Supercritical Hydrothermal Synthesis of Submicrometer Copper(II

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Supercritical Hydrothermal Synthesis of Submicron Copper(II) Oxide: Effect of Reaction Conditions Panpan Sun, Shuzhong Wang, Tuo Zhang, Yanhui Li, and Yang Guo Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 14 May 2017 Downloaded from http://pubs.acs.org on May 16, 2017

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Industrial & Engineering Chemistry Research

Supercritical Hydrothermal Synthesis of Submicron Copper(II) Oxide: Effect of Reaction Conditions Panpan Suna, Shuzhong Wanga,*, Tuo Zhanga, Yanhui Lia, Yang Guoa,b a

Key Laboratory of Thermo-Fluid Science and Engineering, Ministry of Education,

School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi, 710049, China b

Xi’an Jiaotong University Suzhou Academy, Suzhou, Jiangsu, China

∗

Corresponding author. E-mail address: [email protected] (Shuzhong Wang)

Key words: copper(II) oxide, submicron particles, supercritical hydrothermal synthesis

Abstract Copper(II) oxide ultrafine particles are of great interest as a new material for multiple applications. This paper expounds the synthesis and characterization of copper(II) oxide submicron particles formed by a facile and simple supercritical hydrothermal synthesis method. The copper(II) nitrate, copper salt, was used as a precursor aqueous solution heated by a preheated sand bath to reach supercritical conditions. The effects of process operating parameters, such as temperature, pressure, the addition of sodium hydroxide,

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precursor concentrations on the morphology and the size of copper(II) oxide submicron particles have been investigated. The copper(II) oxide particles formed, with particle sizes of ca. 100 nm, were hexagon-flake-like and spindle-like and free of impurities. The average particle size decreased with the increase of temperature under subcritical conditions and decrease of pressure. It decreased with the increase of precursor concentration at lower concentration conditions and revealed an adverse trend at higher concentrations. Sodium hydroxide accelerates the conversion of copper ions.

1. Introduction Ultrafine metal oxides are of great research significance to research scholars and engineers due to their particular properties, such as catalytic, magnetic, electronic and optical properties. More and more researchers studied the applications of nanoscale metal oxides in chemical engineering, physical engineering and obtained dramatic achievements.1,

2

The great significance in the applications of ultrafine metal oxide

materials is owing to the so-called “interfacial effect” and “large specific surface area”, which contributes to the remarkable differences in chemical and physical properties from normal materials with same chemical compositions. Among transition metallic oxides, copper(II) oxide draws much attention from both the domain of science and industrial fields because of its great potential in applications as a semiconductor, high-temperature superconductor and giant magneto resistance material.3, 4 Numerous methods have been explored to synthesize various copper(II) oxide ultrafine 2


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structures, such as chemical precipitation,5 electrochemical,6 spray pyrolysis,7, 8 thermal oxidation9 and sonochemical synthesis10. However, the processes of these methods, usually complicated and requiring a long period of reaction time, are not suitable for large-scale production of submicron copper(II) oxides. On the whole, large-scale synthesizing of high-quality and low-cost ultrafine particles by conventional methods is difficult and even becomes a critical issue in the development of manufacturing ultrafine copper(II) oxides. Therefore, a lot of researches are needed to improve a synthesis methods that can provide good performance in larger-scale and high-quality production. Properties of water near its thermodynamic critical point (Pc = 22.1 MPa, Tc = 373.9 °C) are very different from those liquid water at ambient condition.11 Density of supercritical water is much lower than that of water at ambient conditions. Also, the dielectric constant of water decreases with increase in temperature and decrease in pressure, and is generally below 10 under supercritical conditions, in which it is extremely close to that of polar organic solvent. As a result, supercritical water (SCW) behaves like many organic solvents, leading to a high organic compound solubility in near-critical water and a complete miscibility in SCW. The primary cause of these changes is dramatic decreases in the quantity and strength of hydrogen bonds. Accordingly, the solubility of some inorganic salt undergoes a sudden reduction when it is approaching supercritical conditions. Supercritical hydrothermal synthesis (SHS), first proposed by Adschiri,12 is a method to produce metal crystals and metal oxide crystals from metal salt aqueous solutions by rapidly heating the aqueous solutions to 3



