Hydrothermal Synthesis of ZnO Crystals from Zn(OH)2 Metastable

Article pubs.acs.org/crystal

Hydrothermal Synthesis of ZnO Crystals from Zn(OH)2 Metastable Phases at Room to Supercritical Conditions Frédéric Demoisson,*,† Romain Piolet,† and Frédéric Bernard† Laboratoire Interdisciplinaire Carnot de Bourgogne, UMR 6303 CNRS-Université de Bourgogne, 9 avenue A. Savary BP 47870, F-21078 Dijon, France S Supporting Information *

ABSTRACT: The originality of this work is to highlight the effect of temperature and pressure on the size and morphology of hydrothermal ZnO particles from ambient to supercritical conditions (T > 374 °C and P > 221 bar) using a unique continuous one-step process. Experiments were carried out from zinc nitrate (Zn(NO3)2) and potassium hydroxide (KOH) solutions in the ranges of 1−300 bar and 30−400 °C. The as-prepared particles of ZnO (flower, ellipsoid, and sphere) and ε-Zn(OH)2 (polyhedral) sized from nano to micrometers were characterized by X-ray diffraction and electronic microscopy. The wulfingite phase (ε-Zn(OH)2) was detected inside some powders especially at room temperature for higher pressures. The proportion of each phase was determined by the quantitative Rietveld analysis. On the basis of Loüer’s method, ZnO crystals exhibiting a hexagonal structure were considered as cylinders with a diameter D and a height H. The D/H parameter, also known as the aspect ratio, was correlated with electronic microscopy observations to investigate the ZnO nanoparticle morphology evolution according to the temperature and pressure. Thus, a schematic synoptic summarizing these results is presented.

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according to pH variations;27 thus, a high pH synthesis (pH = 13.5) promotes the formation of flowers consisting of ZnO nanorods. On the contrary, a lower pH synthesis (pH = 12) enables the formation of quasispherical ZnO nanoparticles. The variety of ZnO particle morphologies observed as a function of pH can be explained by evolutions in the nature and the proportion of zinc-soluble species in solution involved in the crystal nucleation and growth process.27 Zhang et al. also studied the influence of the temperature at various pH values and reported the formation of flowers made of primary rods for a temperature above 200 °C and at a pH value of 13.5, whereas the particle growth is noticed between 160−200 °C at a pH value of 12. The formation of ZnO flowers also was reported elsewhere.34 Concerning the ZnO supercritical hydrothermal synthesis (T > 374 °C and P > 221 bar), the effect of temperature on the particle size was also investigated, and the results show that an increase of temperature enhances the formation of spherical nanoparticles.35−38 Note that the supercritical hydrothermal synthesis (SHS) route has numerous advantages such as its environmental friendliness, large scale production at a lower cost, catalyst-free crystal growth, no need for vacuum or carrier gas, and finally, the possibility to reach very low reaction times and high yield for the well-crystallized particle formation. However, it is very difficult to compare the

INTRODUCTION Over the past decade, zinc oxide (ZnO) was extensively investigated owing to its outstanding optical and electrical properties, such as its wide direct band gap of 3.37 eV and high exciton binding energy of 60 meV at room temperature.1 Many applications of ZnO already were reported and primarily concern the field of electronic energy and health (optoelectronic transducer, photocatalyst, UV-blocking agent, or biomedical applications).2−7 Another advantage of ZnO is that the particle morphology can be tuned according to its final application; for instance, the production of ZnO wires, flowers, rods, tubes, sheets, ribbons, plates, dumbbell, disks, and springs was reported.2−12 Many elaboration routes were studied, such as the sol−gel method, hydrothermal and solvothermal synthesis, chemical vapor deposition (CVD), thermal evaporation, or spray pyrolysis, and their parameters were adjusted to enable the production of such materials.13−19 In the case of hydrothermal (and solvothermal) synthesis, the ZnO size and morphology can be tuned by a variation of the temperature, pressure, reaction time, nature, and concentration of the metallic precursor and the alkali solution (pH effect). The number of studies about the hydrothermal synthesis of monodisperse ZnO powders increased in the late 1990s and early 2000s, even if some works were published in the 1980s.20,21 Most of them report the influence of the hydrothermal synthesis conditions on the ZnO particle size and morphology.22−33 Furthermore, a growth mechanism of ZnO nanoparticles was suggested in subcritical conditions © 2014 American Chemical Society

