CuO Rapid Synthesis with Different Morphologies by the Microwave Hydrothermal Method

Abstract

CuO structures were synthesized by microwave hydrothermal treatment using two different mineralizing agents (NaOH and NH₄OH) and were evaluated as photocatalysts. The materials were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and Brunauer-Emmett-Teller (BET) surface area analysis. The XRD patterns indicated the formation of the monoclinic phase in both samples with 13.78 and 14.23 nm crystallite size. SEM analysis showed different agglomerates morphologies based on the mineralization agent employed. The CuO nanostructure synthesized with NH₄OH presented agglomerated like-plates which results in a spherical shape, whereas the material synthesized with NaOH presented an agglomerate of larger plates. Both samples showed photocatalytic activity against RNL azo dye. The quasi-spherical shape CuO material reached 93 % of the discoloration.

Keywords:
copper oxide; microwave hydrothermal method; photocatalytic

1. Introduction

One copper oxide (CuO) stable form is the tenorite with cation valence +2¹. This oxide is a p-type multifunctional semiconductor, which has narrow band gap values (1.2 - 1.5 eV)^2-4. Its monoclinic structure presents interesting characteristics, such as, thermal superconductivity, thermal stability⁵, photovoltaic properties and antimicrobial activity⁶. This metal oxide has been extensively investigated in recent years due to several applications such as an electric superconductor⁷, battery electrodes for lithium-ion cell⁸, solar sensors⁹, gas sensors¹⁰, catalysts¹¹ and wastewater treatments¹².

The synthesis of CuO nanostructures has been obtained by several synthesis methods that apply high temperatures or extensive heat treatments, such as solid state thermal decomposition method¹³, electrochemical¹⁴, thermal oxidation¹⁵ and chemical precipitation¹⁶. The conventional hydrothermal method¹⁷ applies low temperatures, however, it requires very long synthesis times, and the microwave hydrothermal (MH) method¹⁸ has been receiving greater attention due its higher heating rate compared to the conventional hydrothermal method. It presents a reaction kinetics increased by one or two orders of magnitude resulting in energy and time savings^19-21. The reactions are thus completed in minutes or even seconds²². Lee et al.²³ stated that this is viable method to solve economic and environmental problems because it is a closed system that saves energy and time.

Typically oxides synthesized through the MH method present differentiated morphologies such as CuO in the form of sea urchin¹⁸; the BaMoO₄ in the form of micro octahedra²⁴ and the CaTiO₃ in the form of microtubes²⁵ and multi-linked ZnO rods²⁶. However, these morphologies were achieved due to the use of so-called shape controlling agents such as templates or surfactants like polyethileneglicol MW 200 or special reactants. Therefore, in this communication, we report a synthesis of CuO with two different morphologies without surfactants or templates, using different hydroxyl source (NH₄OH or NaOH) at 130 ºC for five minutes synthesis.

The degradation of organic pollutants has received special attention in recent years due to the numerous negative effects on the environment and human health. In particular, industrial dyes have been the most common water pollutants^27,28. These organic dyes have very stable complex structures, which difficult degradation. Among all processes used in these dyes degradation, advanced oxidative processes (AOPs) result in the generation of hydroxyl radicals (•OH), which are used to oxidize organic pollutants to form CO₂, H₂O or some less toxic inorganic small molecule^29,30. AOPs have been the most effective technology in removing organic pollutants for water treatment.^31,32. Currently, semiconductor photocatalysis is a newly developed advanced oxidation process followed by the removal of dye pollutants³³. Several experiments were carried out to study the photocatalytic activity of some semiconductors, such as ZnO, CdS, SnO₂ and ZrO₂ in the degradation of dyes. Among the range of inorganic semiconductors, CuO has attracted attention due to its easy production, as well as its high chemical and thermal stability, and adjustable electronic properties^33,34. As a photocatalyst for discoloration of organic dyes, CuO was evaluated for the degradation of rhodamine B, methyl orange, tartrazine and methylene blue^3,35-36. Either way, the CuO has presented high catalytic activity, being attributed to morphological and superficial aspects.

Thus, in this work, CuO nanoparticles were obtained by the MH method using two different mineralizing agents aiming to observe their influence on the morphology of the obtained material. It was performed photocatalytic bench tests to analyze the material morphology influence on the degradation of the RNL azo dye.