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supercritical conditions. During SHS method, supersaturation condition is created via low metal oxide solubility in SCW. Hydrothermal conditions provided by SCW are quite suitable for crystal formation. SHS is supposed to be one of the most promising synthesis methods for inorganic metal oxide ultrafine particles owing to the following advantages: 1) it yields extremely high reaction rate and high nucleation rate because of the high temperature, reducing the process time, 2) it’s an environmentally benign process with harmless water as the main solvent, 3) it facilitates the formation of highly crystalline small crystals because particle growth is restrained when the metal salt solution being quickly heated to proper a high temperature in an fairly short time and 4) it’s an method appropriate for commercial application in large-scale synthesis of inorganic metal ultrafine particles. Recently, a large number of researches have been done and many metal particles and metal oxide ultrafine particles have been synthesized, such as Ni,13, 14 Ag,13 Cu,13, 15 ZrO2,16 Li4Ti5O12,17 CeO2,18, 19 TiO2,20, 21 ZnO22, 23 and AlOOH24. A number of researches about synthesizing copper(II) oxide ultrafine particles in SCW have been done. Kim et al.13 synthesized copper(II) oxide micrometer particles at 400°C and 300bar, using copper nitrate with a 23 ml stainless steel batch reactor. However, the reaction time (including heating time) is 10 min, which is too long for a quick SHS process. Sue et al.25 studied the synthesizing process of copper(II) oxide nanoparticles by using a continuous hydrothermal synthesis system with a T-type micro mixer, and the effect of the residence time (0.002-2 s) was demonstrated. Outokesh et al.26 made a research on manufacture of CuO nanoparticles in a 200 ml stainless steel batch reactor, and the residence time were 4


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1-3 h. Above all, detailed effects of the operation parameters on the metal oxide ultrafine particles prepared in supercritical water have not been fully studied yet and the comprehensive knowledge in published literatures can hardly been found. To our best knowledge, the growth mechanism of different ultrafine structures obtained in supercritical water has not been reported. To fill this gap, in this paper, the comprehensive influence of the operating parameters (the addition of NaOH, temperature, pressure and precursor concentration) on the morphologies and sizes of the products have been investigated and the growth mechanism of the copper(II) oxide ultrafine structures in supercritical water has been expounded.

2. Materials and methods 2.1 Materials and procedures Copper (II) nitrate (Cu(NO3)2•3H2O, purity>99.9wt%), Sodium hydroxide (NaOH, purity>99.5wt%) and ethanol were obtained from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China), which were used as received without further purification. The metal salt aqueous solution was prepared by using distilled and deionized (DDI) water. High-pressure tube reactors with the inner volume of 5.2 mL (inner diameter and length of 8.7 mm and 80 mm respectively) were adopted to get quick heating (about 2 min) as far as possible. These reactors made of stainless steel 316 with design pressure 35 MPa, were used for synthesis of copper(II) oxide particles. The reactors were loaded with a 5



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predetermined amount of prepared copper nitrate solution and sodium hydroxide in order to reach specific pressure within the reactors when they reached the preset temperature. The pressure was calculated based on the water density in an enclosure space. Then, the reactors were well sealed and soaked into a fluidized sand bath, which was electrically preheated and maintained at an expected temperature. When the expected residence time (5 min) reached, the reaction was immediately terminated with the reactors transferred into a cold water bath. The solid products and liquid products in the reactors were collected separately, and the solid products were purified through several cycles of centrifugation and washing by deionized water and analytically pure ethanol to remove undesired ions. Finally, the obtained solid products after vacuum drying for 24 h at 80 °C, was used for evaluation on performance of copper(II) oxides. 2.2 Analysis means The copper ions remained in the recovered aqueous solutions after the reaction was determined by inductively coupled plasma emission spectroscopy (Thermo, ICP-MS X2, USA) and the percent conversion (%) of the copper ions from metal salt into solid products was calculated by using the following equation (1): X=

(1)

× 100

in which represents the initial copper ion concentration of precursors, represents the copper ion concentration of effluent solutions after reaction. Phase distributions of the products were identified by XRD (PANalytical X’pert MPD Pro diffractometer with a Ni-filtered Cu Kα irradiation). Morphology characteristics of 6


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the products obtained were analyzed by using a field emission scanning electron microscope (FESEM, Hitachi su-8010, Japan) and a transmission electron microscope (TEM, NEC JEM-200CX, Japan). The average particle size (APS) and the corresponding standard deviation (SD) of the products were analyzed based on the above FESEM images obtained and the size of 150 particles manually measured. As for nonspherical particles, the longest length was measured to obtain the APS. For example, as for plate-like products, the longest diagonal of polygon was measured. The crystallite size of products is calculated through Scherrer equation using the full width at half maximum of the peaks of the XRD patterns.