Received: March 24, 2014 Revised: September 30, 2014 Published: October 1, 2014 5388

dx.doi.org/10.1021/cg500407r | Cryst. Growth Des. 2014, 14, 5388−5396

Crystal Growth & Design

Article

Figure 1. Scheme of the experimental continuous hydrothermal setup used to synthesize ZnO powders from room to supercritical conditions (T > 374 °C and P > 221 bar). literature data,35−37 zinc nitrate hexahydrate (98% purity, SigmaAldrich) and potassium hydroxide (≥95% purity, Sigma-Aldrich) were selected as reactants. In the process, both aqueous metal salt precursor (Zn(NO3)2) (Figure 1a) and alkaline solutions (KOH) (Figure 1b) (initial concentrations: 0.06 M and 0.24M, respectively) were fed into the reactor and counter-currently mixed with preheated demineralized water (Figure 1c). The geometry of the mixing zone inside the patented reactor is disclosed elsewhere.46 The three solution flow rates were maintained at 20 mL min−1, which corresponds to a residence time of a few tens of seconds inside the reactor, for example, a residence time of 11 s in supercritical domain (400 °C, 300 bar) was evaluated thanks to Computational Fluid Dynamics simulations. Note that the residence time fluctuates according to experimental conditions because it is a function of the fluid properties such as density and viscosity. In these experimental conditions, the measured pH value (at room conditions) of the resulted ZnO suspensions at the outlet of the system (Figure 1d) was in the range of 12.5−13.0. The recovered suspensions were centrifuged and then washed with demineralized water under ultrasonication. Once suspensions were free of unreacted precursors (zinc salt and KOH in excess) and byproducts (in this case, KNO3), they were freeze-dried in order to recover well-dispersed (nano)powders. Powder Characterization. Dry powders were characterized by TEM, SEM, and XRD. TEM and SEM observations were carried out on a Jeol JEM-2100 LaB6 operating at 200 kV and on a Jeol JSM6400F operating at accelerating voltage in the range 0.5−30 kV, respectively. The TEM observations were performed in bright field mode, and the micrographs were acquired and processed thanks to a Gatan US1000 charge-coupled device (CCD) camera and DigitalMicrograph software (Gatan, Inc.). Samples were prepared from freeze-dried powders dispersed in ethanol onto a 300-mesh carbon film coated copper grid, with a subsequent solvent evaporation in contact with ambient air at room temperature. SEM samples were prepared by disposing a sufficient amount of freeze-dried powders dispersed in ethanol onto a carbon film. XRD data were collected by a D8 Advance diffractometer equipped with a Vantec linear detector using the Kα1 and Kα2 copper radiations. The XRD patterns were scanned over the angular range of 20°−90° (2θ). The step size was selected to be constant over the whole angular range and equal to 0.025°. The counting time was chosen to be 2s step−1. In addition to wurtzite phase (ZnO), wulfingite phase (ε-Zn(OH)2) was detected inside some nanopowders. The proportion of each phase (from ICDD numbers 036−1451 and 038−0385 for ZnO and ε-Zn(OH)2, respectively) was determined by the whole pattern fitting quantitative Rietveld analysis using Topas software.47 The goodness-of-fit is

