New ultrasonic assisted co-precipitation for high surface area oxide based nanostructured materials

Dereck N. F. Muche a, Flavio L. Souza *ab and Ricardo H. R. Castro *a
aDepartment of Materials Science & Engineering, University of California, Davis, CA 95616, USA. E-mail: rhrcastro@ucdavis.edu
bCentro de Ciências Naturais e Humanas (CCNH), Universidade Federal do ABC, Av. dos Estados No. 5001, Bangu, Santo André, São Paulo, CEP 09210-580, Brazil. E-mail: flavio.souza@ufabc.edu.br

Received 3rd November 2017 , Accepted 6th February 2018

First published on 6th February 2018


Owing to their enhanced properties as compared to bulk materials, the prospective applications for nanomaterials have experienced unprecedented growth, gaining attention from all levels of industry, from medical to electronics, chemistry, catalysis and mechanics. However, one of the greatest challenges of the nanomaterial industry lies in developing a production system that assures low cost and high production capabilities while maintaining quality standards. Here, we show a new method for the synthesis of metal oxide nanoparticles based on an aqueous precipitation method. The system makes use of ultrasonic probes and continuous precipitation chambers which allow it to operate continuously. Catalyst support materials, such as MgAl2O4 and γ-Al2O3, were synthesized showing high BET surface areas of 338.61 and 366.10 m2 g−1, hollow spherical morphologies and crystallite sizes as small as 3.2 and 2.1 nm, respectively.


1. Introduction

Continuous synthesis processes can enable more cost effective implementation of nanomaterials in energy solutions. Most commonly, nanoparticles are produced by chemical batch processes, which, although they are very versatile and provide fine control, present drawbacks relating to equipment downtime, reconfiguration, batch consistency and increasing production costs.1 Combining the benefits of physical–chemical routes regarding control over shape, size, size distribution and the degree of agglomeration of the nanoparticles in a reliable continuous flow process could well serve the chemical and pharmaceutical industries in reducing the cost of ‘nanotechnology’.

Sol–gel,2 chemical vapor deposition (CVD),3 plasma or flame spraying synthesis,4 laser pyrolysis5 and co-precipitation methods6,7 are among typical physical–chemical processes utilized to produce oxide nanoparticles relevant for catalysis and photocatalysis applications. Also, continuous flow and ultrasonic assisted microsystems8–10 have been proven to be versatile for use in applications where strict chemical and physical properties are required, such as in pharmaceutical applications, but still remain a challenge to scale up for applications where high production is a concern. Most of those rely on the availability of specific “metal–organic” molecules as precursors or complex apparatus which may lead to a high cost for large scale production. In this regard, co-precipitation offers fundamental advantages as it may use aqueous based reagents, simple reactors and relatively low temperatures, while providing weakly agglomerated, fine, homogeneous and uniform nanoparticles.6

Herein, we report a continuous co-precipitation method capable of fabricating oxide materials with nanoscale features and well-controlled spherical shapes. The developed process is analogous to a continuously stirred-tank reactor (CSTR) setup, where the output solution and the reacting materials are in contact inside the reactor. By making use of an ultrasonic probe injector, uniformly shaped particles of high surface area are obtained. To evaluate the versatility and quality of the nanomaterials processed using the developed reactor, different oxides were tested considering their relevance for industrial applications. Considering applications as gas sensors,11 catalysts in fuel cell technology,12–16 catalytic wet oxidation,17,18 photocatalytic water oxidation19 and many others,20–22 nanometric spheres of both pure and gadolinium doped cerium oxide were synthesized. Considering their extensive interest as catalytic supports,23,24 magnesium aluminate spinel and gamma alumina were also manufactured with an impressive surface area. To highlight the quality of the synthesized oxides careful investigation was conducted in terms of their structure, morphology and thermal evolution.

2. Experimental procedures

2.1. Synthesis by UAAC

A schematic of the Ultrasonic Atomization Assisted Co-precipitation (UAAC) apparatus is shown in Fig. 1. The proposed setup consists of an atomizing chamber with an ultrasonic nozzle working at 1.9 MHz coupled with an air inlet and a vapor outlet. The atomizing chamber is filled with the ionic solution at the right stoichiometry to produce the ‘ionic’ vapor. This vapor is composed of microsized droplets of ionic precursor solution and is carried by synthetic air directly into a 5 mm diameter tube, bubbling at a rate of 2 bubbles per second in three precipitating chambers filled with ammonia solution. The precipitation reaction happens at the interfaces between the air bubbles and the ammonia solution when the liquid micro droplets meet the air–liquid interface. The three precipitating chambers are connected in series because the reaction time is limited by the relatively short residence time of the bubbles within each individual chamber. This ‘in series’ arrangement increases the probability of reaction and increases the yield. The excess gas, which does not contain droplets any more, is released by the venting port located in the last precipitating chamber. This air may contain gas-phase co-products of the precipitation reaction.
image file: c7re00183e-f1.tif
Fig. 1 System apparatus. A schematic diagram showing the entire setup and synthesis process.

