, in which metal salt crystals are replaced with their solutions. Steinbock and co-workers have shown that below a threshold concentration, hydrodynamic plumes form in the iron(II)–sulfide system, which at higher concentrations turn into hollow tubes with distinct length and diameter.7 Concerning the dynamics of tube formation, three growth regimes have been distinguished within the oscillatory tube growth mechanism: popping, jetting, and budding.8 To achieve a better control of the tube growth, the flow-injection technique has been coupled with buoyancy-driven bubble-guidance. Such a method results in the formation of hollow, strictly regulated, straight precipitate tubes.9–12 Consequently, several decimeter long tubes with a quasi constant radius can be achieved in a reproducible way.
Since those precipitate tubes develop in a 3D fashion, their interior part cannot be investigated during the formation. Therefore, as an alternative, some growth experiments have been performed in horizontal confined geometries allowing the follow up of both the interior and exterior surfaces of the evolving membranes.13 Additionally, a zoo of different patterns, like spirals, flowers, worms, and filaments, has been found when any of the following pairs of reactants were used: cations of calcium, cobalt, copper, and nickel with silicate or carbonate anions. In the similar calcium–carbonate system, the existence of a critical concentration of reactants has been shown, above which the amount of precipitate drops due to membrane formation.14,15 Even more insight was achieved when straight membranes were produced and investigated in real time in microfluidic channels eliminating the buoyancy-driven mixing of the injected solutions.16
Besides the complex hydrodynamics of the tube growth systems, precipitation performed under far-from-equilibrium conditions shows promising possibilities. For example, precipitate growth out of a seed can lead to the formation of biomorphs in a solution with a mixture of carbonate and silicate ions in the presence of barium ions.17 It was also shown that either polymorph or crystallite type selection may be achieved by applying a simple flow-injection technique even without the usage of any additives, which draws the attention of materials science.18–20 Furthermore, the selection of appropriate conditions (type of reactants, concentration, flow rate, etc.) can result in self-sustainable precipitate tubes that are stable against time even after drying. This gives the opportunity for producing composite tubes with ingredients being, e.g., catalytically relevant. In addition, the intrinsic far-from-equilibrium synthesis conditions may result in such membrane structures that are not available otherwise. Overall, small-scaled self-assembled reactors with tailored gradient composition can be produced and engineered if the mechanism of the growth is understood.
In this context, throughout our previous studies applying the flow-injection technique, we have developed complex precipitate patterns growing vertically or horizontally in the copper–phosphate system.21 In this work, we focus on the regime where hollow precipitate tubes form and quantitatively describe their growing properties emphasizing the role of osmosis. The microstructure of the synthesized copper phosphate is also characterized and compared to that of copper silicate produced under similar conditions.
To avoid the presence of any additional ions while adjusting pH, the different protonation states were maintained by using solutions of trisodium phosphate, disodium hydrogen phosphate, and sodium dihydrogen phosphate. Solutions were prepared from analytical grade solid reagents: Na3PO4·12H2O (Molar, Sigma), Na2HPO4·12H2O (Reanal), NaH2PO4·2H2O (Fluka), and CuSO4·5H2O (Sharlau) using deionized water as solvent. Technical-grade sodium silicate, in aqueous solution, was purchased from VWR. All chemicals were used as received.
Since buoyancy obviously plays an important role when chemical garden tubes evolve, the density of each solution was repeatedly measured using an Anton Paar DMA 500 Compact Digital Density Meter with 10−4 g cm−3 precision.