3. Results and discussion Experimental conditions and results such as the conversion rate calculated based on copper ion concentration decided by ICP, particle size measured from FESEM micrographs, crystallite size calculated from XRD peak, are summarized in Table 1. Table 1 Summary of experimental conditions and results obtained Precursor

Temperatu

Pressure

Sodium hydroxide

Percent

Average

Standard

Crystallite

concentration

re (°C)

(MPa)

concentration

Conversi

particle

deviation

size (nm)

(mol/L)

on (%)

size (nm)

(nm)

(mol/L) 0.1

400

30

0

24.3

-

-

-

0.1

400

30

0.05

33.6

112.1

52.6

75.4

0.1

400

30

0.1

53.1

107.7

48.8

26.0

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4

0.1

400

30

0.2

96.0

96.5

28.8

23.6

5

0.1

400

30

0.3

96.1

97.2

29.8

38.7

6

0.1

250

30

0.2

97.0

-

-

39.7

7

0.1

300

30

0.2

96.0

-

-

51.6

8

0.1

350

30

0.2

97.4

100.2

44.1

30.6

9

0.1

450

30

0.2

97.8

104.6

29.3

25.5

10

0.1

400

22

0.2

91.6

83.9

44.5

14.1

11

0.1

400

24

0.2

92.3

86.2

34.6

15.6

12

0.1

400

26

0.2

97.1

97.9

24.9

14.3

13

0.1

400

28

0.2

96.7

107.0

27.6

20.1

14

0.01

400

30

0.02

99.8

161.6

43.1

28.3

15

0.02

400

30

0.04

98.3

119.5

46.1

24.5

16

0.05

400

30

0.1

97.3

112.1

48.0

22.1

17

0.2

400

30

0.4

96.0

168.6

51.4

25.5

3.1 Effects of NaOH concentrations Based on the relation between conversion and NaOH concentration shown in Fig. 1, it can be seen clearly that the addition of base has a significant influence on the conversion of copper ions. The conversion degree of copper ions increased with an increase in additive NaOH concentrations as the concentration is below 0.2 M. At the absence of base, the percent conversion of copper ions was just 24.3%. When the molar

8


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concentration ratio of NaOH/Cu(NO3)2 increased to 2.0, the copper ions almost transformed into copper(II) oxides completely. These results are similar with the result obtained by Kiwamu Sue,27 who made a research for fabricating zinc oxides by employing SHS and found that the conversion rate of zinc ions increased when the molar ratio of base to zinc ions in precursors increased from 2:1 to 4:1. The positive and negative ions of metal salts, temperature and pH value of reaction conditions have a significant influence on the hydrolysis properties of metal salts. Generally, with the temperature and pressure of metal salt aqueous solutions approaching the near-critical condition of water, the concentration of OH− increased, thus the hydrolysis rate of metal salt was accelerated.28 However, A research performed by Linda J. Cote29 indicated that it was impossible to obtain cobalt oxides without the base at 400 °C. In the hydrolysis of Cu(NO3)2, the acceleration of high temperature and pressure was not enough, thus the addition of the base further facilitated the hydrolysis of Cu(NO3)2. The hydrolysis reaction expressed in equation (2) was promoted toward right due to the high concentration of OH−, further increasing the percent conversion of copper ions.

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Fig. 1. The relation between conversion of copper ions (● represents), average particle size (■ represents) and NaOH concentrations at 400 °C and 30 MPa with a precursor concentration of 0.1 mol/L

The XRD results of experiments No. 1 to No. 5 are shown in Fig. 2. The FESEM micrographs of the products with different NaOH concentrations are shown in Fig. 3. For experiments without NaOH, the average particle size of products obtained was at micron order (The corresponding FESEM picture is provided in Supporting Information). However, the average particle size and crystalline size decreased with an increase in NaOH/Cu(NO3)2 molar ratio, and respectively reached to 96.5 nm and 23.6 nm when the ratio was up to 2.0. The SD revealed a same trend, of corresponding decrease from 52.6 nm to 29.8 nm. These indicated that addition of sodium hydrate into copper nitrate in 10


