Optimization Study on Formation and Decomposition of Zinc

Article pubs.acs.org/IECR

Optimization Study on Formation and Decomposition of Zinc Hydroxynitrates to Pure Zinc Oxide Nanoparticles in Supercritical Water Seyed Javad Ahmadi,† Morteza Hosseinpour,*,‡ Farzad Javadi,§ and Reza Tayebee§ †

Nuclear Science and Technology Research Institute, End of North Karegar Ave., Tehran, 1439951113, Iran Department of Chemical Engineering, Faculty of Engineering, University of Tehran, P.O. Box 11365-4563, Tehran, Iran § Department of Chemistry, School of Sciences, Hakim Sabzevari University, Sabzevar, 96179-76487, Iran ‡

S Supporting Information *

ABSTRACT: In the present study, synthesis and decomposition of zinc hydroxynitrates, i.e., Zn(NO3)(OH)·H2O, Zn5(NO3)2(OH)8·2H2O, and Zn3(OH)4(NO3)2 into ZnO nanoparticles were investigated at supercritical conditions by combination of Taguchi experiment design method, analysis of variance (ANOVA), and complementary verifying experiments. First, the effect of several important parameters such as temperature, residence time, and initial nitrate salt concentration and its pH on “purity” of synthesis particles (e.g., ZnO) was studied using an orthogonal experiment design methodology coupled with instrumental analysis such as powder X-ray diffraction, Fourier-transform infrared spectroscopy, transmission electron microscopy, and thermo-gravimetric analysis. Then ANOVA was performed on the data and the best conditions for the synthesis of pure ZnO nanoparticles were attained. The second aim was the elucidation of formation and decomposition mechanism of hydroxinitrates by combining the statistical results and additional experiments which influential parameters were investigated independently. It was found that the initial pH of zinc nitrate alongside temperature had the most effects on formation of Zn5(NO3)2(OH)8·2H2O as the first product of the formation reaction, while at higher temperatures it was converted to Zn3(OH)4(NO3)2 which in turn decomposed to ZnO nanoparticles as an ultimate product.

1. INTRODUCTION Recently, interest in ZnO nanomaterials has increased unexpectedly. The large surface to volume ratio of nanocrystalline ZnO makes it a brilliant candidate for gas sensing as well as heterogeneous catalysis.1,2 ZnO is a broad band gap (3.37 eV) semiconductor which is appropriate for ultraviolet LEDs and lasers.3 The surface and the quantum confinement effects of ZnO in nanoscale make it unique in optical, electrical, mechanical, and chemical properties.4−8 Furthermore, current applications of nanocrystalline ZnO in solar cells, field effect transistors, and nanogenerators make it a flexible material.9 A large amount of physicochemical synthesis techniques have been used to produce ZnO nanostructures such as: sol−gel,10,11 chemical vapor deposition,12 pulsed laser ablation,13 electrodeposition,14 ultrasonic irradiation,15 thermal evaporation,16 template growth,17,18hydrothermal methods,19,20and recently hydrothermal synthesis in supercritical water.21,22Among all the methods mentioned above, hydrothermal synthesis in supercritical water is unique which is due to the fact that in this method, size, morphology, and crystal structure of the nanoparticles are adjustable. Supercritical water (SCW) offers a relatively simple method that is inherently scalable and more environmental friendly than the other synthesis methods. The second property is due to the fact that no organic solvent is used in this method. The most important advantages of SCW over the conventional hydrothermal method are faster kinetics, smaller particle size, and the ability of tuning the process that is a result of the drastic change of water physical properties in the vicinity of its critical point.23,24 © 2012 American Chemical Society

