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DOI: 10.1039/C6RA16553B (Paper) RSC Adv., 2016, 6, 85083-85090

Synthesis of CuO and Cu2O nano/microparticles from a single precursor: effect of temperature on CuO/Cu2O formation and morphology dependent nitroarene reduction†

V. Vinod Kumara, A. Dharania, Mariappan Mariappanb and Savarimuthu Philip Anthony*a
aDepartment of Chemistry, School of Chemical & Biotechnology, SASTRA University, Thanjavur-613401, Tamil Nadu, India. E-mail: philip@biotech.sastra.edu
bDepartment of Chemistry, SRM University, Chennai-603203, Tamil Nadu, India

Received 27th June 2016 , Accepted 31st August 2016

First published on 1st September 2016

Abstract

CuO and Cu2O nano/microparticles with pure phases have been synthesized from the same precursor by a hydrothermal method. Hydrothermal heating of Cu(OAc)2 produced CuO at 125 °C whereas pure Cu2O was obtained at 175 °C. Heating at 150 °C gave a CuO/Cu2O mixture. In contrast, Cu(acac)2 produced only Cu2O at all three temperatures. The pure phases of Cu2O and CuO nano/microparticles were confirmed by PXRD and XPS characterization. The mechanistic studies indicate that decomposition of the organic anion/ligand of the Cu-precursor played a key role in the formation of CuO/Cu2O nano/microparticles from Cu(OAc)2/Cu(acac)2. FE-SEM studies revealed the formation of CuO with a microsphere morphology (125 °C) and a micro-cup for Cu2O at 175 °C. Nanowires and micron-sized elliptical cylinders were observed for Cu2O synthesized from Cu(acac)2. However, calcination of Cu(OAc)2, Cu(acac)2 and Cu(NO3)2 at 500 °C produced crystalline CuO nano/microparticles with various sizes and morphologies. Further, CuO nano/microparticles investigated for industrially important aromatic nitro to amine conversion showed morphology dependent nitro group reduction. Smaller spherical CuO nano/microparticles obtained from Cu(acac)2 exhibited the highest catalytic activity. The reusability studies indicate that CuO nano/microparticles can be used for up to six cycles. Thus we have presented a simple method to synthesize Cu2O or CuO from the same precursor and demonstrated the morphology dependent catalytic activity of CuO nano/microparticles.

Introduction

Copper oxide (CuO) and cuprous oxide (Cu2O) semiconductor nanomaterials have drawn significant attention in recent years owing to their fundamental importance, such as photochemical and photoconductive properties that make them potential materials in several applications such as in gas sensing, water splitting, lithium ion batteries, field emission transistors, solar cells, metal–insulator–metal resistive switching memory, magnetic data storage and environmental science.1–7 Further CuO nanomaterials can also be used as a heterogeneous catalyst in several important chemical reactions such as degradation of nitrous oxide, selective reduction of nitric oxide with ammonia and oxidation of carbon monoxide.8 The release of surface lattice oxygen makes Cu2O a better carbon monoxide oxidation catalyst compared to Cu.8 This application potential aroused strong interest to develop simple methods to synthesize CuO and Cu2O nanoparticles in pure phases. Cu2O nanocubes were prepared by reducing Cu2+ using sodium ascorbate in presence of CTAB as a stabilizing agent.9 Theja et al. used phenolic chelating ligands as structure controlling agents in the synthesis of Cu2O nano/microcrystals with different shape that exhibited shape dependent tunable band gap.10 Cu2O nanocrystals with nanocube, truncated nanocubes, cuboctehedra, and octehedral morphology has been obtained using polyvinylpyrrolidone as a shape-directing agent and ascorbic acid as a reducing agent.11 Reducing copper–citrate complex with glucose produced uniform Cu2O nanocubes.12 Selective synthesis of Cu2O using Cu(NO3)2 source by solvothermal approach using three different surfactants have been reported by Giannousi et al.13 Zhang et al. prepared Cu2O nanoparticles and converted to CuO by gas phase oxidation at 200 °C.14 Song et al. reported the transformation of Cu2O to CuO nanoparticles by increase of solution pH.15 In contrast, reductive transformation method has often been employed to convert CuO nanoparticles to Cu2O using different reducing agents.16 In hydrothermal conditions, controlling hydrolysis rate with different concentration of precipitating agent produced nanocrystalline CuO with different morphology.17 Hydrothermal treatment of Cu(OH)42− precursor with hydrazine and glucose at 120 °C resulted in nanorods and nanotubes of CuO.18 Most of these methods report the synthesis of either CuO or Cu2O nanomaterials alone in pure phases first and then transform CuO into Cu2O or vice versa. However, synthesis of CuO and Cu2O nanoparticles with pure phases from same precursor by a simple method has scarcely been reported. Recently, Prasad et al. demonstrated the synthesis of CuO and Cu2O nanoparticles from same precursor in microwave method by adjusting pH of the reaction medium.19