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SHS facilitated the production of CuO with a uniform and smaller size. The decreases of crystallite size and SD may be resulted from the increasing supersaturation of metal oxides30 due to the addition of the base. The addition of hydroxyl ions leaded to more copper(II) oxide nucleus in the SCW. According to the definition of supersaturation, the supersaturation of copper(II) oxide became higher. The higher supersaturation, the more copper ions consumed in nucleation, then the smaller and more uniform particles produced. At the crystal growth stage, crystallites grown to be bigger crystals in the force of high free energy at high-temperature. That is why there exist a difference between average particle size and crystallite size. Overall, it is significant to add base to the supercritical hydrothermal synthesis of CuO from the viewpoint of conversion of copper ions and size of the products. As we can see from Fig. 3, morphology of products changed. When the additive NaOH concentrations were 0.05 M and 0.1 M, the products synthesized were irregular polygon flake-like; when the NaOH concentrations increased to 0.2 and 0.3M, the morphology of products turned out to be hexagon flake-like. The formation of irregular polygon flake-like CuO crystals can be explained as the immature products of hexagon flake-like crystals because of the lower supersaturation inducing by lower NaOH concentrations (0.05M, 0.1M). The growth mechanism of hexagon flake-like crystals will be explained in section 3.2.

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Fig. 2. XRD results with different NaOH concentrations 1) 0 mol/L; 2) 0.05 mol/L; 3) 0.1 mol/L; 4) 0.2 mol/L; 5) 0.3 mol/L at 400 °C and 30 MPa with a precursor concentration of 0.1 mol/L

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Fig. 3. FESEM images of products with the addition of different NaOH concentrations (a) 0.05 mol/L; (b) 0.1 mol/L; (c) 0.2 mol/L; (d) 0.3 mol/L at 400 °C and 30 MPa

3.2 Effects of temperature We have studied SHS of copper(II) oxide submicron structures at different temperatures from 250°C to 450°C on the presence of base (the molar ratio of NaOH to Cu(NO3)2 in aqueous solutions was 2:1). The XRD spectrograms are shown in Supporting Information, which demonstrates that all the products belong to tenorite without the appearance of any impurities. The percent conversions of copper ions are shown in Table 1 from experiment No. 6 to No. 9, where it can be seen that the reaction temperature has a little effect on the percent conversion of copper ions in SCW with the 13



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addition of NaOH. The FESEM pictures of the products obtained at different reaction temperatures and 30MPa are shown in Fig. 4. The crystallite sizes of the products obtained at subcritical conditions ( 250 °C, 300 °C, 350 °C) were 39.7 nm, 51.6 nm and 30.6 nm respectively, are bigger than 23.6 nm produced at supercritical conditions (400 °C), and then increased to 25.5 nm when the reaction temperature reached to 450°C. The TEM micrograph of products synthesized at 250 °C is shown in Fig. 5 (a). It can be seen that the spindle-like products are consistent of many crystallites, whose sizes are about 50 nm. The standard deviation of particle size was bigger at subcritical conditions than that at supercritical conditions, namely the homogeneity of the products synthesized at subcritical conditions was not satisfactory. In addition, the products synthesized at low temperatures were inclined to aggregate.

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Fig. 4. FESEM pictures of copper(II) oxide produced at different reaction temperatures, i.e., (a) 250 °C (b) 300 °C (c) 350 °C (d) 450 °C and 30 MPa

Our results are similar with those of some scholars,14, 31 which indicates that the size of products will decrease with the increasing temperature from subcritical to supercritical conditions. The dielectric constant of water decreases with the increase of temperature, thus the dissolution power of inorganic substances in water reduces and the reaction rate increases. The solubility of copper(II) oxide increases with an increase of temperature from 250 to 350 °C, and then immediately decreases around the critical point of water32 in a great speed. Accordingly, as the temperature increased from 350 °C to 400 °C, the supersaturation of intermediate products in dehydration reaction increases, giving rise to 15