The main goal of this study was to precipitate ZnO particles directly at supercritical water. There was a target parameter: “purity of the product” that should be optimized in the SCW process as the functions of at least four controlling factors, namely, “temperature”, “concentration of zinc nitrate solution”, “residence time”, and eventually “pH”. Fortunately, adequate mathematical tools are now available for solving such a difficult multivariable problem. We used the Taguchi orthogonal experiment design method, which has proven its capacity in solving many similar problems.25,26 In addition, to make the results of the Taguchi method more robust, we fortified it with a strong statistical tool, namely, analysis of variance (ANOVA). Finally, some complementary experiments were carried out to verify the outcomes of the Taguchi-ANOVA analysis. Besides optimization, the current study also aimed at revealing the mechanisms of the effects of the controlling factors (e.g., temperature) on the above-mentioned target. In our primary study on ZnO nanoparticles by SCW synthesis, three zinc hydroxinitrates were found to present along with the produced zinc oxide: Zn3(OH)4(NO3)2, Zn(OH)(NO3)·H2O and Zn5(OH)8(NO3)2·2H2O. Subsequently, the mechanism involved in the formation and decomposition of hydroxinitrates named “impurity” and conversion to ZnO nanoparticles were achieved. Received: Revised: Accepted: Published: 1448

September 29, 2012 December 19, 2012 December 26, 2012 December 26, 2012 dx.doi.org/10.1021/ie3026218 | Ind. Eng. Chem. Res. 2013, 52, 1448−1454

Industrial & Engineering Chemistry Research

Article

2. EXPERIMENTAL METHOD 2.1. Synthesis of Nanoparticles. Zinc(II) nitrate hexahydrate (Merck AG. Fur synthesis) was used as the initial substance in this study. Hydrothermal synthesis of the nanoparticles was performed in a stainless steel batch reactor. A schematic sketch of the apparatus is shown in Figure S1 (see in the Supporting Information), which was planned especially to endure working temperature and pressure up to 550 °C and 610 atm, respectively. The capacity of the reactor was 10 cm3, but it was always loaded with half of its volume in order to keep the sufficient safety margin. In the synthesis procedure, concentration of Zn(NO3)2 solution varied from 0.1 to 0.5 mol·dm−3; the temperature ranged from 350 to 500 °C, and the residence time was 15−60 min. The initial pH of the solution was adjusted in the range of 5.0−5.7 by accurate addition of NaOH or HNO3 solutions. The reaction vessel after removing from furnace was quenched rapidly by cold water, and the obtained nanoparticles were separated from the solution through a high speed centrifugation. Afterward, the particles were transferred to Petri dishes and dried at atmospheric conditions. 2.2. Characterization. The synthesized nanoparticles were characterized by scanning electron microscopy (SEM, LEO1455VP), transmission electron microscopy (TEM, LEO 912AB) for evaluating the morphology, X-ray diffractometry (XRD, Philips PW 1800) with Cu Kα line for chemical and crystallographic analysis of the samples. Fourier-transform infrared spectroscopy (FTIR) spectra were recorded on a Thermo Nicolet Smart Golden Gate MKII single reflection ATR spectrometer from 4000 to 500 cm−1, and thermogravimetric analysis (TG, SATA 1500 Scientific Rheometric) from 30 to 400 °C at a heating rate of 10 C min−1 in flowing nitrogen. 3. RESULTS AND DISCUSSION 3.1. Influential Factors of Taguchi Experiment Design. By reviewing the works of previous researchers about the formation of metal oxide nanoparticles,25,26 four independent variables (factors) of the Taguchi experiment design were found to be: temperature, residence time, initial Zn(NO3)2 concentration, and eventually its pH. The variables and levels of a design matrix L9 (34) are given (see Table S1 in the Supporting Information) and the XRD diagrams of the first nine experiments of the Taguchi design are presented in Figure 1a. Beside created zinc hydroxyl nitrates, i.e., Zn(OH)(NO3).H2O, Zn3(OH)4(NO3)2, and Zn5(OH)8(NO3)2·2H2O which are abbreviated as Zn1, Zn3, and Zn5, respectively, no other impurity was detected in the XRD pattern of the ZnO product. FTIR spectra of selected samples are also in agreement with the established structures of the materials. As it was shown in Figure 2, the OH stretch of the hydroxyls appears between 3000 and 3700 cm−1 with the highest frequency reflecting a lower level of hydrogen bonding in the hydroxy salt. The IR spectra of the Zn1, Zn3, and Zn5 are fully consistent with the structures described in the literature.27−29 Figure 3a shows the results of average responses (S/N) of the synthesis system to the various levels of independent factors. In this Figure, “Irel” denotes the ratio of the highest intensity of XRD peak of the main impurity (i.e., Zn1, Zn3, and Zn5) to the corresponding peak of ZnO.

