ARTICLE pubs.acs.org/IECR
Hydrothermal Synthesis of CuO Nanoparticles: Study on Effects of Operational Conditions on Yield, Purity, and Size of the Nanoparticles M. Outokesh,† M. Hosseinpour,† S. J. Ahmadi,†,§ T. Mousavand,*,‡ S. Sadjadi,† and W. Soltanian† † ‡
School of Energy Engineering, Sharif University of Technology, Azadi Ave., P.O. Box 113658639, Tehran, Iran Department of Mechanical Engineering, McGill University, 817 Sherbrooke Street, West Montreal, QC H3A 2K6, Canada ABSTRACT: Hydrothermal synthesis of CuO nanoparticles under near-critical and supercritical conditions was investigated from two different standpoints in the current study. The first standpoint was optimization of “yield”, “purity”, and “size of the nanoparticles” that were optimized at T = 500 °C, time = 2 h, [Cu(NO3)2] = 0.1 mol dm-3, and pH 3. This was achieved by undertaking an orthogonal experiment design methodology and performing different instrumental analyses, such as X-ray diffractometry, inductively coupled plasma spectrometry, and transmission electron microscopy, along with treatment of the data by analysis of variance (ANOVA). The second goal of the study was elucidation of the mechanisms of effects of operational conditions (e.g., temperature) on the above-mentioned target parameters, through application of the appropriate mechanisms of formation of nanoparticles. Nanoparticles are suggested to form initially in the liquid phase as Cu(OH)2, which are later transformed to Cu2(OH)3NO3, through which CuO product is obtained. Decomposition of nitric acid also plays role in this mechanism. Fabricated nanoparticles are effective catalysts for the synthesis of benzoheterocycle compounds in the pharmaceutical industries.
1. INTRODUCTION Aggregations of atoms or molecules that have at least one of their dimensions in the nano size scale (i.e., between 1 nm and 100 nm) indicate properties that are fundamentally different from the properties of the individual atoms or bulk matter. In general, it is the preparation method of these nanomaterials (particles, fibers, or coatings) that determines their principal characteristics, such as size, morphology, and surface, which, in turn, are of high importance in the subsequent applications.1,2 As for the special case of inorganic nanoparticles, so far, numerous methods have been developed, including (among others) solid-state reactions, sol-gel method, coprecipitation, and hydrothermal techniques. The first three methods, although used widely, suffer from some major shortcomings. Solid-state reactions are generally associated with poor composition and morphology control. Sol-gel methods allow excellent control of composition and morphology, but its process is costly. Additional calcination and milling steps, which are required in the coprecipitation method, make it less desirable than the singlestep synthesis processes.3 Among different methods of preparation of metal oxide nanoparticles, supercritical water (SCW) synthesis is unique, because, in this method, the size, morphology, and crystal structure of the nanoparticles are adjustable. SCW offers a relatively simple route that is inherently scalable, and, because no organic solvents are used, environmentally more benign than the other synthesis pathways. The most important advantages of SCW over the conventional hydrothermal method are these: faster kinetics, smaller particle size, and tuning ability of the process that is a result of the drastic change of physical properties of water in the vicinity of its critical point. So far, the SCW method has been successfully used for the synthesis of many significant inorganic nanoparticles (see Table 1). r 2011 American Chemical Society
One of the most important metal oxides, from the standpoint of catalytic usage, is copper(II) oxide (CuO). CuO is an oxidationreduction catalyst whose application in organic chemistry dates back to the 19th century. In its nano form, CuO has found a broader range of application in high-critical-temperature (highTc) superconductors,5 gas sensors,6 catalysts for the water-gas shift reaction,7 steam reforming,8 and CO oxidation of automobile exhaust gases.9 Recently, our research group has succeeded in the synthesis of tetrahydrobenzofurans and benzoheterocycle compounds using a CuO nanocatalyst.10,11 These organic compounds are important intermediates in the preparation of a wide range of synthetic derivatives with diverse biological and pharmaceutical applications.12-15 For instance, 1,4-benzothiazin-3(4H)-ones are antihypertensive drugs, calcium antagonists, and highly potent inhibitors of low-density lipoprotein (LDL) oxidation.16 To date, several techniques have been used for the fabrication of CuO nanoparticles. These methods include the metalorganic chemical vapor deposition (MOCVD) template method,17 the wet-chemistry route,18 sonochemical preparation,19 alkoxidebased preparation,20 and solid-state reaction in the presence of a surfactant.21 Nevertheless, a supercritical hydrothermal method has never been used for this purpose. If conducted under an optimal condition, SCW produces a nanometric product with a high reaction yield. However, what exactly does the term “optimal” mean in this context? To answer this question, we must go through more details in the next paragraph.
