Nucleation and Growth of Cobalt Oxide Nanoparticles in a Continuous

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Crystal Growth & Design

Nucleation and growth of cobalt oxide nanoparticles in a continuous hydrothermal reactor under laminar and turbulent flow Clément J. Denisa, Christopher J. Tigheb, Robert I. Gruara, Neel M. Makwanaa and Jawwad A. Darra,* a) Department of Chemistry, University College London, 20 Gordon Street, London, WC1H 0AJ, United Kingdom

b) Department of Chemical Engineering, Imperial College London, South Kensington, London, SW7 2AZ, United Kingdom

Cobalt oxide (Co3O4) nanoparticles were synthesized from aqueous solutions of cobalt(II) acetate using a laboratory scale continuous hydrothermal flow synthesis reactor incorporating a confined jet (co-axial) mixer. By changing the concentration of the precursor combined with operating under flow rate conditions expected to result in a laminar or turbulent mixing, the size of the crystallites could be controlled in the range of 6.5 to 16.5 nm (median). A quench stream was employed to rapidly cool down the nascent stream of nanoparticles and to elucidate the mechanisms of nucleation and growth. The results show a clear correlation between increasing precursor concentration and crystallite size, which at lower concentrations in particular decreased in laminar flow and increased in turbulent flow. The smallest particles of 6.5 nm (median) were produced at a precursor concentration of 0.1 M (at a rate of 20 g.h-1). The materials were characterized using a range of analytical methods including powder X-ray diffraction and transmission electron microscopy.

Figure. Parameters of the size distribution (D10, D50 and D90) of cobalt oxide nanoparticles vs. precursor concentration (logarithmic scale). D10, D50 and D90 are the tenth, fiftieth and ninetieth percentile of the size distribution, respectively.

*

Prof Jawwad A. Darr [BSc, DIC, PhD, MRSC]

University College London, Department of Chemistry, 20 Gordon Street, London, WC1H 0AJ, United Kingdom

ACS Paragon Plus Environment Telephone: +44(0)207 679 4345 E-mail: [email protected]

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Nucleation and growth of cobalt oxide nanoparticles in a continuous hydrothermal reactor under laminar and turbulent flow Clément J. Denisa, Christopher J. Tigheb, Robert I. Gruara, Neel M. Makwanaa and Jawwad A. Darra,* a) Department of Chemistry, University College London, 20 Gordon Street, London, WC1H 0AJ, United Kingdom b) Department of Chemical Engineering, Imperial College London, South Kensington, London, SW7 2AZ, United Kingdom

Cobalt oxide (Co3O4) nanoparticles were synthesized from aqueous solutions of cobalt(II) acetate using a laboratory scale continuous hydrothermal flow synthesis reactor incorporating a confined jet (co-axial) mixer. By changing the concentration of the precursor combined with operating under flow rate conditions expected to result in a laminar or turbulent mixing, the size of the crystallites could be controlled in the range of 6.5 to 16.5 nm (median). A quench stream was employed to rapidly cool down the nascent stream of nanoparticles and to elucidate the mechanisms of nucleation and growth. The results show a clear correlation between increasing precursor concentration and crystallite size, which at lower concentrations in particular decreased in laminar flow and increased in turbulent flow. The smallest particles of 6.5 nm (median) were produced at a precursor concentration of 0.1 M (at a rate of 20 g.h-1). The materials were

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characterized using a range of analytical methods including powder X-ray diffraction and transmission electron microscopy.