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a larger quantity of crystallization nuclei. Based on the classical nucleation theory, the greater the number of nucleation nuclei is, the smaller the particle size and size distribution width tend to be, that is the reason for the variation of crystallite size with different water conditions. In Fig. 5, the products were spindle-like at lower reaction temperatures (250 °C and 300 °C), whereas at higher temperatures (350 °C and 450 °C), the morphology of products changed to be hexagon flake-like. In Fig. 5, the TEM detection results of the as-prepared products at 250 °C and 30 MPa are shown, and the corresponding FESEM micrographs are shown in Fig. 4(a). The TEM image of individual spindle-like CuO crystal is presented in Fig. 5(a). It is found that each spindle-like copper(II) oxide crystal is consisted of many small particles. The elongated spots in the selected area electron diffraction (SAED) image (left side of Fig. 5(a)) indicate that the CuO crystal has a defective single-crystal-like feature. This suggests that the crystallographic orientations among particles within CuO architecture are nearly the same as a single-crystal-like structure, which maybe attributed to the oriented attachment growth mechanism determined by the self-assembly of the smaller nanoparticles. The length direction of the _

spindle is [200] and its width direction is [202]. The HRTEM image shown in Fig. 5(b) reveals the crystalline nature of a few CuO submicron particles with straight (200) lattice fringes. As temperature increased from 250 °C to 400 °C, the rate of temperature rising inside the stainless steel reactor is higher at initial stage. More crystal uncles formed instantaneously. The TEM image of hexagon flake-like products is showed in 16


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Fig. 6(a) and the SAED image is shown in Fig. 6(b). The clear diffraction spots in the pattern demonstrated that each hexagon flake-like product was a single crystal. These flake-like CuO submicron structures might be formed through the Ostwald ripening by consuming rather smaller nanoparticles at higher temperatures.

Fig. 5. TEM and SAED examination of the materials obtained at 250 °C and 30 MPa with precursor concentration being 0.1 M (corresponding to Expt No. 6)

Fig. 6. TEM and SAED examination of the materials obtained at 400 °C and 30 MPa with precursor concentration being 0.1 M (corresponding to Expt No. 4)

3.3 Effects of pressure Up to now, there are still numerous disputes for effects of reaction pressure on the products, and, unified understandings has still not reached. For different metal oxides, 17



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there are various impacts.33, 34 In order to explore the effects of reaction pressure on synthesizing copper(II) oxide ultrafine particles in SHS, we studied the process of synthesizing copper(II) oxide submicron materials at different reaction pressures from 22 MPa to 30 MPa. XRD detection demonstrates that all the products are monoclinic CuO, as shown in Supporting Information. The FESEM views of products obtained at different reaction pressures are shown in Fig. 7. With the increase of reaction pressure from 22 MPa to 26 MPa, the average particle size and crystallite size did not have a significant change, however, the standard deviation decreased from 44.5 nm to 24.9 nm. The results of experiment No. 4, No. 12 and No. 13 (shown in Table 1) demonstrate that the influences of reaction pressure on particle size and SD are almost negligible when the reaction pressure is above 26 MPa. The showed results tell us that the reaction pressure mainly affect the uniformity of the copper(II) oxide products fabricated from copper nitrate using supercritical hydrothermal synthesis. This might be due to the solubility change of copper(II) oxide with the reaction pressure. Water density increases with the pressure under supercritical conditions, which results in an increase in copper(II) oxide solubility and thus some bigger crystallite can be gained. In order to get high-quality (small sizes, regular morphologies) CuO crystals, 26 MPa of reaction pressure is perfect considering the high-pressure apparatus need more safe security in the manufactural and operation processes.

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Fig. 7. FESEM plain-views of copper(II) oxide produced at different reaction pressures (a) 22 MPa; (b) 24 MPa; (c) 26 MPa; (d) 28 MPa at 400 °C

3.4 Effects of precursor concentration Precursor concentration plays an important role in controlling the size of products. Five different copper ions concentrations, such as 0.01 M, 0.02 M, 0.05 M, 0.1 M and 0.2 M, were investigated in the SHS reaction respectively. For present experimental group, the reaction temperature and pressure maintained at 400 °C and 30 MPa respectively. The obtained XRD results are shown in Supporting Information, indicating that all the products are CuO. Fig. 8 depicts the FESEM micrographs of copper(II) oxide products from the precursors with different copper ion concentrations. The average particle size 19



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and SD number were measured based on the length of the spindle and were shown in Table 1. The products shown in FESEM micrographs is secondary particles consisting of crystallite. The crystallite size of the products was 28.3 nm when the precursor concentration was 0.01 M, with increasing the precursor concentration to 0.02 M and 0.05 M, the crystallite sizes decreased to 24.5 nm and 22.1 nm, while the average particle size decreased from 161.6 nm to 112.1 nm. The average particle size of products that synthesized at precursor concentration being 0.1 M (experiment No. 4) was 96.5 nm, while it further rose to 168.6 nm when increasing the precursor concentration to 0.2 M.