size and morphology of ZnO particles because of many discrepancies in residence times, ranges of pressure and temperature, reactor types (batch or continuous process), designs (T-shaped mixer, micro swirl mixer, pipe-in-pipe), and reactant solution concentration and nature, for example, the counterion nature of the alkaline (Na+, K+, Li+) and metallic precursor (nitrate, acetate, and sulfate) solutions between studies from the literature.22−41 In this study, a continuous one-step hydrothermal process was employed to synthesize ZnO nanoparticles from room to supercritical conditions (1−300 bar and 30−400 °C). The effect of pressure and temperature on the size and morphology of ZnO particles is investigated hereafter; thus, the flow rates, nature (Zn(NO3)2 and KOH), and concentrations of the reactant solutions were held constant. The as-prepared samples were characterized by X-ray diffraction (XRD) and electronic microscopy (transmission electron microscopy (TEM) or scanning electron microscopy (SEM)). ZnO crystal was considered a cylindrical crystallite of diameter D and height H (considering the hexagonal structure of the ZnO phase) using the Loüer et al. method.42 The D/H parameter, also known as the aspect ratio, was correlated with electronic microscopy observations to investigate the ZnO nanoparticle morphology evolution according to the temperature and pressure.

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EXPERIMENTAL SECTION

Powder Synthesis. ZnO powder was synthesized from a continuous hydrothermal synthesis process shown in Figure 1 and accurately described elsewhere.43,44 This process was equipped with high-pressure pumps, a back-pressure regulator, and tubular furnaces, which allowed the pressure and temperature in the system to be modified independently; thus, experiments were carried out with the pressure and temperature varying in the ranges of 1−300 bar and 30− 400 °C to reach supercritical conditions (T > 374 °C and P > 221 bar). Three thermocouples (type K) were placed inside the patented reactor and, more specifically, in the reactant stream to monitor the temperature in the system.45 The temperature considered in the following sections is the one recorded at midreactor (Tmidreactor). To evaluate the effect of pressure and temperature on the size and morphology of ZnO particles, the precursor concentrations, solution pH, and flow rates were maintained constant. On the basis of the 5389



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Table 1. Pressure Effect on the ZnO or ε-Zn(OH)2 Hydrothermal Formation at Room Temperature (Phase Proportion Determined by Topas Software Using World Pattern Matching) experiments

P (bar)

Exp.2−29 Exp.101−30 Exp.202−31 Exp.304−31

2 101 202 304

± ± ± ±

Tmidreactor (°C)

1 5 5 5

29 30 31 31

± ± ± ±

phase proportion

0 0 0 0

85% 17% 3% 1%

ZnO−15% ZnO−83% ZnO−97% ZnO−99%

ε-Zn(OH)2 ε-Zn(OH)2 ε-Zn(OH)2 ε-Zn(OH)2

Table 2. Combined Effect of the Pressure and Temperature on the ZnO and ε-Zn(OH)2 Hydrothermal Formation. Phase Proportion Was Calculated by Topas Software after the Whole XRD Pattern Was Refined. Crystallite Characteristics Were Determined from the Langford et al. Method48 experiments

P (bar)

Exp.2−29 Exp.2−50 Exp.2−86 Exp.46−253 Exp.102−305 Exp.168−353

2 2 2 46 102 168

± ± ± ± ± ±

Tmidreactor (°C)

1 1 1 1 1 1

29 50 86 253 305 353

± ± ± ± ± ±

0 3 2 2 4 3

phase proportion 85% ZnO−15% ε-Zn(OH)2 100% ZnO

D (nm)

H (nm)

D/H

18 20 39 33 41

40 49 34 48 59

0.45 0.41 1.15 0.69 0.69

Table 3. Temperature Effect on the ZnO and Zn(OH)2 Hydrothermal Formation at 300 Bar in Both Liquid and Supercritical Domains of Water. Phase Proportion Was Calculated by Topas Software after Refining the Whole XRD Pattern. Crystallite Characteristics Were Determined from the Langford et al. Method48 experiments Exp.304−31 Exp.306−52 Exp.300−98 Exp.300−210 Exp.300−294 Exp.299−396a a

P (bar) 304 306 300 300 300 299

± ± ± ± ± ±

5 6 1 1 1 2

Tmidreactor (°C) 31 52 98 210 294 396

± ± ± ± ± ±

0 1 2 1 2 5

phase proportion 1% ZnO−99% ε-Zn(OH)2 42% ZnO−58% ε-Zn(OH)2 traces of β-Zn(OH)2 100% ZnO

D (nm)

H (nm)

D/H

23 28 31 22

55 38 37 22

0.42 0.74 0.84 1.00

Supercritical domain.