1. INTRODUCTION Cobalt oxides have a wide range of applications as supported catalysts for the Fischer-Tropsch process1, supercapacitor electrodes2,3, anodes for Li-ion batteries4,5 water oxidation catalysts6,7, magnetic materials8,9 and in biomedicine10,11. The phase, crystallite size, size distribution, crystallinity faceting and specific surface area of these materials at the nanometer scale are strongly correlated to their fundamental properties and their potential applications. For example, as a Fischer-Tropsch catalyst, cobalt oxide particles < 10 nm have better turnover and lower reduction temperatures than larger particles12 (> 20 nm) as well as improved catalytic activity1,13. Similarly, the surface area of an electrode of a supercapacitor greatly influences its performance14. Therefore, there is a significant amount of interest in the ability to control the particle size of cobalt oxides below 10 nm, in a scalable manufacturing process. The synthesis routes for cobalt oxides nanoparticles such as CoO and Co3O4 include solid state synthesis15, soft chemistry16, gas phase synthesis17, thermal decomposition18 and hydrothermal synthesis19-22. Some of the aforementioned synthesis methods are not able to consistently deliver small size nanoparticles with a narrow size distribution using a process that is readily scalable. For example, a synthesis method yielding very small (≥ 3.5 nm) cobalt oxide nanoparticles was developed by Xu and Zeng23 but it is very long (> 72 hours) and requires the use of surfactants. Consequently, there is a need to develop green, rapid, inexpensive and scalable routes for the synthesis of small cobalt oxide nanoparticles. For the synthesis of inorganic nanoparticles, the continuous hydrothermal method, first reported by Adschiri et al.24, offers many advantages over batch or other synthesis processes by


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combining independent control over reaction parameters (pressure, temperature, concentration, reaction time) and consistency of product, commensurate with a continuous process and the ability to produce a range of nanoparticle compositions in a short time25-28. Crystallite size control using any hydrothermal synthesis method is commonly achieved by changing the concentration or counter-ion of the precursor (aqueous metal salt), temperature of the reaction, pH, mixing regime, or through the addition of dopants29-31. Surfactants are also often used to alter the particle size, morphology and surface properties of products8,32. In the continuous hydrothermal flow synthesis (CHFS) process, nanoparticles form when a flow of aqueous metal salt is mixed with supercritical (or superheated) water, typically at up to 450 °C and 24 MPa. This results in the rapid hydrolysis of the metal salt in the mixture (at up to 335 °C) immediately followed by dehydration, thus forming the oxide33. The rapid crystallization of nanoparticles is facilitated by the change in the properties (density, dielectric constant, ion product) of water under supercritical and sub-critical conditions, which affect the solubility of species in solution, thus forcing precipitation. Even though the temperature of the mixture is in the sub-critical region, the water properties still allow for rapid nucleation of nanoparticles30,31. Some of the authors have developed a confined jet mixer34, in which the precursor is entrained in a jet of supercritical water, enabling rapid, momentum-driven mixing while minimizing undesirable pre-heating of the incoming precursor and restrictions in the flow of the nanoparticle-laden product, where blockages can otherwise occur35. Another feature of this configuration is that mixing improves when the flow of supercritical water is increased above the flow rate of the precursor (so-called unbalanced flows) to produce a hotter reaction temperature, which can yield a different and more desirable crystalline phase of the oxide without the need for


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a heat treatment21. This compares to e.g. the buoyancy driven counter-current reactor36 where such unbalanced flows have been shown to result in undesirable penetration of the supercritical water into the oncoming flow of precursors37. The computational fluid dynamic simulations of Ma et al.

35,38,39

suggest that mixing was complete within a distance of 5 inner pipe diameters

downstream of the outlet of the supercritical water, corresponding to a residence time of 10 MΩ.cm). A schematic of the three-feed CHFS process is presented in Fig. 1; a similar system has been described in greater detail elsewhere34. Briefly, three identical high-pressure diaphragm pumps P1 to P3 (Primeroyal K, Milton Roy, Pont-Saint-


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Pierre, France) were used simultaneously, providing three pressurized feeds as 24 MPa: 0.3 wt.% hydrogen peroxide in DI water at a flow rate of 50 to 80 mL.min-1 (P1), cobalt(II) acetate solution at a flow rate of 50 to 80 mL.min-1 (P2) and DI water at a flow rate of 160 mL.min-1 (P3), used as a quench stream in a subset of the experiments. The flow of 0.3 wt. % aqueous hydrogen peroxide from P1 was heated to 450 °C using a 7 kW electrical heater. This hot stream was mixed with the flow of aqueous cobalt acetate at room temperature in a confined jet mixer, shown in the lower part of Fig. 2, where the nanoparticles were formed.