In this manuscript, we report the synthesis of CuO and Cu2O nano/microparticles with pure phases from same precursor by hydrothermal method. Cu(OAc)2 produced crystalline CuO, CuO–Cu2O and Cu2O nano/microparticles depending on the reaction temperature in hydrothermal treatment whereas Cu(acac)2 gave only Cu2O nano/microparticles. However, calcination of Cu(OAc)2, Cu(acac)2 and Cu(NO3)2 at 500 °C produced crystalline CuO nano/microparticles. The formation of pure phases of Cu2O and CuO were confirmed by PXRD and XPS characterization. FE-SEM studies were performed to analyze the size and morphology. The reaction mechanism for the formation of CuO or Cu2O from Cu(OAc)2 has been established by controlled studies. Further, CuO nano/microparticles has been used as catalysts for industrially significant aromatic nitro to amine conversion. Different morphology of CuO nano/microparticles allowed us to study the morphology dependent nitro group reduction. The reusability of the CuO catalyst has also been demonstrated. Thus simple calcination method allowed us to explore precursor dependent CuO catalytic properties whereas hydrothermal method has been used to control the structure of copper oxide nano/microparticles.

Experimental section

Cu(CH3COO)2·H2O (Cu(OAc)2), Cu(NO3)2, sodium borohydride (NaBH4) and 4-nitrophenol (4-NP), 2-nitroaniline (4-NA), 3-nitroaniline (3-NA), 4-nitroaniline (4-NA) and copper acetylacetonate (Cu(acac)2) were obtained commercially and used as received without further purification.

Hydrothermal reaction

1 g of Cu(OAc)2 or Cu(acac)2 was mixed in 20 mL distilled water and stirred at room temperature for 30 min. Cu(OAc)2 dissolve in water and produce clear solution whereas Cu(acac)2 is not soluble in water. Then the aqueous solution was transferred into a Teflon container of hydrothermal set-up and heated at different temperatures (125 °C, 150 °C, 175 °C) for 12 h. After that the reaction was brought to room temperature. The formed precipitate was centrifuged, washed with water and dried for further studies. Cu(NO3)2 salt did not produce any precipitate in the hydrothermal treatment.

Calcination

1 g of Cu(OAc)2 or Cu(acac)2 or Cu(NO3)2 powdered sample was heated at 500 °C for 3 h in presence of air. After that it was brought to room temperature. The formed precipitate was collected, washed with water and dried for further studies.

Characterization

The particle size and morphology of CuO and Cu2O nanoparticles were studied by field emission scanning electron microscopy (FE-SEM) (Q400 SEM). The phase and the crystallographic structure were identified by X-ray diffraction (XRD, Brucker, Cu-Kα: λ = 0.1540598 nm) at a scanning rate of 0.07° s−1 with 2θ ranging from 20° to 80°. X-ray photoelectron spectroscopic (XPS) analysis was performed using a K-Alpha instrument (XPS KAlpha surface analysis, Thermo Fisher Scientific, U.K.). UV-visible measurements were performed in a Perkin Elmer model Lambda 1050.