Figure 2. XRD patterns collected on ZnO and ε-Zn(OH)2 powders synthesized at room temperature. Each XRD pattern was adjusted by the whole pattern fitting quantitative Rietveld analysis to determine the proportion of each phase.

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Figure 3. Overview of TEM and SEM (indicated by ∗) images of ZnO and ε-Zn(OH)2 powders synthesized at room temperature. appreciated from GoF values; therefore, the scale factor, crystallite sizes, and microdistorsions were adjusted while the structural data were fixed (using crystallographic information files for both phases). Moreover, an XRD pattern decomposition method was used in which each Bragg reflection was modeled by Voigt functions (fitted by the least-squares method, Diffract-AT software) to investigate the crystallite size and shape of ZnO powders.42 This method allows the line broadening to be estimated and thus the microstructural properties of the material to be obtained.48 To get information about each reflection, the XRD data were treated; especially, the Kα2 contribution from the incident beam was stripped using a Kα2/Kα1 ratio equal to 0.504.49,50 Also, a well-crystallized BaF2 powder was used as reference material to evaluate the instrumental line broadening. Each reflection is characterized by a position (2θ), an integral breadth (β), and a shape factor (ϕ). Langford et al. demonstrated the relation between these parameters and the height H and diameter D of a cylindrical crystallite (considering the hexagonal structure of the ZnO phase).48 Note that no aspect ratio of ZnO crystallites is determined when another phase such as ε-Zn(OH)2 is also present because in such a case, the Langford et al. method is not valid.48

Each XRD characterization was correlated with TEM and SEM observations presented in Figure 3. The electronic microscopy observations for Exp.2−29 reveal the presence of two types of particles: (i) flower submicronic particles, such a morphology was also detected in many studies and was identified as ZnO phase and (ii) polyhedral-shaped particles with a size greater than 3 μm.27,52−54 This polyhedral morphology may be attributed to the ε-Zn(OH)2 phase in agreement with literature.55 Indeed, Exp.304−31 led to the almost exclusive formation of micronic polyhedral particles (with traces of flower particles, Figure 3) identified as being a powder composed of 99% wulfingite phase by XRD. The coexistence of these two phases was reported in similar conditions of temperature and pressure by Moezzi et al. while the formation of ZnO nanoparticles from ZnSO4 and NaOH under hydrothermal conditions was investigated.56 In their alkali conditions, the authors showed that ZnO forms via dissolution of primarily formed ε-Zn(OH) 2 and then precipitation of ZnO crystals. Furthermore, they have suggested that this mechanism occurs through the following set of equations (eqs 1−8).56

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RESULTS AND DISCUSSION The presented hydrothermal process allowed the ZnO powder synthesis (sometimes in the presence of ε-Zn(OH)2) to be performed from room to supercritical conditions and thereby to observe several changes in terms of phase, size, and morphology. The experimental conditions, results of phase quantification, and crystallite characteristics (D, H, and D/H) are summarized in Tables 1−3 in the following sections. The pressure effect at room temperature (Table 1), the combined effect of the pressure and the temperature (Table 2), and the temperature effect at 300 bar (Table 3) are investigated. The aspect ratio D/H already was used to observe the change of ZnO crystallite shape according to the temperature in the case of the decomposition of ZnO precursors such as zinc oxalate or zinc acetate.51 Likewise, the correlation between D/H values and TEM or SEM observations is carried out in this study. Pressure Effect at Room Temperature. Some experiments were performed at 30 °C from 2−304 bar, and their conditions are summarized in Table 1. The effect of a pressure increase on the powder phase composition is disclosed in Table 1 and illustrated in Figure 2. An increase of the pressure from 2−304 bar at room temperature promotes the formation of the wulfingite phase ε-Zn(OH)2.