In a typical run, the precipitating chamber is filled up to 1/3 of its volume with 7 M aqueous ammonia solution. The system is kept bubbling for the desired time, in our case 30 min. During this time, the ammonia solution becomes opaque in the precipitating chamber due to the nucleation of metal hydroxides. Without turning off the setup, the precipitated slurry is purged, filtered through filter paper, gently washed with ethanol and dried at 75–80 °C for a day, while ammonia is replaced in the reactor for truly continuous production. In the test experiments, the collection of the slurry and the addition of new ammonia solution were performed every 30 min to minimize the pH fluctuations in the solution that could happen if too much hydroxide is produced, as this would potentially affect the quality of the precipitated products. The obtained precipitates are spherical in shape but very fragile due to their very thin walls. These need careful handling during filtration to minimize the collapse of the morphology, if desired.

This apparatus was used to synthesize magnesium aluminate spinel, MgAl2O4, where stoichiometric amounts of magnesium nitrate [Mg(NO3)2·6H2O, >99%] and aluminum nitrate [Al(NO3)3·9H2O, >98%] (Sigma Aldrich Inc.) at a 1[thin space (1/6-em)]:[thin space (1/6-em)]2 molar ratio were dissolved to obtain a 0.75 mol L−1 nitrate solution placed in the atomizer. The precipitated hydroxides were collected from the precipitating chambers and calcined at 750 °C for 6 h and at 800 °C for 6 h to form the desired oxide phase of spinel. In addition, undoped cerium oxide, CeO2, was also precipitated utilizing a 1 mol L−1 aqueous solution containing Ce(NO3)3·6H2O, and gadolinium-doped CeO2 was similarly produced utilizing stoichiometric amounts of Ce(NO3)3·6H2O and Gd(NO3)3·6H2O mixed in a 1 mol L−1 aqueous solution. The undoped and doped precipitates were calcined at 450 and 500 °C for 4 h, respectively. Alumina oxide nanoparticle precipitation followed the same protocol as for MgAl2O4, utilizing aluminum nitrate alone as the precursor, and calcination was carried out at 800 °C for 2 h to avoid any further coarsening and or agglomeration. This short time is possible because, unlike MgAl2O4, alumina does not require time for the diffusion of multiple components to form the spinel phase.

2.2. Characterization

X-Ray diffraction patterns from the as-collected and calcined co-precipitated powders were acquired using a Bruker AXS D8 Advance powder diffractometer (Bruker AXS, Madison, WI) (CuKα radiation, λ = 1.5406 Å) operated at 40 kV and 40 mA. The pattern profile fitting was carried out using JADE 6.1 (MDI) software to confirm the crystal structure, phase purity and crystallite size. PDF reference #211152 was used as the reference for the profile of MgAl2O4. PDF reference #340394 was used for undoped CeO2 and for Ce0.9Gd0.1O1.95, and for gamma alumina, the PDF was #100425.

Scanning electron microscopy (SEM) micrographs were acquired using a scanning electron microscope, model FEI Nova NanoSEM, operated at 10 kV in high immersion mode. The powders were gently dispersed and placed on carbon tape. The samples were sputter coated with ∼2 nm of gold prior to imaging. Nitrogen gas adsorption was performed using a Micromeritics ASAP 2020 analyzer (Micromeritics Instrument Corporation, Norcross, GA) equipped with a furnace and turbo pumps for degassing. The calcined MgAl2O4 and Al2O3 powders were degassed at 400 °C under 0.01 mmHg for 16 h. For undoped and doped CeO2 samples, the powders were degassed at 250 °C to avoid coarsening. Surface area measurements were calculated using the Brunauer, Emmett and Teller (BET) method. The reported surface area is an average of three consecutive experiments. To investigate the thermal decomposition of the as synthesized powders, differential scanning calorimetry and thermal gravimetric analysis were performed. Samples were subjected to 1200 °C, heating at 10 °C min−1 in 100 μl platinum crucibles under a synthetic dry air flow of 20 ml min−1 using a Setaram SETSYS evolution simultaneous differential scanning calorimeter and thermal gravimetric analyzer (DSC/TGA) (Setaram Instrumentation, Caluire, France).