Catalytic studies

The nitroarene reducing catalytic activity of CuO nanostructures (CuO-1, CuO-2, CuO-3 and CuO synthesized by hydrothermal method at 125 °C from Cu(OAc)2) were explored by freshly preparing a series of solution. Typically, appropriate amounts of 4-NP, 4-NA, 3-NA, 2-NA and NaBH4 were mixed and stirred at room temperature. Then, a certain amount of CuO nano/microparticles catalyst was introduced into the solution and with stirring. The reaction progress was monitored by withdrawing 25 μL that was diluted to 2 mL with distilled water and measuring absorption maximum. The diluted each sample was centrifuged before absorption measurement to remove the CuO nano/microparticles. The typical concentrations of nitroarenes (4-NP, 4-NA, 3-NA and 2-NA), NaBH4 and catalyst were, 1.0 × 10−2 mol L−1, 2.0 mol L−1 and 5 mg L−1, respectively.

Results and discussion

Hydrothermal reaction was performed for both Cu(OAc)2 and Cu(acac)2 precursor at three different temperature (125, 150 and 175 °C). Hydrothermal heating Cu(OAc)2 at 125 and 150 °C produced brownish black precipitate whereas reddish brown precipitate was obtained at 175 °C. Interestingly, PXRD studies revealed the formation of CuO/Cu2O or CuO–Cu2O mixture depend on the temperatures. The PXRD profile of brownish black precipitate obtained at 125 °C can be indexed to monoclinic phase of CuO nano/microparticles (JCPDS card No. 45-0937, Fig. 1a). The peaks at 32.42, 35.47°, 38.67°, 48.52°, 57.92, 61.37, 65.97 and 68.02 are assigned to (−110), (002), (111), (−202), (202) (−113), (022) and (−220) plans of monoclinic phase. However, brownish black powder obtained at 150 °C showed PXRD peaks correspond to both monoclinic CuO and cubic Cu2O nano/microparticles (Fig. 1a). In contrast, PXRD pattern of reddish brown powdered obtained at 175 °C perfectly indexed to a cubic Cu2O phase (JCPDS card number 05-0667, Fig. 1a). The peaks at 2θ values 29.7°, 36.6°, 42.4°, 61.4°, and 73.8° are assigned to (110), (111), (200), (220), and (311) planes of the Cu2O phase. Thus both CuO and Cu2O nano/microparticles with pure phases have been synthesized using more common Cu(OAc)2 precursor under hydrothermal condition by adjusting the reaction temperature. Further to explore the role of precursor, we have used another commonly employed precursor, (Cu(acac)2), under same reaction condition. Interestingly, hydrothermal treatment of Cu(acac)2 at all three temperatures (125, 150 and 175 °C) produced cubic Cu2O (Fig. 1b). PXRD profiles showed clear sharp peaks that could be assigned to (110), (111), (200), (220) and (311) planes of the cubic Cu2O phase (JCPDS card number 05-0667). It is noted that increase of peak intensity with temperature could be attributed to the increase of Cu2O nano/microparticles crystallinity. Hydrothermal heating of Cu(NO3)2 did not produce any precipitate at all three temperature. Calcination of Cu(OAc)2 and Cu(acac)2 at 500 °C produced brownish black precipitate. The PXRD pattern confirmed formation of CuO nano/microparticles with monoclinic (JCPDS data card no. 45-0937, Fig. 2). CuO prepared from Cu(OAc)2 and Cu(acac)2 precursor is denoted as CuO-1 and CuO-2, respectively. Similarly, calcination of Cu(NO3)2 at 500 °C has also produced CuO nano/microparticles with monoclinic phase (CuO-3). The heating of copper salts at high temperature in presence of air leads to CuO formation.
image file: c6ra16553b-f1.tif
Fig. 1 PXRD pattern of CuO/Cu2O nano/microparticles synthesized by hydrothermal method from (a) Cu(OAc)2 and (b) Cu(acac)2 at different temperature.

image file: c6ra16553b-f2.tif
Fig. 2 PXRD pattern of CuO nano/microparticles synthesized by calcination of different copper precursors.