When copper sulfate solution is pumped into such phosphate solution, three distinct growing types of blue copper phosphate precipitate can be identified depending on the concentrations (and thus the density distribution) of the reactants. According to the constructed phase diagram shown in Fig. 1, only short tubes are produced with low copper and high phosphate concentrations (green squares on the left side of Fig. 1). In this region of the phase diagram, the outer electrolyte has greater density than the injected one, which prompts an enhanced upward flow due to the presence of buoyant forces (see the negative Δρ values in Table 1) that overcome the strength of the membrane. Therefore, despite the constant injection rate, precipitate vesicles form periodically at the tip of the tubes, which then pop off and rise to the surface. This tube growth mechanism is called popping.8
Δρ (g cm−3) | [CuSO4] (M) | |||||||
---|---|---|---|---|---|---|---|---|
Δρ = ρ(CuSO4) − ρ(Na3PO4) | 0.1 | 0.2 | 0.3 | 0.4 | 0.5 | 0.6 | 0.7 | |
[Na3PO4] (M) | 0.10 | — | −0.025 | −0.0093 | 0.0064 | 0.0225 | 0.0372 | 0.0524 |
0.15 | — | −0.015 | 0.0007 | 0.0164 | 0.0325 | 0.0472 | 0.0624 | |
0.20 | — | −0.006 | 0.0097 | 0.0254 | 0.0415 | 0.0562 | 0.0714 | |
0.25 | −0.0126 | 0.0033 | 0.0190 | 0.0347 | 0.0508 | 0.0655 | — | |
0.30 | −0.0036 | 0.0123 | 0.0280 | — | — | — | — |
In the opposite case, i.e. with low phosphate and increasing copper concentrations (blue triangles in the lower right part of Fig. 1), the inflowing copper sulfate is denser, hence it tends to advance under the phosphate solution. Precipitation follows this fluid flow, therefore the pattern spreads on the bottom of the dish. At these concentrations, acidic copper sulfate is in stoichiometric excess, thus phosphate ions are protonated to a larger extent in the solution. Those ions favor the formation of separate crystals instead of resulting in amorphous membranes.
Wedged between those two regions lays the concentration range (for a given flow rate) where hollow tubes grow at a steady rate (see the red circles in Fig. 1). In this zone, the higher reactant concentration ensures that the evolving membrane is sufficiently strong to sustain the interior pressure arising due to the combination of injection and buoyancy. Therefore, the membrane breaks up only at its tip and a contiguous precipitate tube forms.
While keeping the reactant concentration and pH unchanged, the tube growth can also be affected by varying the rate of inner electrolyte supply. The experimental results presented in Fig. 1 correspond to a fixed injection rate (w = 0.71 mL h−1) applied for 10–15 min. Such a slow flow leads to the development of 5 cm-long tubes, while a greater flow overcomes the mechanical stability of the membrane. As a consequence, they become tilted or even break regularly resulting in only 2–3 cm long, stable tubes. Therefore, the tube length clearly depends on the flow rate as well.
One may expect that the growing speed of the tubes is also a function of the injection rate. However, inspection of the linear growth rate for tubes with constant reactant concentration demonstrates that it is independent of the injection rates for tubes (see Table 2). Hence the linear growth rate is found to be 30.6 ± 1.8 cm h−1 while the injection rate is multiplied more than three times (raised from 0.4 to 1.3 mL h−1). It is worth noting that for precise measurements, in the case of tilted tubes, the data set is always corrected with the appropriate tilt angle. Furthermore, it is crucial to point out that this vertical tube growth occurs at conditions where the inflowing copper solution has greater density than the phosphate one (see the corresponding positive Δρ values in red in Table 1). Buoyancy counteracts the upward growth, therefore an additional contribution to the tube growth mechanism must be active in the precipitate formation. The volume increase rate of a single tube can be defined as the product of linear growth rate and tube cross section. The latter can be obtained from the measured tube diameter assuming a circular geometry. The volume increase rate of the tubular precipitate is found to be proportional to the flow rate for any reactant concentration applied (see Fig. 2). At a given time, the volume of a tube is found to be ca. twice that of the copper sulfate solution pumped in, suggesting that the wall of the freshly formed tube is permeable to the electrolyte and/or water molecules, and hence a significant amount of the outer electrolyte is engulfed by the closure of the membrane. Linear regression on volume increase rate with respect to pumping rate provides an average slope of 1.89 ± 0.05 taking into account all the measurements with the various reactant concentration pairs. This reveals the presence of inward osmotic flow through the wall with membrane-like properties, since in our earlier work we have shown that the region of steady tube growth dominantly falls in the range where the osmotic pressure of copper sulfate solution is greater than that of sodium phosphate.21 With the formation of stiff and rigid walls, tubes would grow with a volume increase rate similar to that of pumping. Therefore, the ratio would be closely equal to one indicating little contribution from other processes taking place in the growth.