3. Results and discussion

Fig. 2 shows the XRD diffraction patterns of the as collected hydroxides from the continuous co-precipitation process. Fairly broad peaks were observed for all studied compositions, corresponding to the respective hydroxide phases and consistent with very fine powders. The calcination temperatures to allow crystallization of the oxide phases were determined by performing thermal analysis on the hydroxide precipitates using DSC/TG analysis. Fig. 3a shows the heat flow curve for MgAl2O4. The curve is very similar to equivalent data for batch mode co-precipitated powders,25 suggesting the powder is chemically similar. It is worth noting that an endothermic heat effect at 170–180 °C (named A1) is observed and can be explained by the elimination of water and volatile organics weakly bonded at the surface of the precipitate. The following set of peaks, named B1, represents two exothermic peaks likely associated with the oxidation of organics and dihydroxylation of the hydroxides to form magnesium oxide and an amorphous aluminum oxide structure. C1 is an exothermic effect starting at 800 °C and is associated with the full crystallization of the spinel phase.22 The dihydroxylation and crystallization process for the MgAl2O4 precursor into the oxide form can be represented by the reaction equation:
 
image file: c7re00183e-t1.tif(1)

image file: c7re00183e-f2.tif
Fig. 2 XRD profiles for the hydroxide precursors and calcined powders of γ-Al2O3, MgAl2O4, CeO2 and Gd0.1Ce0.9O1.95.

image file: c7re00183e-f3.tif
Fig. 3 Thermal decomposition based on DSC/TG analysis of the precipitated hydroxide precursors for: a) MgAl2O4 and b) CeO2.

The thermal decomposition for magnesium aluminate spinel presents about 37% mass loss, with only MgAl2O4 nanoparticles presenting the final mass of the experiment. The mass loss is in good agreement with stoichiometric calculations, supporting eqn (1) with a deviation of about 3% of the initial mass (such a deviation is mainly associated with the accuracy of the TG result). γ-Al2O3 presented very similar results compared to MgAl2O4, with the exothermic heat associated to the crystallization of the γ-phase (C1) at 780 °C instead. The data is not shown here for brevity.

Fig. 3b illustrates the DSC/TG curves for the CeO2 precursor powder (as precipitated) (SEM images of the as precipitated powder are shown in the ESI). Similar to the spinel powders, the heat flow curve also exhibits an endothermic peak associated to the elimination of volatile moisture on the surfaces, peak A2, followed by peaks B2 and C2 associated to the dihydroxylation and oxidation of organics. Crystallization happens earlier compared to that with MgAl2O4, peak D2, and because it overlaps with peak C2 a ‘shoulder’ peak is observed, labeled as E2. The crystallization is then followed by coarsening, as indicated by the broad peak F2, consistent with literature on co-precipitated CeO2 powders.26 The stoichiometric calculations for the thermal decomposition analysis of the CeO2 precursor powder resulting in oxide nanocrystals are in good agreement with the reaction:

 
image file: c7re00183e-t2.tif(2)
where cerium oxide nanoparticles presented 13.3% mass loss. The results also presented a deviation of about ±3% of the measured initial mass, in good agreement with the accuracy of the TG analysis. A similar analysis was performed for Gd doped CeO2 and the DSC/TG data was statistically the same, suggesting that Gd does not affect the synthesis process and that the method can homogeneously incorporate ionic dopants.

Table 1 summarizes the calcination conditions and crystallite sizes obtained from the XRD refinement (Scherrer method) and the BET surface areas of the obtained samples, as well as literature data on common high surface area synthesis techniques for comparison. The crystallite size determined for MgAl2O4 is about 3.2 nm, and 2.1 nm for gamma alumina. For undoped and Gd-doped CeO2, the sizes are 6.0 and 6.3 nm, respectively. To the best of our knowledge, this is the smallest crystallite size and largest BET surface area ever reported for MgAl2O4, making it a useful material for catalyst supports. While in some cases, such as for Al2O3, higher surface areas have been reported, the adopted method (e.g. aerogel for alumina) is hardly scaled-up for industrial applications.

Table 1 Calcination conditions, crystallite sizes and BET surface areas of the synthesized powders and literature data for direct comparison
Methods Sample Calcination conditions Crystallite size (nm) BET surface area (m2 g−1) References
a Ligand-mediated overgrowth (quasi-spherical nanocrystal). b Organic-medium (cubic nanocrystal).
UAAC MgAl2O4 800 °C/6 h 3.2 ± 0.3 338.6 ± 0.2 This work
UAAC Al2O3 800 °C/2 h 2.1 ± 0.1 366.1 ± 1.3 This work
UAAC CeO2 450 °C/4 h 6.3 ± 0.2 100.9 ± 0.4 This work
UAAC Ce0.9Gd0.1O1.95 500 °C/4 h 6.0 ± 0.2 79.7 ± 0.2 This work
Co-precipitate MgAl2O4 800 °C/8 h 18.0 162.0 Ref. 27
Sol–gel MgAl2O4 700 °C/3 h 7.0 264.0 Ref. 28
Pechini Al2O3 650 °C/15 h Not reported 132.6 Ref. 29
Aerogel Al2O3 800 °C/2 h 431 Ref. 30
Overgrowtha CeO2 400 °C/4 h 7.0 63 Ref. 31
Hydrothermalb CeO2 1000 °C/4 h 7.0 138 Ref. 32