Interestingly, hydrothermal treatment of Cu(OAc)2 in aqueous medium produced pure CuO, CuO/Cu2O mixture and pure Cu2O nano/microparticles at 125, 150 and 175 °C, respectively. Cu(acac)2 gave only Cu2O nano/microparticles at all three temperature. In general, Cu2O nanoparticles were often prepared using reducing agents such as ascorbic acid, hydrazine and glucose.9–12 However, we did not use any such reducing agents in the reaction medium and suggests that anions/organic ligands (acetate (OAc−) and acetylaceonate (acac)) played significant role in the formation of CuO or Cu2O nano/microparticles. We propose the following mechanism for formation of CuO and Cu2O nano/microparticles from Cu(OAc)2 at 125 and 175 °C, respectively (Scheme 1).


image file: c6ra16553b-s1.tif
Scheme 1 The schematic representation of CuO and Cu2O nano/microparticles formation mechanism from Cu(OAc)2 at different temperature.

Cu(OAc)2 in aqueous medium can dissociate into Cu2+ and OAc− ions due to its ionic nature. At 125 °C, acetate ions, a conjugate base, might have been converted into acetic acid upon interacting with water that could produce hydroxide ions. The hydroxide ions might produce Cu(OH)2 that could have been converted to CuO. Previously, it has been shown that hydrolysis of Cu(OH)2 under hydrothermal condition produced CuO nanoparticles.17,18 However, at higher temperature (175 °C), the acetate ions might have been decomposed into formic acid. The formic acid is a weak reducing agent that can convert CuO into Cu2O.20 To support the above hypothesis, we have performed hydrothermal reaction using Cu(NO3)2 precursor with acetic and formic acid. It is noted that hydrothermal treatment of Cu(NO3) alone did not produce any precipitate at both 125 and 175 °C. Similarly, hydrothermal heating of Cu(NO3)2 with acetic or formic acid at 125 °C did not give any precipitate. However, heating Cu(NO3)2 at 175 °C with acetic and formic acid produced reddish brown precipitate. PXRD studies indicate the formation of Cu2O nano/microparticles with cubic phase (JCPDS card number 05-0667, Fig. S1†). Cu(acac)2 is a water insoluble coordination complex with strong hydrophobic acetylacetonate ligand. The ligand might undergo decomposition at 125 °C and produced formic acid. The formic acid can act as reducing agent in the conversion of Cu2+ ions into Cu2O. We expect that there could be a pH change in the reaction medium if acetic acid or acetylacetonate ligand produces relatively strong acidic formic acid. This was confirmed by measuring the pH of Cu(OAc)2 and Cu(acac)2 reaction medium. The aqueous solution of Cu(OAc)2 heated 175 °C becomes relatively acidic (pH = 3.5–4.0) compared to solution heated at 125 °C (pH = 6.0–6.5). The initial solution was nearly neutral (pH = 6.9–7.1). Interestingly, aqueous solution of Cu(acac)2 heated at 125, 150 and 175 °C showed similar pH (4.0–4.5). These studies clearly suggest that acetic acid at higher temperature (175 °C) and acetylacetonate at 125 to 175 °C produces relatively strong acidic formic acid. The obtained Cu2O nano/microparticles are very stable and did not undergo any oxidation even by heating in presence of air (Fig. S2†). However, CuO-1 nano/microparticles has been completely converted to Cu2O by hydrothermal heating in presence of acetic acid (0.5 mL, Fig. S3†). In contrast, hydrothermal heating of CuO at 175 °C without acetic acid did not show any conversion (Fig. S4†). Thus, the presence of organic acid or acidic condition is important for Cu2O formation.