ε‐Zn(OH)2 (s) ⇌ Zn(OH)2 (aq)

K1sp = 3.5 × 10−17 (1)

Zn(OH)2 (aq) → ZnO (s) + H 2O (l) ΔrH2° = 4.07, ΔrG2° = − 29.51 (kJ mol−1)

(2)

Zn(OH)2 (aq) + OH− (aq) → Zn(OH)3− (aq) ΔrH3° = 3.02, ΔrG3° = − 12.78 (kJ mol−1)

(3)

Zn(OH)3− (aq) → ZnO (s) + H 2O (l) + OH− (aq) ΔrH4° = 0.69, ΔrG4° = − 16.73 (kJ mol−1)

(4)

Zn(OH)3− (aq) + OH− (aq) → Zn(OH)4 2 − (aq) ΔrH5° = 0.02, ΔrG5° = −4.78 (kJ mol−1)

(5)

Zn(OH)4 2 − (aq) → ZnO (s) + H 2O (l) + 2OH− (aq) ΔrH6° = 0.67, ΔrG6° = − 11.95 (kJ mol−1) 5391

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Figure 4. XRD patterns collected on ZnO and ε-Zn(OH)2 powders synthesized along the liquid/vapor equilibrium curve of water.

Figure 5. Overview of TEM images of ZnO and ε-Zn(OH)2 powders synthesized at different pressures and temperatures.

half, growth of an ellipsoid, and finally, nucleation and growth of multiple branches (from the former nucleation spot of the first two halves) to form a flower (also called star-like particles).55 Hence, our observations of ZnO flowers at atmospheric pressure are in agreement with the literature.52−57 Then, the explanation of the powder composition evolution observed by XRD (Figure 2) and electronic microscopy (Figure 3) relies on the solid phase solubility variation with increasing the pressure at ambient temperature (Exp.101−30 to Exp.304− 31). Indeed, it could be assumed that an increase of pressure generates a decrease of the ε-Zn(OH)2 (s) solubility and thus promotes the wulfingite crystallization. The precipitation equilibrium (eq 1) would be shifted toward the stabilization of ε-Zn(OH)2 (s) and thus limit the formation of Zn(OH)2 (aq) and reduce the amount of ZnO (s) (eqs 2−6). Another theory may be considered and corresponds to the catalytic effect of hydroxide ions in solution on the ZnO formation.56 This would explain the primary formation of ZnO (85% ZnO−15% ε-Zn(OH)2) at ambient temperature and

ε‐Zn(OH)2 (s) → ZnO (s) + H 2O (l) ΔrH7° = 5.62, ΔrG7° = −4.07 (kJ mol−1)

ZnO (s) ⇌ ZnO (aq)

K8sp = 2.2 × 10−17

(7) (8)

Moezzi et al. showed that this dissolution/precipitation phenomenon occurs until the ε-Zn(OH)2 solubility equilibrium is reached. McBride et al. took a step forward by claiming that ZnO always precipitated in a wulfingite-saturated suspension based on the solubility data of both solids.57 Oliveira et al. also demonstrated that ZnO flowers form from a pre-existing hydroxide phase.55 Furthermore, the authors performed a kinetics study at room temperature in alkali media that revealed the formation mechanism of ZnO flowers. Actually, this mechanism involves the nucleation of ZnO nanocrystals directly inside the zinc hydroxide phase (rather than on its surface), aggregation of those nanoncrystals via orientedattachment to form a half-ellipsoid, nucleation of a second 5392



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Figure 6. XRD patterns collected on ZnO and Zn(OH)2 powders synthesized at 300 bar.