Figure 1. (a) Typical XRD patterns of the produced ZnO product containing impurities and (b) XRD patterns of complementary experiments. (References for hydroxinitrates and zinc oxide according to X-pert high score PDF code are 00-047-0965 (Zn1), 01-070-1361 (Zn3), 00-024-1460 (Zn5), and 00-005-0664 (ZnO), respectively).

Irel =

Imax ,hydroxinitrate Imax ,ZnO

(1)

By denotation of Irel as an index for comparing the impurity quantity of samples, a superior value of Irel indicates higher amounts of hydroxinitrate versus zinc oxide and subsequently an inferior value can be attributed to lower amounts of the major impurity.26 The situation that warrants the application of Irel has been found by performing thermogravimetric analysis (TGA) on three samples, i.e., A3, A6, and A9 with different amounts of Irel. The impurity content of the samples was obtained from the extent of their weight losses in the TGA test (see Figure 4), using the overall following hydroxinitrates decomposition scheme: 1449

dx.doi.org/10.1021/ie3026218 | Ind. Eng. Chem. Res. 2013, 52, 1448−1454


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Figure 4. Typical thermogravimetric analysis (TGA) diagram for ZnO nanopowder that contains impurities. Figure 2. FTIR spectra of three samples with different amounts of hydroxinitrates.

observed TGA trends of A3 and A6 in Figure 4. The amount of impurity that was estimated using the TGA method was consistent with the trend of Irel value of samples. By averaging the responses of system, these results were obtained over different levels of the design matrix which are shown in the first nine rows (see Table S2 in the Supporting Information). For example, in Figure 3a, the response of (Irel) to “temperature” at level 1 was calculated by (Irel1 + Irel2 + Irel3)/ 3 of Table S2, and it was 2.15, while its response to “time” at level 2 was computed by (Irel1 + Irel4 + Irel7)/3 and was about 1.28. The results of ANOVA calculations are illustrated in Figure 3b. Obviously, we conclude form these charts that temperature and pH of the solution by nearly the same order had the most significant effects on the total response of the system (e.g., impurity content). 3.2. Complementary Experiments. In order to verify the previous results about the correct approach for obtaining optimum reaction conditions, a new set of experiments was conducted. Figure 1b shows the XRD diagram of this attempt and Supporting Information Table S2 represents the results of these experiments which also include the data of nine initial Taguchi tests. To make a connection more meaningful, Table S3 (see in the Supporting Information) summarized the effects of different independent factors on the target. In each cell of the table, some couples were incorporated by which one can readily justify the effect of the aimed factor on the inquired target parameter. 3.3. Optimum Condition. By combination of Figure 1b and Supporting Information Tables S2 and S3, one very significant result is improving the values of all controlling parameters at the highest temperature, i.e., 500 °C. In this case, fortunately, higher temperature could compensate the negative effect of “higher initial concentration and pH” which shows them in impurity formation. By considering this result and the three above-mentioned outcomes, optimum operational conditions were found to be: temperature = 500 °C, residence time = 30 min, concentration = 0.1 mol·dm−3, and pH = 5. 3.4. Elucidation of Parameters Effect on Formation and Decomposition of Hydroxinitrates. 3.4.1. Effect of Parameters on Formation of Hydroxinitrates. Knowledge of the previous study on preparation methods of pure Zn1, Zn3

Figure 3. (a) Response of the synthesis system (Irel) to variations of controlling factors at different levels. (b) Significance of the contribution of each controlling factors in the total response of the system (impurity content) calculated by ANOVA analysis.