Figure 1. Simplified flow diagram of the CHFS process. When no quench was employed, the nanoparticles formed in the confined jet mixer were cooled down to room temperature using a pipe-in-pipe countercurrent heat exchanger before passing through a backpressure regulator (BPR, Tescom, Elk River, USA) at the outlet of the process, which maintained the pressure at 24 MPa. The slurry of nanoparticles was collected in a beaker for further separation and washing.


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For the experiments in which a DI water quench was used, a second mixer was added between the confined jet mixer and the heat exchanger, as shown in the upper part of Fig. 2. In the quench mixer, the hot nanoparticle slurry from the confined jet mixer entered the inner pipe and DI water at room temperature flowed up through the annulus at 160 mL.min-1. This diluted the Co3O4 nanoparticle slurry, which as a result was rapidly cooled from 335 °C to < 180 °C before further cooling to room temperature in the counter-current heat exchanger.

Figure 2. Confined jet mixer (lower part) with quench downstream (upper part). Table 1 gives the dimensions of four different sizes of confined jet mixer and quench used in the experiments. The Reynolds numbers of the flow through the outlet pipe of the confined jet mixers were determined by:

Remix =

ρ mixu mix d µmix


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where ρ mix , umix and µmix are the density, velocity and dynamic viscosity, respectively, of the

Inlet pipe O.D. (inch)

Outlet pipe O.D. (inch)

Residence time (s)

Flow rate inlet (mL.min-1)

Flow rate precursor or quench water (mL.min-1)

Remi

Turbulent mixer

3/16

3/8

~ 10a

80

80

6900

Quench after turbulent mixer

3/8

3/4

~ 1b

160

160

3300

Laminar mixer

1/8

3/4

~ 10a

50

50

2000

3/4’

3/8

~ 3b

100

160

4600

Quench mixer

after

laminar

x

mixture and d is either the inner diameter of the outlet from the confined jet mixer or the quench.

Table 1. Mixer dimensions, flow rates and Remix used in the synthesis of cobalt oxide nanoparticles.

a

= time before the mixture enters cooler

b

= time before the slurry meets the

quench feed. The physical properties of the mixture were assumed to be those of pure water, determined using the model of Wagner and Pruß46. From Table 1, the lowest Reynolds number at the mixer, Remix= 2000, classically would be considered to be on the boundary of the transition from laminar to turbulent flow. In fact, the presence of nanoparticles in suspension has been found to increase the Reynolds number of this transition47,48. Consequently, this is referred to as the “laminar mixer” whilst the geometry giving Remix = 6900 is referred to as “turbulent mixer”. These terms refer to the flow after the superheated water and precursor have mixed. The feed of supercritical water was highly turbulent in both geometries (Rehot ~ 26000 for both mixers). Complete mixing of the hot water and precursor streams is therefore expected to occur very rapidly in both mixing geometries. Further details about the mixer geometries used for this work are provided in the supplementary information.


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The nucleation and growth of cobalt oxide was studied in the laminar and turbulent mixer, with and without a water quench downstream of the confined jet mixer, giving four unique flow configurations. Experimental runs using each of these four configurations comprised six precursor concentrations increasing from 0.005 M to 0.2 M. 2.2. Purification of nanoparticles. Nanoparticles were recovered from solution by flocculating the aqueous suspension obtained from the CHFS process with 50 g.L-1 of sodium chloride. Samples were allowed to settle overnight, decanted into 50 mL centrifuge tubes then centrifuged (Sigma 4-15, Sigma, Osterode am Harz, Germany) at 4300 RCF for 5 minutes. The concentrated sediment was re-dispersed in 50 mL of DI water and transferred to dialysis membrane tubing (Medicell International Ltd, London, UK) where it was dialyzed against DI water as a buffer. This water was replaced regularly until the conductivity of the dialate reached values < 50 µS.m-1, as measured by a HI98311 dissolved solid analyzer (Hanna Instruments, Woonsocket, USA). The cleaned wet solids were freeze-dried in a Vitris Genesis 35 XL Freeze Drier (SP Scientific, New York, USA) by first freezing to -40 °C followed by freeze-drying at 60 °C for ca. 23 hours at 3.10-7 MPa. The yield of the reaction was measured in two ways: weighing the solid and quantitative absorbance spectroscopy of the supernatant. The mass yield of the reaction was determined by comparing the recovered mass of dried powder against the theoretical yield if all the cobalt acetate was converted to cobalt oxide. Because of the nature of the cleaning procedure, an error in the mass yield would be expected due to loss of product. This is especially important for lower concentration samples for which only small amounts were collected. The absorbance yield was determined by measuring the absorbance (UV-2401 PC, Shimadzu Corporation, Kyoto, Japan) of the supernatant after centrifugation of the flocculated samples to determine the concentration of