2. Experimental

2.1 CuO synthesis

Monoclinic copper (II) oxide was synthesized by the microwave hydrothermal method in a RMW 1 (INOVTEC) reactor.

The applied reagents were Cu(NO₃)₂3H₂O (98%, PROQUIMS), NaOH (98%, ISOFAR) and NH₄OH (28%, PROQUIMS).

Initially, in 90 mL of copper nitrate solution (9 x 10^-3 mol.L^-1) was added 8 mL of the solution (5 mol.L^-1) of two different mineralizing agents (NaOH and NH₄OH). Both precursors’ solutions with precipitate were individually transferred into a sealed Teflon autoclave and placed in a domestic microwave (f = 2450 MHz, maximum power = 900 W). The reaction system was treated at 130 ºC with a heating rate of 10 ºC.min^-1 for 5 minutes. The autogenous pression in the sealed auclave was stabilized at 1.2 kg.cm^-2 resulting in a black precipitate. Both samples were washed by centrifugation (400 rpm) for ten min three times with distilled water until neutral pH and then dried at 110 ºC for 12 h. The sample synthesized using the mineralizing agent NaOH was named CuNa05 and another one applying NH₄OH was CuNH05.

2.2 Characterization

Both samples were characterized by X-ray diffraction (XRD), FT-IR spectroscopy (IR), scanning electron microscopy (SEM) and textural analysis.

The XRD patterns were obtained from Shimadzu (XRD 6000 model) operating with KαCu radiation (λ = 1.5406) in the region of 20-80º with a step of 0.02º and a step time of 2 seconds. IR spectra were recorded in the region of 400-800 cm^-1 via a Prestige-21 IR Spectrophotometer from Shimadzu. SEM (backscattered-electron) images were obtained on a JEOL-300 microscope. The textural analysis were performed from the adsorption / desorption isotherms of N₂ (using the BET equation) on a Bel Japan Belsorp mini II automatic surface area measurement device.

2.3 Photocatalytic and adsorption tests

The CuNa05 and CuNH05 samples were used as photocatalysts for the degradation of Remazol Golden Yellow dye (RNL). The molecular structure of the RNL dye is shown in Figure 1.

Figure 1
Molecular structure of Remazol Golden Yellow dye (RNL). Adapted from Alvarenga et al.³⁷.

The photocatalytic tests were performed in a homemade reactor with dimensions of 10 x 20 x 100 cm using UVC lamp (λ = 254 nm, 20 W, ILUMISAMPA). The experiments were carried out in triplicate using 10 mg of the photocatalyst and 15 mL of the dye solution with a concentration of 10 mg.L^-1 at pH = 6. Petri dishes containing the dye solution and the photocatalysts were photoirradiated for 1 h. After photocatalysis, the mixtures were centrifuged and filtered.

The percentage of discoloration and degradation of the dye solutions were quantified using a UV-vis spectrophotometer (Model UV-2550, Shimadzu) in absorbance mode by measuring the absorbance of the solution at 411 and 240 nm, respectively³⁸. The photocatalytic efficiencies of the photocatalysts were calculated by calibration curves; the initial absorption of the untreated dye solution and the respective concentrations before and after photocatalytic treatment were considered. In addition, tests were performed in the dark, using the same conditions of the photocatalytic tests, to evaluate the discoloration solely due to the adsorption of the dye on the surface of the material.

3 Results and Discussion

3.1 Material characterization

Figure 2 presents the XRD patterns of the synthesized samples. The diffraction peaks were indexed according to JCPDS N° 45-0937, indicating that the monoclinic CuO phase was obtained for both samples. No commonly impurities’ peaks such as Cu(OH)₂ or Cu₂O were detected from this pattern, confirming the pure monoclinic phase for both samples.

Figure 2
XRD patterns of CUNa05 and CuNH05 samples obtained at 130 °C for 5 minutes.

Lattice constants were calculated using the method of the least squares, starting from the values arranged in the JCPDS Nº 45-0937 card.

Using the values of the diffraction angles and the indexing planes, lattice constants, unit cell volume and average crystallite size were calculated. Results are shown in Table 1.