Fig. 8. Micrographs of copper(II) oxide for precursors with different copper ion concentrations at 400 °C and 30 MPa (a) 0.01 M; (b) 0.02 M; (c) 0.05 M; (d) 0.2 M 20


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Our results about the influence of precursor concentration in the lower range on the particle size are similar with those of some research scholars.15, 24 The supersaturation of reaction intermediates changed with the precursor concentrations. It is well known that average size of nucleation particles is obviously affected by the supersaturation. For the lower concentration range (< 0.1 M), when the supersaturation increases with precursor concentrations, more copper ions will be inclined to nucleate rather than to be consumed for the further growth of nucleation particles. Thus smaller particles can be formed with increasing the precursor concentrations. As the concentration increased to 0.1 M, the above-mentioned low concentration effects disappeared, i.e., the particle size did not decrease with precursor concentrations any more, but present an increasing trend. These results are consistent with those of Sue K,27 in which as the Zn(NO3)2 concentrations changed from 5×10-4 mol/kg to 5×10-3 mol/kg and 5×10-2 mol/kg, at first the particles size decreased from 49 nm to 28 nm, and then increased to 44 nm. The nucleation rate and the subsequent growth rate have a competitive impact on the ultimate particle size of the formed products. At higher concentrations, there might be many copper ions failing to nucleate, which promote the growth of initially formed nucleuses in later reaction, thus results in the formation of many bigger product particles. From the micrographs Fig. 8 (a-c), it can be seen that for precursor concentration 0.01 M to 0.05 M the products were all rod-like. The growth mechanism of rod-like CuO crystals can be account for the different growing rates of different directions growth units belonged to the growth units. With the presence of NaOH solution, the growth units of 21



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CuO crystals are supposed to be Cu(OH)64- (coordinating octahedron) based on anionic coordinative polyhedra theoretical model.35 Cu2+ is surrounded by six OH-, four located at the square plane, and other two located at its axis. The binding energies of the four hydroxy located at plane are higher than those located at axis, therefore, the two hydroxy located at axis are easily dehydrated to form copper(II) oxide crystallites. The growth rates in axis are higher than those of plane, leading to the formation of rod-like crystals. 3.5 Production of copper(II) oxide submicron materials The XRD diffractograms of the solid products synthesized at different conditions are shown in Fig. 2 and Supporting Information, also demonstrating that all the products belong to tenorite crystalline structure. The phases of products are all monoclinic CuO (JCPDS file no. 48-1548) in present experiments, and belong to the C space group.

No diffraction peaks other than those of CuO were observed, which indicates that CuO particles with a high purity were obtained. Above all, the optimum condition for synthesizing CuO in supercritical water is taken on 400 °C and 26 MPa with the precursor concentration being 0.1 M and sodium hydrate concentration being 0.2 M (experiment No. 12). In 1990s, Adschiri12 creatively put forward the supercritical hydrothermal synthesis (SHS) to characterize the production of metal oxides from metal salts in supercritical water, proposing that reaction mechanism of hydrothermal synthesis in supercritical water contains two steps: firstly, the hydrolysis of metal salts to produce intermediate hydrous metal oxides; secondly, the dehydration of hydrous metal oxides to produce metal oxide24, as shown in equations (2) and (3): 22


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Cu(NO3)2 + 2OH- → Cu(OH)2 + 2NO32-

(2)

Cu(OH)2 → CuO + H2O

(3)

Based on our experimental results, submicron CuO can be synthesized. However, nanoscale CuO is needed because of the excellent performance. In order to synthesis of nanoscale CuO using supercritical hydrothermal synthesis, the less heating time is essential. This can be realized through continuous supercritical hydrothermal synthesis system, which is our next step of research. 4. Conclusion With copper nitrate as precursor, the fabrication of copper(II) oxide submicron structures by SHS is investigated, and the influences of reaction parameters, such as addition of NaOH, temperature, pressure, and precursor concentrations, on the average particle size, morphologies and phases of the products, and percent conversion of the reaction are examined. For SHS, it is quite important to add NaOH into the initial copper nitrate solutions, which can determine the product yield directly. The appropriate molar ration between NaOH and Cu(NO3)2 is 2:1. The reaction temperature and pressure have a significant influence on the average particle size and homogeneity of the products obtained from SHS. Precursor concentration is also an important parameter for SHS technology industrialization. This paper demonstrates that 0.1 M is a relatively optimal concentration for present precursor, namely copper nitrate. The growth mechanism of different CuO submicron structures might be oriented attachment and Ostwald ripening, and the temperature and precursor concentration have an influence on the degree of 23