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unreacted precursor left in solution. The absorbance yield was determined by comparing the absorbance at 502 nm of the product with a set of cobalt acetate reference solutions. This method measures the concentration of unreacted precursor and assumes that there are no byproducts influencing the absorbance of the solution at the set wavelength. It also assumes that cobalt acetate will solely react to form cobalt oxide. The pH of the samples as produced by the CHFS reactor was measured with a pH probe (sympHony, VWR, Radnor, USA). 2.3. Characterization of nanoparticles. Powder X-ray diffraction (XRD) patterns were collected using a STOE StadiP diffractometer in transmission mode (STOE, Hilpertstraße, Germany) employing a Molybdenum source (Mo Kα1, λ= 0.7107 Å) followed by a Germanium(111) monochromator and a DECTRIS Mythen 1k silicon strip detector (DECTRIS, Baden, Switzerland). Yttria (Y2O3) was used as the standard for estimation of instrumental peak broadening. Samples were prepared for Transmission Electron Microscopy (TEM) by dispersing the particles ultrasonically in methanol (Sigma-Aldrich, Dorset, UK) and dropping onto Holey carbon film grids (300 mesh, Agar Scientific, UK). A JEOL JEM-2100 Transmission Electron Microscope (200 keV accelerating voltage) was used for generating TEM micrographs and images were captured using a CCD camera (GATAN). Measurements of particle size and determination of the circularity of the 2D projections of the nanoparticles were carried out using the freely available ImageJ software (http://imagej.nih.gov/ij/). Measurements were made on 300 individual nanoparticles to give a statistical distribution. The tenth, fiftieth and ninetieth percentile values (D10, D50, D90, respectively) were employed as indicators of particle size distribution. Circularity, C, was determined by:


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A C = 4π 2 P where A and P are the area and perimeter, respectively, of the 2D projection of the nanoparticle. The diameter of an equivalent sphere of perimeter P was selected as the characteristic size for comparing C as function of particle size.

3. RESULTS AND DISCUSSION 3.1. Synthesis. Table 2 lists the experimental conditions, pH of the product along with mass and absorbance yield. The products from all experiments were acidic. This is due to the formation of acetic acid during the hydrolysis of cobalt(II) acetate into cobalt hydroxide, which is the first step in the hydrothermal synthesis of Co3O4:

Co(CH3COO)2 + 2H2O → Co(OH)2 + 2(CH3COOH) The product from the laminar mixer tended to be slightly more acidic than the product from the turbulent mixer for the same precursor concentration. The yield, as determined by both mass and absorbance methods, tended to be slightly higher for the laminar mixer than the turbulent mixer both with and without the addition of the quench. Therefore a higher concentration of acetic acid was produced from the same concentration of precursor and the pH of the product from the laminar mixer was typically 0.2 – 0.6 points lower than the product from the turbulent mixer. Without the quench, the pH of the product decreased with increasing precursor concentration for both the laminar and turbulent mixing. This is to be expected because more acetic acid and Co3O4 is formed in the reactions with increasing precursor concentration. When the quench was added to the outlet of the turbulent mixer, the pH increased by 0 - 0.4 points for all precursor concentrations; this equates to a 2.5 decrease in [H+]. This is reasonably


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consistent with a 1:1 dilution by the quench, which should have halved [H+] and increase pH by 0.2 points. When the quench was added to the outlet of the laminar mixer, the pH increased by 0.4 – 0.7 points for precursor concentrations ≤ 0.05 M – a factor of up to 6 decrease in [H+]. This is reasonably consistent with a 1:1.6 dilution by the quench, which should reduce [H+] by a factor 3 and increase pH by 0.4 points. However for the laminar mixer the pH increases considerably on addition of the quench by 1.1 and 1.6 points (reduction of [H+] by factor of 12 and 40) for precursor concentration of 0.1 and 0.2 M respectively. This must be because in this case, the product from the laminar mixer was still undergoing reaction when the quench was added, which was stopped in its tracks by dilution and cooling. This is supported by the significant decrease in yield on addition of the quench, as measured by both the mass and absorbance methods. A similar decrease in mass and absorbance yield was observed when the quench was added to the product from the turbulent mixer for concentrations > 0.1 M, although in this case it was not associated with a significant increase in pH.