The size and morphological transformation of CuO to Cu2O with increasing temperature (125 °C to 175 °C) from Cu(OAc)2 precursor has been explored by FE-SEM. CuO obtained at 125 °C revealed micro spheres with size range between 1–2 μm (Fig. 3a). Higher magnification revealed growth of smaller nanocrystals (less than 50 nm) on the surface of spheres (Fig. 3b and c). Interestingly, CuO micro-spheres transformed to micro-cup with similar size (1–2 μm) and formed Cu2O when heated at 175 °C (Fig. 3d). The magnified image showed nano-domains with rough surface in the curved area of the cup (Fig. 3e and f). Cu2O prepared from Cu(acac)2 exhibited different morphologies including nanowires (4 μm length and 100 nm diameter), multi-shaped micro-spheres (2–4 μm) and elliptical cylinders (Fig. 4 and S5†). The elliptical cylinders were further self-assembled into square pyramidal and square planar shape. The elliptical cylinders length and diameter was ranged between 10–15 μm and 3–5 μm, respectively. CuO-1, CuO-2 and CuO-3 showed aggregated particles with different sizes (Fig. 5). CuO-1 showed featureless aggregated microparticles with domain size range between 300–500 nm. Similarly CuO-3 also showed highly aggregated featureless microparticles with domain size between 150–300 nm. CuO-2 showed relatively smaller spherical nanoparticles (50–100 nm) that were further aggregated and showed more void space than CuO-1 and CuO-2. Thus CuO obtained from Cu(OAc)2 in hydrothermal showed microspheres that was converted to micro-cup of Cu2O by increasing temperature. Cu(acac)2 produced self-assembled elliptical cylinders and nanowires. Calcination of copper salts produced featureless aggregated with different domain size for CuO-1, CuO-2 and CuO-3.


image file: c6ra16553b-f3.tif
Fig. 3 FE-SEM images of Cu(OAc)2 produced (a–c) CuO and (d–f) Cu2O nano/microparticles synthesized by hydrothermal method.

image file: c6ra16553b-f4.tif
Fig. 4 FE-SEM images of Cu2O nano/microparticles synthesized from Cu(acac)2 by hydrothermal method.

image file: c6ra16553b-f5.tif
Fig. 5 FE-SEM images of CuO nano/microparticles synthesized from (a) Cu(OAc)2 (CuO-1), (b) Cu(NO3)2 (CuO-3) and (c, d) Cu(acac)2 (CuO-2) by calcination.

In addition, CuO and Cu2O nano/microparticles phase have also been confirmed by X-ray photoelectron spectroscopy (XPS). XPS spectra of CuO nano/microparticles (obtained from Cu(OAc)2 by hydrothermal heating at 125 °C) is shown in Fig. 6. The XPS binding energy at 935.28 and 955.28 eV with 20.0 eV splitting could be assigned to Cu (2p3/2) and Cu (2p1/2) (Fig. 6a). This could be attributed to the Cu2+ oxidation state.21 Further the appearance of shakeup satellites centred at 944.18 and 962.78 eV indicate that the nano/microparticles are oxidized form of copper. The binding energy peak at 531.38 eV corresponds to O 1s (Fig. 6b). The XPS spectrum of Cu2O nano/microparticles showed different binding energy peaks for Cu (2p) (Fig. 7a). The peak at 937.18 eV is corresponding to the binding energy of Cu (2p3/2) and peak at 957.18 eV could be assigned to Cu (2p1/2). This is in good agreement with reported data for Cu2O nanoparticles.22 The O 1s binding energy peak for Cu2O nano/microparticles was observed at 532.28 eV (Fig. 7b). As a representative example, only CuO and Cu2O nano/microparticles prepared from Cu(OAc)2 by hydrothermal heating at 125 and 175 °C, respectively, has been chosen for analyzing by XPS.


image file: c6ra16553b-f6.tif
Fig. 6 XPS spectra of hydrothermally synthesized CuO nano/microparticles from Cu(OAc)2 at 125 °C (a) Cu 2p and (b) O 1s spectrum.

image file: c6ra16553b-f7.tif
Fig. 7 XPS spectra of hydrothermally synthesized CuO nano/microparticles from Cu(OAc)2 at 125 °C (a) Cu 2p and (b) O 1s spectrum.