are not considered here. In other words, ε-Zn(OH)2 phase is thermodynamically metastable with respect to ZnO at room temperature (ΔrG7° = −4.07 kJ mol−1) and its transformation rate to ZnO is low and occurs preferentially through dissolution/precipitation (endothermic reactions). The disappearance of polyhedral particles in favor of ZnO flowers is also observed in Figure 5 when the temperature increases from 29−50 °C (Exp.2−29 and Exp.2−50). Then, a modification of the ZnO particle morphology is observed between 50−86 °C. Submicrometric particles with an ellipsoidal shape are observed in Figure 5 (Exp.2−86). Oliveira et al. also observed such morphology for ZnO prepared at 60 °C by hydrothermal method.55 As mentioned in the previous section, they suggested that at ambient temperature, this ellipsoidal morphology is an intermediate state toward the formation of ZnO flowers and that the ellipsoids need more time to grow additional branches. Consequently, they stated that a temperature rise might lead to limitation of nanocrystal aggregation into a more complex morphology such as flowers.55 Therefore, to increase the temperature hinders the flower branches growth and promotes the nucleation of ZnO nanocrystals; hence, our observation of ellipsoids at 86 °C is in agreement with the literature.55 At higher temperatures, that is, at 253 °C (Exp.46−253) and 305 °C (Exp.102−305), a dramatic reduction of the particle size is observed in Figure 5, and the particles become spherical with an aspect ratio (D/H) close to 1 (Table 3). Hence, the limitation of the particle growth in favor of the nucleation as aforementioned when the temperature increases is confirmed; however, an increase of the particle size with a change of morphology is observed above 253 °C. Indeed, at higher temperature (Exp.168−353), sticks with a 150−200 nm length and a width of approximately 40 nm are observed. Note that the combined effect of the pressure could be responsible for this morphology change. Temperature Effect at 300 Bar. The next section concerns the evaluation of the temperature effect on the particle morphology at 300 bar. Thus, some experiments were

atmospheric pressure. An increase of the pressure could reduce the catalytic activity of OH− species and thus generate a lower ZnO formation rate, which would result predominantly in the formation of wulfingite. In summary, the ZnO dissolution/ precipitation formation mechanism seems to be greatly affected by an increase of pressure at ambient temperature since a very low amount of wurtzite was detected inside the sample elaborated at 304 bar. Combined Effect of the Pressure and Temperature. Some experiments were performed along the liquid/vapor equilibrium curve of water from 2−168 bar and from 30−353 °C and are summarized in Table 2. XRD patterns for these experiments are presented in Figure 4, while TEM images illustrating particle morphologies are shown in Figure 5. First, the phase composition analysis reveals that, except for Exp.2− 29, all samples exhibit a ZnO wurtzite phase. As mentioned in the previous section (see Figure 3), Exp.2−29 primarily presents ε-Zn(OH)2 polyhedral particles with a low amount of ZnO flowers (15 wt % according to XRD analyses, Table 1). Note that a small increase of temperature between 29−50 °C at atmospheric pressure promotes the transformation of the wulfingite phase into the oxide phase. Such a result is in agreement with the literature since it was stated that the conversion of zinc hydroxide to ZnO is increasingly favored at higher temperatures, both from a free energy and kinetic perspective.56,58 Indeed, values of standard enthalpy of reaction (ΔrH2° to ΔrH7°) and standard free enthalpy (ΔrG2° to ΔrG7°) at ambient temperature and atmospheric pressure for eqs 2−7 were reported.56 Slightly negative values of ΔrG° are noticed, while the reactions of eqs 2−7 are endothermic (ΔrH° > 0) at room temperature. From a thermodynamic standpoint, the dissolution/precipitation appears to be the privileged path for the formation of ZnO (s) from ε-Zn(OH)2 (s) rather than the solid phase transformation since eqs 2−6 present lower values of ΔrH° (absolute value) and greater negative values of ΔrG° than does eq 7. This does not mean that solid phase transformation cannot occur since the differences between values of standard enthalpy of reaction are low and the kinetics 5393



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Figure 7. Overview of TEM images of ZnO and ε-Zn(OH)2 powders synthesized at 300 bar.