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[Co] (mol. L-1)

Mass Yield (%)

Abs. Yield (%)

pH

No Quench/ Quench

No Quench/ Quench

No Quench/ Quench

71 / 65

78 / 81

4.0 / 4.4

0.01

74 / 68

90 / 86

3.9 / 4.1

0.025

66 / 81

88 / 88

3.8 / 3.8

0.05

89 / 75

92 / 92

3.5 / 3.7

0.1

78 / 85

94 / 91

3.4 / 3.5

0.2

91 / 71

95 / 85

3.2 / 3.4

75 / 62

96 / 90

3.5 / 4.0

0.01

63 / 69

92 / 89

3.5 / 3.9

0.025

60 / 79

92 / 92

3.2 / 3.9

0.05

75 / 77

95 / 85

3.2 / 3.8

0.1

90 / 80

97 / 71

3.1 / 4.2

0.2

75 / 52

98 / 78

3.0 / 4.6

0.005

0.005

Mixing regime

Turbulent

Laminar

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Table 2. Experimental conditions, yield and product pH of cobalt oxide nanoparticles from CHFS process. 3.2. Crystallography. Figures 3 and 4 are powder X-ray diffraction (XRD) patterns of the samples listed in Table 2. All the patterns demonstrated a phase pure Co3O4 cubic spinel structure (similar to PDF reference pattern 01-076-1802). The Miller indices of each reflection are noted above the reference pattern. Figure 3a) and b) show the XRD patterns for the set of reactions using the turbulent mixer without and with a water quench, respectively. It can be seen by comparison that the crystallinity of all samples appeared to be lower when the quench was employed, as indicated by the lower intensity of the diffraction peaks. This suggests that


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although the reaction forming Co3O4 appeared to have stopped, as discussed in the previous section, crystallization of the nanoparticles was still occurring when the quench was added.

Figure 3. XRD patterns of the cobalt oxide samples produced on the CHFS process under turbulent mixing conditions. Figure 4a) and b) show the XRD patterns for the set if reactions using the laminar mixer with and without the water quench, respectively. For the samples synthesized with a precursor concentration < 0.1 M, the crystallinity of the samples remained similar whether or not the quench was used. At concentrations ≥ 0.1 M, a decrease in crystallinity was observed when the quench was used with the laminar mixer. This indicates that for precursor concentrations < 0.1 M both reaction and crystallization had stopped when the quench was added. In contrast, at


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concentrations ≥ 0.1 M, the product from the laminar mixer was still undergoing both reaction and crystallization.

Figure 4. XRD patterns of the cobalt oxide samples produced on the CHFS process under laminar mixing conditions. 3.3. Electron microscopy. The TEM images, exemplified by Fig. 5, often revealed cubic nanoparticles with a distribution of sizes mixed with nanoparticles which appeared to have morphologies closer to that of a sphere. It was observed that the spheroids tended to be smaller than the cubes.


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Figure 5. TEM micrograph of Co3O4 nanoparticles synthesized at precursor concentration of 0.2 M under laminar conditions with the quench, showing a range of particle sizes and morphologies. Figure 6 shows the circularity of the 2D projections of the nanoparticles plotted against the equivalent sphere diameter. It can be seen that as particles get larger the circularity tended to reduce from 0.92 (a spheroid) to 0.78 (a square); this suggests that the nanoparticles nucleate as spheres before undergoing anisotropic growth to form cubes.


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Figure 6. Circularity of nanoparticles synthesized at a concentration of 0.2 M using a laminar mixer with quench as a function of equivalent sphere diameter. Additionally, the predominant morphology of each sample produced was dependent on the average particle size (and therefore amount of growth). Samples with larger crystallites tended to consist more of cuboids, as shown in Fig. 7.