Znx(OH)y (NO3)z ·nH 2O → ZnO + H 2O + NO + NO2 + O2

(n = 0, 1, 2)

(2)

According to Biswicka et al.,27 Zinc hydroxy nitrate decomposes into ZnO in two highly overlapped steps between 200 and 320 °C which have a good agreement with the 1450



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Water dissociation

and Zn5 may guide us in predicting what really happens in high temperature water. According to the procedures reported by Newman and Jones,28 Zn5 was prepared as the result of ionic reaction by dropwise addition of quantified aqueous sodium hydroxide (0.75 M) to aqueous zinc nitrate (3.5 M) at room temperature:

H 2O → H+ + OH−

(5)

Zn5 formation Zn(NO3)2 + OH− → Zn5(OH)8 (NO3)2 ·2H 2O(s) + NO3− (6)

5Zn(NO3)2 + 12OH−

Indeed, higher residence time helps the progress of the formation reaction, but its suppressing effect on growth which is particle agglomeration occurs at long durations. Consequently we should take an optimum time to allow the reaction to go forward sufficiently by keeping the particles in nanoscale. Increasing both initial concentration and pH seems to be in favor of Zn5 yield (eq 6) but is normally associated with some adverse phenomena such as agglomeration of nanoparticles. It seems to be advantageous to limit both of them under rather low values of 0.1 mol·dm−3 and 5, respectively. 3.4.2. Decomposition of Hydroxinitrates. Previous reports demonstrate the steps associated with the thermal decomposition of Zn5 and Zn1 using TGA analysis in a dry atmospheric environment.33−35They suggested that Zn5 decomposes into ZnO via two intermediates, anhydrous Zn 5 (i.e., Zn5(OH)8(NO3)2) and Zn3:

hydrolysis method

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Zn5(OH)8 (NO3)2 ·2H 2O(s)

(3)

similarly, crystals of Zn5 were synthesized by precipitation from homogeneous solution of zinc nitrate (2 M in water) with urea (2−3 M) at 63 °C.29 Unlike Zn5, the synthesis reaction of which occurred in aqueous media, the preparation of Zn1 and Zn3 was carried out through evaporation method. Auffredic et al.30 reported synthesis of Zn3 which was obtained in crystalline form by slowly evaporating the zinc nitrate hexahydrate salt melted at 120 °C. Chouillet et al.31 achieved the same product in a different melting point (40 °C). Moreover, Zn3 was prepared by this method in 170 °C in about 12 h. Synthesis of Zn1 was accomplished in the same way by heating zinc nitrate hexahydrate at 100 °C for 12 h according to the report27 by Biswicka et al. while Chouillet et al.31 reported a similar product which was obtained at 120 °C during 7 days:

Zn5(OH)8 (NO3)2 ·2H 2O

drying method

Zn(NO3)2 ·6H 2O ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Zn(NO3)(OH) ·H 2O(s) or Zn3(OH)4 (NO3)2(s)

80 − 120 ° C

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Zn5(OH)8 (NO3)2 + 2H 2O (4)

Zn5(OH)8 (NO3)2

It must be regarded that behind these mentioned methods, no literature reported the synthesis of Zn1 or Zn3 in aqueous media. In addition, our attempts to acquire aqueous Zn1 or Zn3 failed. Even if Zn1 or Zn3 are produced along with Zn5 through hydrolysis method at room temperature, the largest amount of standard Gibbs energy of formation (−ΔGf°)for Zn5 among the three products shows that Zn5 will be the first and the main hydroxinitrate product in the aqueous solution. Table 1 gives the values of calorimetric enthalpy of formation, their standard entropy, and standard Gibbs energy of formation of these salts at 298.15 K.32

120 − 140 ° C

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Zn3(OH)4 (NO3)2 + 2ZnO + 2H 2O

ΔH°f / kJ·mol−1

ΔS°f / kJ·K−1·mol−1

ΔG°f / kJ·mol−1

Zn(NO3)(OH)·H2O Zn3(OH)4(NO3)2 Zn5(NO3)2(OH)8·2H2O

−898.18 −1851.04 3754.76

−689.82 −1270.7 −689.82

−692.51 −1472.18 −3079.89

(8)