w (mL h−1) | v l (cm h−1) |
---|---|
0.40 | 28.7 ± 0.9 |
0.72 | 30.6 ± 1.2 |
0.96 | 33.5 ± 0.6 |
1.19 | 31.5 ± 2.4 |
1.43 | 29.0 ± 2.5 |
The linear growth rate of copper silicate tubes was determined to be vl = 50.4 ± 5.4 cm h−1 and it was found to be independent of the flow rate, similar to that of copper phosphate (vl = 30.6 ± 1.8 cm h−1), although it is greater for silicate. Consequently, phosphate tubes evolve with larger diameter than the silicate ones. However, the volume increase rates are found to be equal within experimental errors for a set pumping speed. The ratios of volume increase rate with respect to that of pumping also match well: 2.0 ± 0.2 and 2.1 ± 0.2 for silicate and phosphate, respectively, for this particular concentration. Since the tube radius is constant, the volume growth rate can be expressed as vV = R2πvl. Taking into account that the volume increase rate is the same for the two systems, we deduce that the linear growth rate is inversely proportional to the square of the tube radius, i.e., vl,Si/vl,P = (RP/RSi)2. Hence, the surface increase rate rA will be inversely proportional to the tube radius as rA,Si/rA,P = RP/RSi = 1.29 ± 0.08. Both phosphate and silicate tubes form semi-permeable membrane walls at their tips that allow osmosis to contribute to the sufficiently slow flow. These relations suggest that the walls of the formed copper phosphate tubes are more porous than those of copper silicate, since the same amount of outer liquid is enclosed in the hollow precipitate structure yet their surface area is smaller.
In the reference case, amorphous precipitates form, as depicted by the SEM images. With the spatially separated reactants of the flow-injection technique, the precipitate forming in the mixing zone possesses different face structures due to the presence of concentration gradients. In the case of phosphate, the surface of the outer wall is smooth (see Fig. 3A and C) while the inner wall has complex morphology corresponding to a greater surface area.
It can be seen in Fig. 3B for one set of concentrations that flower-shaped particles are formed with 20 μm size at a lower flow rate. Keeping the concentration unchanged but increasing the injection rate leads to a morphology change as 7–9 μm long needle-shaped crystals grow on the inner faces of the tubes (Fig. 3D). The flower structure is robust as it also covers the inner wall of the tubes obtained under different conditions. For example, when 0.4 M copper solution is flown into 0.2 M phosphate solution, those flowers evolve with an increased size of 25 μm. Although the structure mentioned is not in the nanometer range, the term nanoflower has been assigned to similar flower petal-shaped, micrometer-sized particles having nanoscale features.23 They can also develop in a phosphate-buffered copper sulfate solution containing bovine serum albumin22 or in cellulose stearoyl esters.23 Besides the organic and inorganic ones, protein–inorganic hybrid nanoflowers can also be synthesized.24 The enlarged surface area of copper phosphate has been shown to enhance catalytic activity, as demonstrated in the dehydration reaction of fructose to 5-hydroxymethylfurfural.25
Copper silicate tubes also exhibit a smoother outer surface (cf.Fig. 3A, C and 4A), however, unlike copper phosphate, their inner surface has amorphous character and is covered with compact spherical structures, as shown in Fig. 4B–D. Here again, the size of those spheroids contributing to the fine interior structure significantly depends on the flow rate. At w = 0.39 mL h−1, spheres with d ≈ 1 μm are formed, while at w = 1.41 mL h−1, the average diameter of the particles increases to 15 μm.
Due to the presence of concentration gradients across the wall, the inner and outer surfaces have distinctly different microstructures. The outer electrolyte solution is significantly basic, therefore copper hydroxide together with basic copper phosphate or silicate form, which favor an amorphous structure. Hence, both the phosphate and silicate tubes have a smooth amorphous outer surface, while the inner wall structure of the phosphate has significantly greater surface area covered with nanoflowers unlike the silicate tubes or the phosphate precipitate formed in a well-stirred batch system. The flow-injection technique hence represents a tool in synthesizing hollow precipitate structures with delicate inner surfaces without the application of various additives.
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