Figure 7. TEM micrograph of Co3O4 nanoparticles synthesized using the turbulent mixer with the quench at precursor concentrations of 0.005 (a), 0.01 (b), 0.025 (c), 0.05 (d), 0.01 (e) and 0.2 M (f).


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Figure 8 is a high resolution TEM image of a cubic Co3O4 nanoparticle. The lattice fringes show a d-spacing of 0.28 nm, which corresponds to the (222) plane of cubic spinel Co3O4

Figure 8. High resolution TEM micrograph of a cobalt oxide nanoparticle synthesized at 0.05 M with the laminar mixer with the use of the quench.

3.3. Turbulent mixing. Figure 9a - c) are histograms showing the particle size distributions of selected samples produced under turbulent conditions with and without the quench water. For concentrations of 0.005 M and 0.05 M, as shown in Fig. 9a) and b), the introduction of the quench did not cause the particle size distribution to become significantly narrower, but slightly reduced the average size. At a precursor concentration of 0.2 M, as shown in Fig. 9c), the quench both reduced the average size and caused a significant narrowing of the distribution. These results suggest that the particles were still growing in size when the quench was added. At precursor concentrations ≤ 0.1 M, it was previously shown that the reaction forming Co3O4 had stopped in the product from the turbulent mixer, but the particles continued to crystallize;


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thus it appears that at low concentrations this continued growth must be the result of coalescence and crystallization of the nanoparticles. Conversely, the much more marked effect of the quench on the size distribution at a precursor concentration of 0.2 M, along with the indication that both reaction and crystallization were still proceeding as discussed earlier, suggests that surface growth from solution may also have a role to play in this case.

Figure 9. Particle size histograms (smoothed with bin size = 1 nm) for samples produced at 0.005, 0.05 and 0.2 M metal salt concentrations under turbulent conditions. TEM particle size measurements are plotted in Fig. 10 (detailed size values are available in the supplementary information). D10 correlates to the size of the smaller particles, or nuclei. D50 correlates to particles that have started to grow/coalesce. D90 correlates to the size of the largest particles, and is indicative of growth. The dotted lines in Fig. 10 are a visual aid to highlight the different trends observed.


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Figure 10. Parameters of the size distribution (D10, D50 and D90) of cobalt oxide nanoparticles vs. precursor concentration (logarithmic scale).

Figure 10a) and b) shows that in the turbulent regime, the size of the cobalt oxide nanoparticles increased monotonically with the concentration of the precursor. At the concentration of 0.2 M, D90 increases sharply from 23 to 33 nm. When the quench water was introduced after the turbulent mixer, the nanoparticles were diluted and cooled very rapidly soon after their initial formation (< 1 s after the mixing of the metal salt feed and the supercritical water). At precursor concentrations < 0.2 M, the nanoparticles were ~3 nm smaller than without the quench, which is consistent with the full size distributions in Fig. 9. Interestingly, the sharp increase in size previously observed between precursor concentrations of 0.1 M and 0. 2M disappeared when the quench was introduced.


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For turbulent conditions at concentrations ≤ 0.1 M, when the quench water was used, the crystallinity of the product was reduced, suggesting that the quench stopped a process that was still ongoing by diluting and cooling the nanoparticles slurry. The reduction of the particle sizes and narrowing of the size distribution confirmed that a growth stage was halted by the quench. The use of the quench for those concentrations did not significantly affect the yield of the reaction, indicating that the precipitation of all cobalt oxide nanoparticles had occurred before the introduction of the quench. The growth of the nanoparticles had therefore to be driven by coalescence (or Ostwald ripening) and crystallization. These results suggest that, at precursor concentrations ≤ 0.1 M, the synthesis of cobalt oxide nanoparticles under turbulent mixing conditions occurred through complete nucleation of the nanoparticles followed by growth via coalescence; clearly the final stage of this growth was inhibited by the dilution and cooling effect of the quench. The increase in particle size with increasing concentration might then be explained by the higher probability of particle collisions at higher concentration, thus increasing the rate of coalescence. For the specific case of 0.2 M metal salt concentration, a sharp increase in particle size was observed. This increase was stopped by the introduction of the quench water, suggesting that the quench water halted an ongoing growth. The lowering of the yield observed when the quench was introduced indicates that incomplete conversion of the precursor had not occurred before the quench water was mixed with the nanoparticles slurry, leaving unreacted precursor in solution. This incomplete nucleation may have been the result of the low pH of the product (~ 3), which is known to inhibit nucleation of Co3O449. These results suggest that at precursor concentration ≥ 0.2 M, the synthesis of cobalt oxide nanoparticles under turbulent mixing conditions occurred through incomplete nucleation of particles followed by growth from both solution and