Table 1
Lattice parameters, unit cell volume (V) and crystallite size (D) of both CuNa05 and CuNH05 samples.

Samples	Lattice parameters (Å)				V (Å3)	D (nm)
	a	b	c	β
CuO (Standard)*	4.69	3.43	5.13	99.93	81.2	-
CuNa05	4.30	3.74	5.46	96.33	87.3	13.78
CuNH05	4.31	3.76	5.48	96.17	88.3	14.23

• ^*
  JCPDS card nº 45-0937.

According to the results shown in Table 1, the CuNa05 and CuNH05 samples presented a subtle difference in the network parameters, whereas in comparison with standard (CuO JCPDS Nº 45-0937), the values of a and β were reduced and the values of b and c increased. These variations resulted in an increase in unit cell volume of 6.1 ų for the CuNa05 sample and 7.1 ų for the CuNH05 sample. This may be occurring due to the experimental conditions (treatment temperature, heating rate and / or processing time) employed in the synthesis which are capable of promoting structural defects formation, residual stresses and/or minor distortions in the lattice constants for this oxide³⁹. These small variations may also be due to rapid kinetics in the formation of nanocrystals synthesized through the HMO method^39,40. Chen et al.^41,42 observed a similar result when synthesizing mixed aluminum and zinc oxides.

Results indicate the differentiation of the mineralizing agents did not affect the crystallographic parameters relevantly. The average crystallite size (D) of particles was determined by the Scherrer's formula⁴³ (equation 1).

(1) $\mathit{D}=\frac{\mathit{K}\mathit{\lambda }}{\left(\mathit{L}\cos \frac{\mathit{\chi }}{2}\right)}$

in which λ is the wave-length of the incident x-rays, L the linear dimension of particle, χ/2 the Bragg angle and K a numerical constant for which obtained the value 2(ln 2/π)^1/2 = 0.93.

For the CuNa05 sample D = 13.78 nm and 14.23 nm for CuNH05 sample. Yang et al.⁴⁴ synthesized CuO by the Microwe hydrothermal method at 120 ºC in 5 min using PEG 400 as template resulting in samples with different morphologies, i.e. quasi-spherical nanoplates, willow-leaf-like and rod like. The D parameter ranging from 11.8 to 15 nm. This slightly variation is also observable in this work.

Low temperatures and short times of synthesis are favorable conditions for the non-growth of particles. Luo et al.⁴⁰ reported that in the synthesis of nest like BaMoO₄ nanoparticles; particles grow to a certain extent and then there is re-dissolution in the hydrothermal treatment. This may also be related to the small size of particles in syntheses through the hydrothermal microwave assisted method.

Figure 3 shows the IR spectra for both CuNa05 and CuNH05 samples.

Figure 3
IR spectra of the CuNa05 and CuNH05 samples obtained at 130 °C for 5 minutes.

The vibrational modes located at 506 cm^-1, 570 cm^-1 and 603 cm^-1 are characteristic of CuO monoclinic^45-47. The high frequency mode at 603 cm^-1 and 570 cm^-1 refers to the Cu-O stretch around the direction [-110] while the mode at 506 cm^-1 is associated to vibration along the direction [101]⁴⁸.

Scanning electron microscopy was used to observe the influence of mineralizing agents on the morphology of materials. SEM images of CuNH05 and CuNa05 samples are presented in Figures 4 and 5.

Figure 4
Image obtained by SEM for CuNH05 sample ZONH5 10.000 times.

Figure 5
Image obtained by SEM for CuNa05 sample ZONH5 10.000 times.

SEM images for both samples present plate-like particles morphology, but the difference between the two samples is in the agglomeration. CuNH05 sample the agglomerates present quasi spherical form. It is possible to notice that the material has some smaller quasi-spherical agglomerates and others larger than a micrometer resulting in a small particle size. For the CuNa05 sample the small plates above large plates resulting in the interconnected channels. With these results it is evident that the agglomeration´s morphologies of the copper oxide synthesized was differentiated as a function of the mineralizing agent used.