Received: August 12, 2010 Accepted: January 5, 2011 Revised: December 1, 2010 Published: February 16, 2011 3540
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Table 1. Compounds Produced in Nanoparticles by Hydrothermal Synthesis in Supercritical Water (SCW)4 Particle Dimension product
range
AlOOH
mean size 80-1000
authors Adschiri et al., Hakuta et al.
R-Fe2O3 Fe3O4
50 50
Adschiri et al. Adschiri et al.
Co3O4
100
Adschiri et al.
NiO
200
Adschiri et al.
ZrO2(cubic)
The current study has three new features. It is the first attempt at using SCW for the synthesis of CuO as one the most significant nanometal oxides. But, more importantly, it takes a new approach for optimization (i.e, Taguchi-ANOVA analysis) that has been rarely used in supercritical water technology. 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 parameters. This goal has been achieved through investigation of the mechanisms of formation of the CuO nanoparticles in SCW. The suggested mechanisms are primarily based on the works of the previous researchers; however, because of the particular feature of the CuO synthesis, the authors had to take some new standpoints and suggest a series of new reactions that have not been proposed so far. Also, this work is probably the first attempt in applying elevated temperatures in the range of 500 °C for the SCW synthesis of nanoparticles.
10
Adschiri et al.
10-1000
20
Adschiri et al.
20
Adschiri et al.
CeO2
20-300
18
Hakuta et al.
BaO 3 6Fe2O3 Al5(YþTb)3O12
50-1000 20-600
∼100
Hakuta et al. Hakuta et al.
2. EXPERIMENTAL METHODS 2.1. Synthesis of Nanoparticles. Copper(II) nitrate trihydrate (Merck AG, Fur synthesis) was used as the raw material for the synthesis of CuO nanoparticles. Hydrothermal synthesis was carried out in a stainless steel batch reactor (type 316 L), especially designed to endure a working pressure and temperature of 610 atm and 550 °C, respectively. The capacity of the reactor was 200 cm3; however, to maintain an adequate safety margin, it was always loaded to only one-third of its volume. In the synthesis procedure, concentration of Cu(NO3)2 solution varied from 0.1 mol dm-3 to 0.5 mol dm-3; the temperature ranged from 350 °C to 500 °C, and the residence time was 13 h. The initial pH of the solution was adjusted in the range of 3-3.75 by precise addition of NaOH or HNO3 solutions. After removing from furnace, the reaction vessel was quenched by cold water and produced nanoparticles were separated from the solution using a high-speed centrifuge. Afterward, supernatant was sent for copper analysis by inductively coupled plasma (ICP, Varian 150AX turbo) to result in the yield of reaction; and the obtained nanoparticles were three-fold washed/centrifuged with distilled water and dried under ambient conditions. 2.2. Characterization. The synthesized nanoparticles were characterized by transmission electron microscopy (TEM) (LEO Model 912AB), X-ray diffractometry (XRD) (Philips Model PW 1800), thermogravimetric analysis (TGA) (Model SATA 1500, Scientific Rheometric), and measurement of the specific surface area via nitrogen adsorption testing (i.e., BET, Nova Model 2000e).
TiO2 TiO2(anatase)
LiCoO2
40-400
Hakuta et al.
ZrO
10-1000
Adschiri et al.
20-200
Adschiri et al.
AlO(OH) Ce1-xZrxO2 CeO2
3-5, 4-7
Cabanas et al.
20-200
Adschiri et al.
Fe3O4 þ Fe
40-92
Cabanas et al.
CoFe2O4 NiFe2O4
39-72 28-43
Cabanas et al. Cabanas et al.
NixCo1-xFe2O4
23-42
Cabanas et al.
ZnFe2O4
47-105
Cabanas et al.
AlOOH R-Fe2O3
30-60
13-16
Li et al.
30-40
Cote et al.
25-170 Co3O4
30-60
30-40
20-30 CoFe2O4 ZnO
20 120-320
Cote et al. Cote et al. Viswanathan et al.
39-251 YAG:Tb
14-152
Hakuta et al.
ZrO2
3-5
Galkin et al.