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coalescence. The growth from solution appeared to be stopped by the diluting and cooling of the product by the quench.

3.4. Laminar mixing. Figure 10a - c) are histograms showing the particle size distributions of selected samples produced under laminar conditions with and without the quench water. For concentrations of 0.005 M and 0.05 M, as shown in Fig. 10a) and b), the introduction of the quench did not significantly change either the average size or the broadness of the size distribution. Fig 10c) shows that at precursor concentration of 0.2 M, the introduction of the quench narrowed the particle size distribution and reduced the average particle size significantly. These results suggest that the particles had stopped growing in size when the quench was added; thus reaction, crystallization and growth had all stopped by the time the quench was added for a precursor concentration ≤ 0.05 M. At the higher concentrations, it was shown that the product from the laminar mixer continued to undergo reaction and crystallization. The marked effect of the quench on the size distribution, as with the turbulent mixer at high precursor concentrations, suggests that surface growth may have also occurred.

Figure 10. Particle size histograms (smoothed with bin size = 1 nm) for samples produced at 0.005, 0.05 and 0.2 M metal salt concentrations under laminar conditions.


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It can be seen in Fig. 11c) and d) that, in contrast to the turbulent mixer, for which the particle size increased with increasing concentration of precursor, under a laminar regime the size of the particles decreased with precursor concentration up to 0.05 M. However, for precursor concentrations of 0.1 and 0.2 M, Fig. 11c) shows a sharp increase in particle size with the laminar mixer similar to the one observed with the turbulent mixer at precursor concentration of 0.2 M. In common with the turbulent mixer, introducing quench water after the laminar mixer stopped the sharp increase in particle size at higher concentrations of precursor, resulting in a particle size which decreased monotonically with increasing precursor concentration. For concentrations < 0.1 M, when the quench water was used, the crystallinity of the product was not significantly reduced, suggesting that the growth of the crystallites had already occurred before the quench water and nanoparticles slurry mixed. The particle size, particle size distribution and reaction yield were not significantly affected either by the addition of the quench, confirming that all particles had nucleated and grown before the addition of the quench. These results suggest that, at precursor concentrations < 0.1 M, the synthesis of cobalt oxide nanoparticles under laminar mixing conditions occurred through complete nucleation of the nanoparticles followed by limited growth via coalescence. The decrease of particle size with increasing precursor concentration supports the hypothesis that at low precursor concentrations, an increase in concentration can lead to a higher nucleation rates due to an increase in the supersaturation of the solution upon mixing with the supercritical water, resulting in smaller particles. This hypothesis was first suggested by Lester et al. for cobalt oxide synthesis using a continuous hydrothermal reactor under laminar conditions with a counter current mixer42,50.


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Owing to the streamline nature of laminar flow, the probability of the particle-particle collisions at any concentration would be expected to be much less than in turbulent flow, resulting in a low rate of coalescence. An increased rate of nucleation with limited growth by coalescence would explain the decreasing particle size with increasing precursor concentration. Similar to the turbulent mixer, for precursor concentrations ≥ 0.1 M, a sharp increase in particle size was observed. This increase was stopped by the introduction of the quench water, suggesting that the quench water halted an ongoing growth stage. Lower crystallinity combined with a reduction of particle across the whole distribution, as previously discussed, lends support to the hypothesis that growth was halted by the addition of the quench. The decrease in yield (and significant increase in the pH of the product) when the quench was introduced indicates that complete precipitation of cobalt oxide nanoparticles had not occurred before the quench water was mixed with the nanoparticles slurry, leaving unreacted precursor in solution. This suggests that at precursor concentrations ≥ 0.1 M, the synthesis of cobalt oxide nanoparticles under laminar mixing conditions occurred by incomplete nucleation of particles (due to low pH ~ 3, as with turbulent mixer) followed by growth from solution.