140 − 170 ° C

Zn3(OH)4 (NO3)2 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 3ZnO + 2HNO3 + H 2O (9)

Zn1 has also been reported to decompose into ZnO via a Zn3 intermediate. Auffredic et al.36 also showed that Zn1 completely transforms into Zn3 under nitrogen at 60−90 °C according to the following reaction: 3Zn(OH)(NO3)·2H 2O 60 − 90 ° C 1 ⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Zn3(OH)4 (NO3)2 + HNO3 + 2H 2O 3

Table 1. Standard Enthalpy, Standard Entropy, and Standard Gibbs Energy of Formation for Three Hydroxysalts32 hydroxinitrate

(7)

(10)

the subsequent transformation of Zn3 into zinc oxide (eq 9) occurs between 160 and 200 °C. Study of Zn5 and Zn1 decomposition was re-examined later by Biswicka et al. 27 which demonstrated that in the decomposition of Zn3 as an intermediate product in eqs 8 and 10, an anhydrous zinc nitrate intermediate is also involved:

With the help of the above-mentioned reason for the formation of Zn5 as the initial substance, the effect of four major parameters can now be explained: at high temperatures, dissociation constant (Kw) of water increases considerably and gives rise to higher concentrations of [OH−] ions, and hence, there would be a stronger hydrolysis of the metal cations to the corresponding hydroxinitrate. Moreover, increasing the temperature contributes to a decrease in dielectric constant of water (ε), and accordingly, decreasing the solvent power of water elevates the level of supersaturation of the solution and in turn leads to the creation of a greater number of nucleation centers in their precipitation and, thus, formation of Zn5 according to the following reactions:

4Zn3(OH)4 (NO3)2 160 − 240 ° C

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 9ZnO + 3Zn(NO3)2 + 7H 2O + 2HNO3 (11)

Formation of ZnO was outlined in the following reaction: 260 − 320 ° C

Zn(NO3)2 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ ZnO + NO2 + NO + O2

(12)

Although the suggested mechanisms are primarily based on using TGA analysis in dry atmospheric environment, they can practically be applicable for gas properties of water as particular feature at supercritical conditions. It must be mentioned that several primary experiments were performed by varying all 1451



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parameters at subcritical temperature, but no outcome was detected in the solution which confirms that decomposition of hydroxinitrates occures in a gas-like media of water at supercritical conditions. By considering the variation of “Znx” as the main impurity among all the existing hydroxinitrates together with its relative quantity Irel in Supporting Information Table S3, the following results were deduced: • At lower temperature, Zn5 is more prominent in comparison with other hydroxinitrates. Although the decomposition of Zn5 was accomplished by degradation in Irel, all evidence in Table S3 show that the rate of Zn5 production overwhelmed to conversion of it according to eq 8. It means that by rising the temperature, supersaturations of Zn5 nucleus forming at subcritical conditions (in that case higher solubility power of water) through ionic reaction occurs at supercritical conditions. It must be regarded that according to eq 6, initial concentration and pH addition help the formation of Zn5. • At elevated temperature, the rate of conversion of Zn5 to Zn3 overcomes its formation rate. On the other hand, at higher temperatures, we observe Zn3 more than the other two impurities. This can be made due to the conversion of Zn5 to Zn3 according to the reaction (eq 8), or another mechanism in which zinc nitrate solution is dried and then solid Zn1 and Zn3 are produced through evaporation method (eq 4) and finally conversion of Zn1 to Zn3 (eq 10) occurs. The latter mechanism cannot be ignored because water will considerably lose its ionic property at that time and this environment behaves more like a gas with low polarity (ε < 10) rather than an ordinary liquid which can solve nearly all nitrate salts (ε > 80). Once again Supporting Information Table S3 states that raising the temperature is the only way to compensate the negative effects of other parameters. To sense the effect of high temperature water directly on the size and morphology of particles, an additional visual analysis was performed on three samples with different impurity contents (Irel). Figure 5 shows the scanning electron microscopy (SEM) of A3, A6, and A9 the previous analysis of which (TGA) confirmed the amount of impurity. Although increasing all the parameters except temperature has an adverse effect on size (i.e., growth and agglomeration of particles) and purity of particles, raising the temperature could suppress the opposing effects by decomposition of hydroxinitrates and size reduction due to the decline of dielectric constant . Nevertheless, to prevent all the unfavorable effects mentioned, it is better to operate in an optimized condition to preserve the size of pure ZnO particles in nanoscale. The TEM pattern of A13 together with its related size distribution graph as a sample which satisfies our anticipation indicates the narrow size distribution without any impurity.