Keyson et al.¹⁸ synthesized through the HM method nanostructures similar to sea urchins. In this study the authors synthesized this material with a precursor solution containing polyethylene glycol 400 + CuCO₃.Cu(OH)₂ + NH₄OH at 120°C / 1 h. They reported that polyethylene glycol (MM = 400 g.mol^-1) played a fundamental role in obtaining the oxide with this differentiated morphology. Through the same method, Maul et al.⁴⁹ ​​evaluated the influence using or not NaOH and NH₄OH as mineralizer agents on the CuO morphology. Using NaOH in the precursor solution many small particles and plates were observed. While NH₄OH promoted flower like morphology formed by plates. Without alkali agents small agglomerated particles forming spherical morphologies was observed. These results were similar to those presented in this work, although using time of synthesis twelve times shorter. In another work, Yang et al.⁵⁰ obtained nanoplates and dendrites of CuO. The precursor solution used consisted of copper nitride, PEG 400, NaOH and urea at a temperature of 120 ºC for five minutes. Shi et al.⁵¹ synthesized CuO nanorods from a precursor solution of CuSO4.5H₂O + PEG400 + polyethylene glycol (4: 2) + 10 g of Urea at 120 ºC / 5 min.

It was observed that to obtain structures in the micro or nano scales with differentiated morphologies many authors use to attribute to the role of templates, drivers and/or use of surfactants or same precursor solutions containing chitosan, PEG, Cu₂Cl(OH)₃ + K₂CO₃ + CuCl₂, copper glycinatemonohydrate^51-55. However in this paper it was possible to obtain differentiated morphology, mainly using the NH₄OH mineralizer, without special reagents and templates.

Thus it is observed that this method uses low temperature and short time to obtain the CuO and with possibility of morphology control being influenced mainly to the type of precursor used for the material synthesis.

In this paper, copper oxide II was synthesized through the microwave hydrothermal method in two different ways, resulting two samples with different morphologies distinguishing only two hydroxyl source mineralizing agents: NaOH and NH₄OH. These precursor solutions had pH = 13 and 10, respectively. As no template or driver was used it is suggested that the pH difference as a function of the two distinct bases applied is the main factor responsible for the influence on the final morphology of the two copper oxide samples obtained. The reactions are presented in equations 2 to 8.

(2) $\begin{array}{c}\mathit{Cu}{\left({\mathit{NO}}_3\right)}_2.3{\mathit{H}}_2{\mathit{O}}_{(\mathit{aq})}+2\mathit{NaOH}\to \hfill \\ \mathit{Cu}{\left(\mathit{OH}\right)}_{2\left(\mathit{s}\right)}\downarrow +2{\mathit{NaNO}}_{3\left(\mathit{aq}\right)}\hfill \end{array}$

(3) $\begin{array}{c}\mathit{Cu}{\left(\mathit{OH}\right)}_{2\left(\mathit{orthot}\mathrm{hom}\mathit{bic}\right)}+2{\mathit{OH}}_{\left(\mathit{aq}\right)}^{-}\to \hfill \\ \mathit{Cu}{\left(\mathit{OH}\right)}_{4\left(\mathit{s}\right)\left(\mathit{Tetrahydroxucuprate}\mathit{II}\mathit{Anion}\right)}^{2-}\hfill \end{array}$

(4) $\mathit{Cu}\left(\mathit{OH}\right)_4^{2-}{}_{\left(\mathit{s}\right)}\underset{\raisebox{1ex}{$130°\mathit{C}$}\!\left/ \!\raisebox{-1ex}{$5\min $}\right.}{\overset{\mathit{MWH}}{\to }}{\mathit{CuO}}_{\left(\mathit{s}\right)}\mathrm{agglomerated}\mathrm{sheets}$

⁵¹

(5) $\mathit{Cu}{\left({\mathit{NO}}_3\right)}_2.3{\mathit{H}}_2{\mathit{O}}_{\left(\mathit{aq}\right)}+4{\mathit{NH}}_{3\left(\mathit{aq}\right)}\to {\left[\mathit{Cu}{\left({\mathit{NH}}_3\right)}_4\right]}_{\left(\mathit{aq}\right)}^{2-}$