The morphologies of the synthesized nanometric oxides were evaluated by scanning electron microscopy (SEM) and are shown in Fig. 4. Hollow spheres were obtained as a product of this novel chemical route for all produced compositions. It is worth noting that the shell walls are relatively thin and fragile and so the process of washing/filtering must be conducted carefully to avoid any morphology damage, as is shown to have occurred for MgAl2O4 and γ-Al2O3 in Fig. 4b and c where small residues of particles can be found in open spheres. An important observation is that the sizes of the obtained spheres are consistent with the expected dimensions of the liquid droplets in atomization at the utilized frequency, suggesting that the morphology is maintained during the reaction and calcination. This implies that the wall thickness is primarily determined by the concentration of ions in the droplet, and that it is possible to control the thickness of the shell by tuning the precursor concentrations.


image file: c7re00183e-f4.tif
Fig. 4 SEM micrographs of: a) CeO2 calcined at 450 °C/4 h, b) MgAl2O4 spinel calcined at 800 °C/6 h, c) γ-Al2O3 calcined at 800 °C/2 h and d) Ce0.9Gd0.1O1.95 calcined at 500 °C/4 h.

The mechanisms of formation of the hollow spheres can be explained based on the dynamics of the co-precipitation process. In truth, precipitation occurs when ions located in the droplets are exposed to the highly concentrated ammonium hydroxide solution. Since the droplet surface is the region of contact, the surface is the first to experience a significant pH change, causing high saturation and instantaneous polynucleation of the ion hydroxides, as indicated in the following reaction:

 
image file: c7re00183e-t3.tif(3)

It is also possible that the reaction starts when the droplet meets vaporized ammonia found in the carrier gas instead. Either way, the precipitation causes a gradient of concentration of ions in solution so that the ions diffuse from the center of the droplet towards the surface of the sphere, sending the precipitation front inwards. The precipitation continues until all available ions have been consumed. As the concentration is finite the precipitation results in a hydroxide shell with the same shape and size as the droplet sphere. However, it is likely that this process causes encapsulation of the remaining water inside the sphere. This hypothesis agrees well with the experimental observations, showing holes and ruptures in some spheres and supports that during the calcination or even the drying of the hydroxide spheres, a sudden local increase of vapor pressure of the liquid entrapped in the sphere causes volume expansion and breakage. A careful filtration and drying process with a slow heating rate seems to be enough to control damage to the spheres.

The determined yield of the utilized lab-version setup was 66% in terms of the theoretical mass of the oxides (mass of oxide obtained versus mass predicted if all salt solution were transformed into oxides). The main loss in the reactor is due to the impregnation of the powder onto the wall of the tubes and surfaces of the reactor, which in a small setup, such as the one utilized here, is difficult to remove. However, in a continuous large-scale application, even though powders are accumulated on the tubes, the residues achieve a steady state and later production shows a higher yield.

4. Conclusion

The novel UAAC method is shown as a promising technique to fabricate large amounts of oxide materials in a continuous process. The principle of the technique enables the synthesis of simple and complex oxides as long as they have a suitable precipitation window, that is, the elements co-precipitate at the same pH. The produced powders showed, in general, well-defined characteristics and properties such as spherical shape, high surface area, high purity and an absence of organic contaminants as polymeric surfactants were not utilized. As an example of UAAC versatility, it was successfully applied to fabricate four different systems of technological interest. UAAC brings an entirely new concept to co-precipitation synthesis in which the possibility of a continuous reactor, eliminating the idle time typical of batch systems, brings more efficiency for industrial applications.

UAAC introduces two new approaches to allow continuous efficient co-precipitation of nanostructures: the first is the use of ultrasonic probes to inject dispersed vaporized reagents into the second reagent already in the tank. This brings the presence of more molecular surfaces for reaction since the droplets are now micron-sized and a better dispersion of the reagent, decreasing the agglomeration state of nanooxides. The second novelty is related to the continuous purge of the precipitates in suspension during the reaction. With a more controlled injection of the cationic reagent being employed, a continuous and controlled purge of the suspension from the bottom of the reactor can be carried out, and the compensation can be done with a continuous injection of the base reagent, the reagent that was present in the tank. Thus, this method is highly compatible with CSTR, requiring minor to moderate alterations in already implemented industrial plants, speaking for the suitability and readiness for industrial applications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was funded by the National Science Foundation DMR 1609781 – Ceramics. DM thanks CNPq 236631/2012-8 for the scholarship. FLS thanks FAPESP grant no. 16/02157-2.

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c7re00183e

This journal is © The Royal Society of Chemistry 2018