Figure 5. SEM image of samples with different quantity of impurity: Ireal (A3) = 3.01, Ireal (A6) = 0.83, Ireal (A9) = 0.08, and TEM image of ZnO nanoparticles synthesized at optimized condition (A13) beside the size distribution graphs of A9 and A13.

In our experiments no sign of the Zn(OH)2 (intermediate substance of the other proposed mechanism21−24) was observed in the XRD analysis. Instead, there was often a peak of three hydroxinitrates i.e., Zn(NO 3 )(OH)·H 2 O, Zn5(NO3)2(OH)8·2H2O, and Zn3(OH)4(NO3)2. The suggested mechanisms for the formation of these impurities were found to be formation of Zn5(NO3)2(OH)8·2H2O and then conversion to Zn3(OH)4(NO3)2 which finally decomposes into zinc oxide. The suggested mechanism is the following set of reactions: Water dissociation

H 2O → H+ + OH−

(I)

Hydroxynitrate formation Zn(NO3)2 + 3OH− → Zn5(NO3)2 (OH)8 ·2H 2O + NO3− (II)

Decomposition Zn5(NO3)2 (OH)8 ·2H 2O

4. CONCLUSION The Taguchi experiment design method offers a simple but effective technique for optimization of different chemical processes. The current study demonstrated that this method when supported with the direct analytical techniques (e.g. XRD and TGA) can be a valuable tool for revealing the formation mechanism of materials.

↔ Zn3(OH)4 (NO3)2 ↔ HNO3(g) + H 2O(g) + ZnO

(III)

Besides the above mechanisms, there is still another mechanism in which zinc nitrate solution is dried at first and then solid Zn(NO3)(OH)·H2O and Zn3(OH)4(NO3)2 are produced and then are converted into Zn3(OH)4(NO3)2 which 1452