Figure 8. Schematic synoptic of the ZnO hydrothermal synthesis in alkali conditions: changes in composition, size, and morphology of crystals synthesized from room to supercritical conditions.

disappearance beyond 52 °C. To understand the formation mechanism of β-Zn(OH)2, a temperature-dependent XRD analysis was carried out between 30−130 °C on the sample elaborated at 31 °C and 304 bar (Exp.304−31, see Supporting Information). Results show the transformation of wulfingite to wurtzite as soon as the temperature reaches 100 °C, and the former completely disappears in favor of the latter at 120 °C, but, no trace of β-Zn(OH)2 was detected during this solid phase treatment. It also can be assumed that the β-Zn(OH)2 phase forms by a liquid phase transformation, for which eq 9 could be proposed:

performed from room temperature (Exp.304−31) to the supercritical conditions of water (Exp.299−396) and are summarized in Table 3. XRD patterns for those experiments are presented in Figure 6. As previously mentioned, the experiment carried out at 31 °C reveals the formation of an almost pure ε-Zn(OH)2 phase, but powders elaborated at a temperature greater than or equal to 98 °C exhibit a pure wurtzite phase as shown in Figure 6. This phase transformation occurs in the range of 31−98 °C as 58% of ε-Zn(OH)2 remains in the sample at 52 °C. Moreover, XRD characterization of this sample also shows some traces of β-Zn(OH)2 (not quantified using Topas software), which is a metastable phase of zinc hydroxide;59 thus, β-Zn(OH)2 could be an intermediate phase of the transformation from εZn(OH)2 to pure ZnO at high pressure. This assumption explains the presence of this new phase only as traces and its

ε‐Zn(OH)2 (s) ⇌ Zn(OH)2 (aq) ⇌ β‐Zn(OH)2 (s) → ZnO (s) + H 2O (l) 5394

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Figure 7 shows that ZnO and ε-Zn(OH)2 particles present the same morphologies at 52 °C and 31 °C, that is, flowers and polyhedral particles, respectively. A slight growth of ZnO particles is noticed when the temperature increases from 52−98 °C. Furthermore, once the temperature is greater than or equal to 210 °C (Exp.300−294), TEM observations show a particle size reduction associated with an increase of the number of crystals with sphere-like morphology corresponding to an aspect ratio (D/H) close to 0.84. The size of the ZnO nanoparticles decreases to a value close to 30 nm at 294 °C (Exp.300−294). Finally, the Exp.299−396 highlights the interest to prepare metallic nano-oxides in supercritical water as well-dispersed ZnO particles with homogeneous sizes smaller than 25 nm. XRD analysis indicates a crystallite size of 22 nm with a D/H ratio equal to 1.00 (Table 3). In general, the formation of nanoparticles with a spherical shape implies that the nucleation stage is dominant over the particle growth phenomena during synthesis. In conclusion, the increase of temperature above 210 °C promotes the nucleation of ZnO nanoparticles at high pressure. Schematic Synoptic of the ZnO Hydrothermal Synthesis. In accordance with the data of the literature, a schematic synoptic of the ZnO hydrothermal synthesis on the state diagram of water is suggested in Figure 8.52−57 It includes possible mechanisms of formation and takes into account the nature of the end-product (ZnO, ε-Zn(OH)2, and β-Zn(OH)2), the size (micro- or nanometers), and the morphology of crystallites (flowers, ellipsoids, sticks, and spheres). At ambient conditions (Figure 8a), the mixing of a zinc nitrate solution (Zn(NO3)2) with an alkali solution (KOH) leads to the synthesis of both submicronic ZnO flowers and ε-Zn(OH)2 polyhedrons larger than 3 μm. The path for the formation of these compounds is described by eqs 1−8. Additionally, the presence of OH− species in solution supposedly favors the formation of the oxide phase at the expense of the hydroxide phase in those conditions of pressure and temperature.56 At room temperature, an increase of the pressure (Figure 8b) seems to generate a decrease of the ε-Zn(OH)2 (s) solubility and thus promotes the micronic polyhedron crystallization (only traces of ZnO at 300 bar). On the other hand, an increase of the temperature at high pressure (Figure 8c) unbalances the ε-Zn(OH)2/ZnO dissolution/precipitation mechanism in favor of ZnO flower formation. During this process, the presence of β-Zn(OH)2 is detected (as traces at 52 °C) and is supposedly related to the dissolution/precipitation mechanism at high pressure as the solid phase transformation of ε-Zn(OH)2 to βZn(OH)2 has been ruled out. In addition to this phase composition evolution, a particle size reduction is observed when the temperature increases. This phenomenon is attributed to a limitation of the particle growth process. Indeed, beyond 200 °C (at 300 bar), spherical ZnO nanoparticles form because of the predominance of the particle nucleation phenomena over the growth phenomena. Furthermore, the predominance of the nucleation phenomena becomes so important over 200 °C that particles exhibit a spherical morphology (D/H = 1.00) with size around 20 nm in diameter at 400 °C. Along the liquid/vapor equilibrium curve of water (Figure 8d), a change of morphology from ZnO flowers (ambient conditions) to ellipsoids (at 86 °C) is explained by the increase of temperature, which limits the flower growth by aggregation of multiple branches (ellipsoids) and thus promotes the nucleation of smaller ZnO nuclei.