4. CONCLUSIONS The synthesis of cubic spinel Co3O4 by a CHFS process has been studied at a range of metal salt concentrations, using four different mixer configurations in either laminar or turbulent flow regimes in the pipe downstream of the mixer. The distribution of sizes of the nanoparticles was affected by the flow regime, the concentration of the precursor and the introduction of quench water after the initial precipitation. Mechanisms of complete nucleation followed by growth via coalescence and incomplete nucleation followed by growth from solution were observed. The


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nature of the nucleation and growth were strongly dependent on the mixing conditions as well as the initial cobalt acetate concentration and resulting product pH. The use of the quench feed allowed for a reduction of the average crystallite size without significantly influencing the yield of the reaction when growth occurred via Ostwald ripening. A significant lowering of the yield was observed when growth occurred from solution, although the growth limiting effect of the quench was greater under those conditions. An increase of the crystallite size with increasing concentration was observed for turbulent mixing conditions whereas a decrease in size with increasing concentration was observed for laminar mixing conditions. Table 3 gives a summary of the proposed nucleation and growth mechanisms for the four mixer configurations used.

Low concentrations

pH > 3.2

Turbulent

Turbulent + quench

Laminar

Laminar + quench

Complete nucleation

Size reduced as particles still coalescing when quench introduced

Complete nucleation

No significant effect on size or yield as all nucleation and growth occur before quench

Fast growth from particle coalescence due to high collision rate in turbulent flow

No significant effect on yield as all precursor consumed

Increasing particle size with increasing precursor concentration due to higher rate of coalescence

High concentrations

pH < 3.2

Slow growth from particle coalescence due to low rate of coalescence in laminar flow Decreasing particle size with increasing precursor concentration, due to higher supersaturation at supercritical conditions, thus smaller nuclei

Incomplete nucleation due to low pH

Growth from solution and particle coalescence slowed due to cooling and dilution of the slurry.

Incomplete nucleation due to low pH

Fast growth from particle coalescence and growth from remaining precursor in solution

Lower yield due to precursor left in solution

Growth from remaining precursor in solution

Growth from solution stopped due to cooling and dilution of the slurry Lower yield due to precursor left in solution

Table 3. Summary of the nucleation and growth mechanisms observed for all of the mixer configurations studied.


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The main benefits of the CHFS synthesis method in this case is the ability to produce small nanoparticles with a D50 size of 6.5 nm at a rate of 20 g.h-1 using a laboratory scale continuous reactor. Moreover, the synthesis method is elegant in that it only uses cobalt acetate, water and hydrogen peroxide as reagents. A better understanding of parameters controlling the size of cobalt oxide nanoparticles will prove beneficial for the use of these materials in applications such as energy storage and Fischer-Tropsch catalysis, in which size dependent properties are key.

Supporting information Mixer information (design, dimensions, Reynolds and Froude numbers, temperature profiles), TEM size distribution measurements and additional electron micrographs, yield measurements methodologies, zeta potential measurement. This material is available free of charge via the Internet at http://pubs.acs.org

AUTHOR INFORMATION Corresponding Author

Tel: +44(0)207 679 4345. E-mail: [email protected]

Notes

The authors declare no competing financial interest.


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For Table of Contents Use only, Manuscript Title: Nucleation and growth of cobalt oxide nanoparticles in a continuous hydrothermal reactor under laminar and turbulent flow Author list: Clément J. Denis, Christopher J. Tighe, Robert I. Gruar, Neel M. Makwana and Jawwad A. Darr

Table of content graphic Synopsis: The work presented focuses on the continuous hydrothermal synthesis of cobalt oxide nanoparticles and the study of the parameters influencing their nucleation and growth. Crystallinity, microscopy and yield observations of the effect of mixing conditions, precursor concentration and temperature highlight the nucleation and growth mechanisms involved in the continuous hydrothermal synthesis of cobalt oxide nanoparticles.


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Nucleation and Growth of Cobalt Oxide Nanoparticles in a Continuous

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