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arrays and their optical and field emission properties. Nanotechnology 2005, 16, 2567. (15) Qian, D.; Jiang, J. Z.; Hansen, P. L. Preparation of ZnO nanocrystals via ultrasonic irradiation. Chem. Commun. 2003, 1078. (16) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Nanobelts of semiconducting oxides. Science 2001, 291, 1947. (17) Shen, X. P.; Yuan, A. H.; Hu, Y. M.; Jiang, Y.; Xu, Z.; Hu, Z. arrays. Nanotechnology 2005, 16, 2039. (18) Zhu, P.; Zhang, J.; Wu, Z.; Zhang, Z. Microwave-Assisted Synthesis of Various ZnO Hierarchical Nanostructures: Effects of Heating Parameters of Microwave Oven. Crys. Growth Des. 2008, 8, 3148−3153. (19) Baruwati, B.; Kumar, D. K.; Manorama, S. V. Hydrothermalsynthesis of highly crystalline ZnO nanoparticles: A competitive sensor for LPG and EtOH. Actuactors Sen. B 2006, 119, 676−682. (20) Shim, J. B.; Chang, H.; Kim, S. O. Rapid Hydrothermal Synthesis of Zinc Oxide Nanowires by Annealing Methods on Seed Layers. J. Nanomater. 2011, 25. (21) Adschiri, T.; Kanazawa, K.; Rapid, Arai K. and Continuous Hydrothermal Crystallization of Metal Oxide Particles in Supercritical Water. J. Am. Ceram. Soc. 1992, 75, 1019−1022. (22) Sue, K.; Murata, K.; Kimura, K.; Arai, K. Continuous synthesis of zinc oxide nanoparticles in supercritical water. Green Chem. 2003, 5, 659−662. (23) Viswanathan, R.; Gupta, R. B. Formation of zinc oxide nanoparticles in supercritical water. J. Supercrit. Fluids 2003, 27, 187−193. (24) Viswanathan, R.; Lilly, G. D.; Gale, W. F.; Gupta, R. B. Formation of Zinc Oxide−Titanium Dioxide Composite Nanoparticles in Supercritical Water. Ind. Eng. Chem. Res. 2003, 42, 5535−5540. (25) JafariNejad, Sh.; Abolghasemi, H.; Moosavian, M. A.; Golzary, A.; Maragheh, M. G. Fractional factorial design for the optimization ofhydrothermal synthesis of lanthanum oxide nanoparticles under supercriticalwater condition. J. Supercrit. Fluids 2010, 52, 292. (26) Outokesh, M.; Hosseinpour, M.; Ahmadi, S. J.; Mousavand, T.; Sadjadi, S.; Soltanian, W. Hydrothermal Synthesis of CuO Nanoparticles: Study on Effects of Operational Conditions on Yield, Purity, and Size of the Nanoparticles. Ind. Eng. Chem. Res. 2011, 50, 3540− 3554. (27) Biswicka, T.; Jonesa, W.; Pacuab, A.; Serwickab, E. Podobinski. The role of anhydrous zinc nitrate in the thermal decomposition of the zinc hydroxy nitrates Zn5(OH)8(NO3)2 - 2H2O and ZnOHNO3 -H2O. J. Solid State Chem. 2007, 180, 1171−1179. (28) Newman, S. P.; Jones, W. J. Comparative study of some layered hydroxide salts containing exchangeable interlayer anions. J. Solid State Chem. 1991, 48, 26. (29) Stählin, W.; Oswald, H. R. The topotactic reaction of zinc hydroxide nitrate with aqueous metal chloride solutions. J. Solid State Chem. 1971, 3, 252. (30) Auffredic, J. P.; Louër., D. A calorimetric study of the thermal decomposition of zinc hydroxynitrate Zn3(OH)(NO3)2. Thermochim. Acta 1978, 22, 193−196. (31) Chouillet, C.; Krafft, J. M.; Louis, C.; Lauron-Pernot, H. Characterization of zinc hydroxynitrates by diffuse reflectance infrared spectroscopystructural modifications during thermal treatment. Spectrochimica Acta: Part A 2004, 60, 505−511. (32) Auffrdic, J.; Louër, E. D. Enthalpies de formation des hydroxinitrates. Thermochim. Acta 1976, 16, 223−229. (33) Staḧlin, W.; Oswald, H. R. The Infrared Spectrum and Thermal Analysis of Zinc Hydroxide Nitrate. J. Solid State Chem. 1971, 2, 252− 255. (34) Auffredic, J. P.; Louër, E. D. Controlled microstructural properties of zinc oxide powder formed by the thermal decomposition of zinc hydroxinitrate. Reactivity Solids 1987, 4, 105−115. (35) Biswick, T.; Jones, W.; Pacu, A.; Serwicka, E. Synthesis, characterisation and anion exchange properties of copper, magnesium, zinc and nickel hydroxy nitrates. J. Solid State Chem. 2006, 179, 49−55.

turns into zinc oxide according to reaction III. The experimental evidence of this study revealed that the formation of Zn5(NO3)2(OH)8·2H2O occurs through ionic reactions in the solution at lower temperature, while the two other hydroxinitrates prefer high temperature water where formation occurs far from ionic reaction.

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ASSOCIATED CONTENT

S Supporting Information *

Details of the Taguchi experiment design (Tables S1, S2, and S3), as well as supercritical water reactor scheme (Figure S1). 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]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We appreciatively express our gratitude to Dr. Amir Charkhi and Dr. Soudeh Sadjadi for their helpful discussions on reactions mechanisms. We also acknowledge Ms. Samira Hafeziyeh for the grammatical revision support of this research.

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REFERENCES

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