Therefore, this study shows that two areas can be distinguished on the state diagram of water in alkali conditions. The first one corresponds to the zone in which there is a competition between the ZnO growth and nucleation phenomena, that is, for a temperature under 200 °C, approximately. Over this threshold temperature, the predominance of the nucleation phenomena causes a reduction of ZnO particle size and an increase of the temperature in this domain promotes the formation of spherical particles.

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CONCLUSION In summary, ZnO powders (sometimes in the presence of Zn(OH)2) were prepared by hydrothermal synthesis from zinc nitrate (Zn(NO3)2) and potassium hydroxide (KOH) as raw materials. The effect of the pressure and temperature on the size and morphology of ZnO crystallites was investigated. Experiments were performed from room to supercritical conditions by a continuous one-step hydrothermal process that allows the pressure and temperature in the system to be independently adjusted. The as-prepared samples were characterized by XRD and electronic microscopy (TEM or SEM). In addition to the wurtzite phase (ZnO), hydroxide phases as ε-Zn(OH)2 and β-Zn(OH)2 were detected inside some powders, especially at 30 °C when the pressure increases and at 300 bar and 50 °C, respectively. Furthermore, the results of this study reveal that the ZnO formation mechanism corresponds, first, to the precipitation of Zn(OH)2 and then to the dissolution of this phase and the precipitation of the final product. The conditions of temperature and pressure have a significant impact on this mechanism; ZnO material with various morphologies such as flowers, ellipsoids, sticks, and spheres was observed from nano to micrometric size depending on experimental parameters. In accordance with the data of the literature, a schematic synoptic of the ZnO hydrothermal synthesis was established and took into account the nature of the end-products and the crystallite characteristics. This work tends to prove that, in the case of ZnO hydrothermal synthesis, the nucleation phenomenon becomes predominant over the growth phenomena for temperatures above 200 °C.

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ASSOCIATED CONTENT

S Supporting Information *

Temperature-dependent X-ray diffraction analysis of the sample Exp.304-31. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +33 380 395 906. Fax: +33 380 396 167. Author Contributions †

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. These authors contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Burgundy Regional Council, the Welience Company, and the CNRS organization. The authors also thank the DTAI of the ICB laboratory for their assistance in the process development and material 5395



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characterization, especially Nicolas Geoffroy for the XRD pattern fitting quantitative Rietveld analysis.

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