TiO2
7-9
Galkin et al.
1% Pd/ZrO2
3-5
Galkin et al.
1% Pd/TiO2 R-Fe2O3, Fe3O4
7-9 14 -25
Galkin et al. Takami et al.
AlOOH
45-500
Mousavand et al.
La2CuO4
70-120
Galkin el al.
There are three target parameters—“yield of the reaction”, “size of the nanoparticles”, and “purity of the products”—that must be optimized in the SCW process, as the functions of at least four controlling factors, namely, “temperature”, “concentration of copper nitrate solution”, “residence time”, and eventually “pH”. Fortunately, to solve such a difficult multivariable problem, adequate mathematical tools are now available. We made use of the Taguchi orthogonal experiment design method, which has proven its capacity in solving many similar problems.22-25 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.
3. RESULTS AND DISCUSSION 3.1. Optimization of the Synthesis Process. Optimization of the synthesis process was the prime goal of the current study, and therefore it is discussed first in this section. The outcomes of this section are specific operational conditions in which the three target parameters of the study (i.e., purity, yield, and size of the nanoparticles) attain their optimum values. The section starts by describing the procedure of designing the experimental array, which is followed by analysis of the responses of the system using Taguchi-ANOVA analysis. Complementary experiments then will be discussed that, together with the foregoing statistical methods, specify the optimum reaction conditions. 3.1.1. Setting of the Experiment Design Matrix. The extensive studies of the previous researchers had revealed4,26,27 that, under supercritical conditions, the controlling factors whose variations 3541
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Figure 1. Response of the synthesis system to variations of controlling factors at different levels: (a) impurity content, (b) yield of reaction, and (c) size of nanoparticles. In all figures, X represents temperature (°C), Y represents time (h), Z represents concentration (mol dm-3), and, eventually, W represents the pH of the initial solution.
significantly influence the characteristic of the nanoparticles are: “temperature”, “residence time”, “initial Cu(NO3)2 concentration”, and, eventually, its “pH”. Ranges of variations of these factors in the current study were restricted by certain physicochemical limitations. For instance, application of excessively low temperatures (i.e., < 300 °C) led to a rather low reaction yield. On the other hand, temperatures that were too high (T g 500 °C) gave rise to an excessive steam pressure, the handling of which demanded special safety precautions. For pH, limitations were of another type. Using a pH value of >4 led to an immediate precipitation of Cu2þ ions under ambient conditions and prior charging of the solution into the reactor. Conversely, a pH value of 450 °C), where Cu2(OH)3NO3 is extremely unstable, the two bulleted mechanisms merge into each others and, in that case, only reaction set 4-6 occurs. One significant point that has not been emphasized by the previous researchers is the important role of the decomposition of nitric acid to N2, H2O, etc. The occurrence of this reaction guarantees the stability of formed nanoparticles, because of the removal of their solvent (HNO3) at the end of the reaction. At the end, it seems worthwhile to briefly discuss the generality and applicability of the above findings. The present study exploited the batch mode of the hydrothermal synthesis for fabrication of the CuO nanoparticles. The main advantage of the batch mode is the possibility of controlling of oxidation states of the elements and preparation of a system of mixed oxides in which a polycrystalline structure of high homogeneity is formed. The mixed oxides phases are of great importance in the manufacturing of the major industrial catalysts, and in the other applications of nanotechnology. The current article, besides synthesizing of one of the important nanomaterials, has provided the following: (1) The trends of the responses of the target parameters to variations in the controlling factors, and the weight of the 3549
dx.doi.org/10.1021/ie1017089 |Ind. Eng. Chem. Res. 2011, 50, 3540–3554
Industrial & Engineering Chemistry Research influence of each controlling factor in the total response of the system. For instance, the results of this study demonstrated that the effect of temperature on size of the nanoparticles is more pronounced than that of pH. Using the knowledge acquired in this study, the authors have attempted to synthesize some other metal oxides in other batch reactors that had different geometries. A brief summary of this investigation is presented in Table 77. In either case, the same trends of variation of the target parameters with controlling factor were observed, even though the numerical results were, to an extent, different. Some of the results of the current batch experiments can be used for parameter setting of the large-scale continuous process. For example, for the first time, our batch experiments exploited temperatures as high as 500 °C for the supercritical water (SCW) synthesis, which was an experience that was proved advantageous. Now, based on this experience, it is advisible to increase the temperature of the continuous SCW reactors from its present values (normally