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aggregation growth. Acknowledgments We thank the supports from the National Natural Science Foundation of China (51406146) and Jiangsu Province Natural Science Foundation of China (BK20140406). Reference (1) Spencer, M. J. S. Gas sensing applications of 1D-nanostructured zinc oxide: Insights from density functional theory calculations. Prog. Mater. Sci. 2012, 57, 437-486. (2) Comini, E.; Baratto, C.; Faglia, G.; Ferroni, M.; Vomiero, A.; Sberveglieri, G. Quasi-one dimensional metal oxide semiconductors: Preparation, characterization and application as chemical sensors. Prog. Mater. Sci. 2009, 54, 1-67. (3) MacDonald, A. H. Superconductivity - Copper oxides get charged up. Nature. 2001, 414, 409-410. (4) Anandan, S.; Yang, S. H. Emergent methods to synthesize and characterize semiconductor CuO nanoparticles with various morphologies - an overview. J. Exp. Nanosci. 2007, 2, 23-56. (5) Zhu, J. W.; Bi, H. P.; Wang, Y. P.; Wang, X.; Yang, X. J.; Lu, L. CuO nanocrystals with controllable shapes grown from solution without any surfactants. Mater. Chem. Phys. 2008, 109, 34-38. (6) Yuan, G. Q.; Jiang, H. F.; Lin, C.; Liao, S. J. Shape- and size-controlled electrochemical synthesis of cupric oxide nanocrystals. J. Cryst. Growth. 2007, 303, 400-406. (7) Morales, J.; Sanchez, L.; Martin, F.; Ramos-Barrado, J. R.; Sanchez, M. Nanostructured CuO thin film electrodes prepared by spray pyrolysis: a simple method for enhancing the electrochemical performance of CuO in lithium cells. Electrochim. Acta. 2004, 49, 4589-4597. (8) Jian, G. Q.; Liu, L.; Zachariah, M. R. Facile Aerosol Route to Hollow CuO Spheres and its Superior Performance as an Oxidizer in Nanoenergetic Gas Generators. Adv. Funct. Mater. 2013, 23, 1341-1346. (9) Chen, J. T.; Zhang, F.; Wang, J.; Zhang, G. A.; Miao, B. B.; Fan, X. Y.; Yan, D.; Yan, P. X. CuO nanowires synthesized by thermal oxidation route. J. Alloy. Compd. 2008, 454, 268-273. (10) Anandan, S.; Lee, G. J.; Wu, J. J. Sonochemical synthesis of CuO nanostructures with different morphology. Ultrason. Sonochem. 2012, 19, 682-686. (11) Adschiri, T.; Lee, Y. W.; Goto, M.; Takami, S. Green materials synthesis with supercritical water. Green. Chem. 2011, 13, 1380-1390. (12) Adschiri, T.; Kanazawa, K.; Arai, K. RAPID AND CONTINUOUS HYDROTHERMAL CRYSTALLIZATION OF METAL-OXIDE PARTICLES IN SUPERCRITICAL WATER. J. Am. Ceram. Soc. 1992, 75, 1019-1022. (13) Kim, M.; Son, W. S.; Ahn, K. H.; Kim, D. S.; Lee, H. S.; Lee, Y. W. Hydrothermal synthesis of metal nanoparticles using glycerol as a reducing agent. J. Supercrit. Fluids. 2014, 90, 53-59. (14) Sue, K.; Kakinuma, N.; Adschiri, T.; Arai, K. Continuous production of nickel fine particles by hydrogen reduction in near-critical water. Ind. Eng. Chem. Res. 2004, 43, 2073-2078. (15) Kubota, S.; Morioka, T.; Takesue, M.; Hayashi, H.; Watanabe, M.; Smith, R. L. Continuous supercritical hydrothermal synthesis of dispersible zero-valent copper nanoparticles for ink applications in printed 24


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Graphical abstract

26


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27


Supercritical Hydrothermal Synthesis of Submicrometer Copper(II

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