(6) ${\left[\mathit{Cu}{\left({\mathit{NH}}_3\right)}_4\right]}_{\left(\mathit{aq}\right)}^{2-}+4{\mathit{OH}}_{\left(\mathit{aq}\right)}^{-}\to \mathit{Cu}\left(\mathit{OH}\right)_4^{2-}{}_{\left(\mathit{aq}\right)}+4{\mathit{NH}}_{3\left(\mathit{aq}\right)}$

(7) $\mathit{Cu}\left(\mathit{OH}\right)_4^{2-}{}_{\left(\mathit{aq}\right)}\to \mathit{Cu}{\left(\mathit{OH}\right)}_{2\left(\mathit{aq}\right)}+2{\mathit{OH}}_{\left(\mathit{aq}\right)}^{-}$

(8) $\mathit{Cu}{\left(\mathit{OH}\right)}_{2\left(\mathit{aq}\right)}\underset{\raisebox{1ex}{$130°\mathit{C}$}\!\left/ \!\raisebox{-1ex}{$5\min $}\right.}{\overset{\mathit{MWH}}{\to }}{\mathit{H}}_2\mathit{O}+{\mathit{CuO}}_{\left(\mathit{s}\right)}$

sheets agglomerated form quasi-spherical mosphologiy⁵⁶.

Different mineralizing agents, due to the basic strength of alkalis, lead to different pH. As result the synthesized materials presented different morphologies. High concentrations of copper cations and hydroxyde anions produces copper complex ions, such as [Cu(OH)₄]^2- or [Cu(NH₃)₄]²⁺ which are converted to CuO⁵⁶, in this case by HM. So, the two forms of CuO materials can be synthesized by using different precursors solutions ions.

Nikan et al.⁵⁷ synthesized nano-belt like morphology by the microwave-assisted wet chemical synthesis at pH =10 from cupper acetate, 100 µL of NaOH (0.1 mol.L^-1) and benzyl alcohol for 1 minute synthesis. Low pH lead to Cu₂O cubes morphology.

Other materials like ZnO varies its the morphology according to solution pH values. Ram et al.⁵⁸ synthesized ZnO by the microwave hydrotermal method (100 ºC/ 2 min) at different pH values; 8.5 and 10 using ammonia solution (NH₄OH) as mineralizer agent and pH 10 and 13.5 using NaOH, without using growth agents or templates. The morphology of the materials varies with pH increase, starting from nanoflakes, than tapered hexagonal rods, and hexagonal nanorods. This last presenting different size distributions. Also to reinforce the influence of pH on the syntheses of oxides. Wu et al.⁵⁹ obtained the ZnO oxide by the HC at 200 ºC / 15 h increasing the pH value, at pH = 8.92 rods structure; 11.78 nanorods; 12.81 sea urchin like and 12.89 flower like morphology. The excess of [(OH-)] favors nucleation.

Figure 6 presents the N₂ adsorption and desorption isotherms of the CuNa05 and CuNH05 samples.

Figure 6
N₂ adsorption / desorption isotherms of CuNa05 (a) and CuNH05 (b).

The plot of adsorbed volume of N_2(g) against relative pressure (P/P₀) for both samples exhibited Type III isotherm (IUPAC) as show in Fig. 6a and 6b ⁶⁰. The two samples showed same hysteresis behavior, ie, narrow hysteresis loop. In set Fig. 5(a) and (b) shows pore size distribution. The sample CuNa05, agglomerated sheets, exhibited larger pore distribution (max. 34.43 nm) than CuNH05 which exhibited morphology in form agglomerated sheets resulting quasi-spherical morphology with max pore size 19.04 nm.

Both materials are nanoporous, the relatively low pore diameter in both samples may be attributed to the closely packing of the small particles (sub-units) resulting their final micro-structure. Bhuvaneshwari et al. 2016⁶¹ synthesized via hydrothermally, ie, without microwaves CuO snowflake, flower, hollowsphere and urchin morphologies. All samples also exhibited Type III isotherm.

From these results, surface area (S_BET), pore volume (Pv) and average pore size (Tp) were estimated and the values are presented in Table 2.

Table 2
Textural characteristics of CuNa05 and CuNH05 samples.

Sample	SBET (m2.g-1)	Pv (cm3.g-1)	Ps(nm)
CuNa05	23.486	5.396	10.561
CuNH05	11.758	2.701	9.7188

According to Table 2, CuNa05 presented twice the surface area (S_BET) than CuNH05 sample. Yang et al.⁵² synthesized the CuO with different morphologies like tadpole-shaped, spindly, leaf / sphere-like, and fusiform CuO nanoparticles. The specific surface ranging from 18 to 71 m².g^-1. The CuNa05 specific surface area is inside this interval and CuNH05 is out of low limit (11.758 m².g^-1).

Average pore volume (Pv) of the sample alkalinized with NaOH was higher than the sample basified with NH₄OH. The average pore size (Ps) was practically uninfluenced by the mineralizing agent. This pore size non-variation possibly is associated with the use of inorganic bases (NaOH and NH₄OH) as a mineralizing agent because they are similar chemical substances (bases). According to the literature, variations in pore size were observed in materials synthesized by hydrothermal and hydrothermal microwave methods that used templates such as PEG 400⁶² or organic substances, such as hexane⁶³, decane⁶⁴, as pore expansion agents.

3.2 Photocatalytic efficiency

The UV-Vis spectra and the percentage discoloration of the RNL dye after the adsorption and photocatalysis processes are shown in Figure 7(a) and (b), respectively.

Figure 7
UV-vis spectra of RNL azo-dye, after adsorption (a) and after photocatalytic. test (b), in the presence of CuNa05 and CuNH05 (inset: percentage of discoloration).

The quantification of RNL dye degradation is performed by monitoring the absorbance at 240 nm and 411 nm. The first mode refers to the transitions of the aromatic groups of the UV-Vis spectrum, while the latter is associated with the azo bond (-N=N-). Thus, the decrease in the intensity of the band at 411 nm implies the breakdown of the azo bond or reduction of the dye concentration in solution⁶⁵.

The UV-vis spectra of the RNL dye after the tests with the CuNa05 and CuNH05 samples in the absence of light (Fig. 7a) showed a decrease in signal intensity at 240 nm and 411 nm, keeping the same profile of the spectrum of pure dye, the discoloration occurred due to adsorption of the dye molecules on the surface of the CuO particles. It was observed that the percentage of discoloration of azo dye RNL due to adsorption, practically did not change, maintaining the values of 43% and 45%, for CuNa05 and CuNH05, respectively. On the other hand, the UV-vis spectra of the RNL dye after the tests performed in the presence of radiation (Fig. 7b) showed that there was photodegradation and that the discoloration reached 85% for CuNa05 and 93% for CuNH05.

When evaluating the photocatalytic efficiency of the CuO synthesized in this work, it is noticed that the CuNH05 sample showed an increase of 11% in the RNL discoloration in relation to the CuNa05 sample. According to literature^3,34,36, the change in morphology and surface properties, including higher specific surface area, may be the cause of the enhanced activity. But in this case the shape of particles also influences the photodegradation. Although the CuNa05 sample had a larger surface area than the CuNH05 sample, 23.486 and 11.758 m².g^-1 respectively, but the photodegradation of the latter sample was slightly higher, demonstrating that the morphology of the particular agglomerates is more influential than the surface area of the catalyst.

4. Conclusion

The copper oxide (CuO) was synthesized by microwave hydrothermal method under mild conditions of low temperature and short time synthesis. A significant change in the structures of the CuO was observed for different precursors without surfactants or templates, varying only alkaline mineralizes NaOH or NH₄OH in addition to Cu(NO₃)₂.3H₂O solutions, separately. The morphologies are affected by the mineralizing agent, the sample, alkalinized with NH₄OH, is in the form of agglomerated plate-like, which results in a quasi-spherical shape. CuNa05 sample, alkalinized with NaOH, which presents in the form of smaller plates-like on much larger plates. The heterogeneous photocatalysis of these samples was more influenced by the morphology of copper oxide than the surface area, since the alkalinized sample with NH₄OH, quasi-spherical agglomerates and lower surface area, presented greater degradation of the RNL azo dye. This material is an candidate to be used in photocatalysis applications.

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Publication Dates

• Publication in this collection
  11 Oct 2018
• Date of issue
  2018

History

• Received
  27 Mar 2018
• Reviewed
  29 June 2018
• Accepted
  16 Aug 2018