In recent years, exploring CuO nano/microparticles catalytic activity especially in nitroarene reduction received considerable interest. The low cost and easy fabrication with controlled size and shape is expected to make CuO nano/microparticles as alternative for other costly catalysts presently used in chemical reactions.23 Further, the conversion of aromatic nitro compounds to their corresponding amines is also an interesting topic in industry because amines serve as intermediates for several vital pharmaceuticals, polymers and herbicides.24 The synthesize of CuO nano/microparticles with different size and morphology from different precursor allowed us to investigate nitro group reduction and correlate with CuO size and morphology. Scheme 2 shows the structure of different aromatic nitro compounds explored for catalytic activity, The conversion of nitro to amine with different CuO nano/microparticles was monitored by measuring time dependent absorption change. For example, aqueous solution of 4-nitrophenol (4-NP) with NaBH4 exhibits strong absorption at 400 nm (Fig. 8a). It was observed that 4-NP absorption with NaBH4 did not change even after standing a day. However, after adding CuO nano/microparticles, absorption at 400 nm slowly decreased and a new peak at 300 nm that is attributed to the characteristic absorption of 4-aminophenol (4-AP) (Fig. 8).25 Interestingly CuO-1, CuO-2 and CuO-3 nano/microparticles exhibited different catalytic ability. 4-NP absorption (λmax = 400 nm) with CuO-1 did not disappear completely. Although CuO-3 addition lead to complete disappearance of 4-NP absorption at 400 nm, the characteristic absorption for 4-AP was not observed. However, CuO-2 nano/microparticles catalytic reaction showed complete disappearance of 4-NP peak and appearance of clear peak at 300 nm corresponds to 4-AP. In contrast to CuO-1, CuO-3 and CuO nano/microparticles synthesized from Cu(OAc)2 precursor by hydrothermal reaction (at 125 °C) showed clear reduction of 4-NP to 4-AP (Fig. S6†). The comparison of catalytic activity suggests that CuO-2 and CuO nano/microparticles synthesized by hydrothermal method showed strong nitro group reduction whereas CuO-1 exhibited weak catalytic activity (Fig. 9). The difference in the catalytic activity of CuO nano/microparticles could be attributed to the size and morphological differences (Fig. 3 and 5). The smaller nanoparticles are known to exhibit increased catalytic activity due to increased surface area.25 The nitro group reduction of 4-NP could be assumed as a pseudo-first order kinetics since NaBH4 has been used in excess concentration. Hence the reaction rate is assumed to be dependent on 4-NP concentration. The rate constant (k) for conversion of 4-NP to 4-AP in presence of different CuO catalysts were calculated to be 0.1002 min−1 (CuO-1), 0.2060 min−1 (CuO-2) and 0.1555 min−1 (CuO-3) that again confirms the strong activity of CuO-2. The increase of catalyst amount showed linear enhancement of reduction rate (Fig. S7†). The reusability of CuO-2 nano/microparticles catalyst has also been evaluated for reducing 4-NP (Fig. S8†). The catalyst was recovered by simple filtration after each reaction and washed with ethanol–water mixture to remove any absorbed products. CuO-2 maintained good catalytic activity up to four cycles and slightly decreased in the fifth and sixth cycles.


image file: c6ra16553b-s2.tif
Scheme 2 Catalytic studies of CuO nano/microparticles for the reduction of different nitroarenes.

image file: c6ra16553b-f8.tif
Fig. 8 Monitoring the conversion of 4-NP to 4-AP using different CuO nano/microparticles by absorption spectroscopy.

image file: c6ra16553b-f9.tif
Fig. 9 Comparing 4-NP reducing capacity of different CuO nano/microparticles.

Further other aromatic nitro compound has also been investigated to expand the scope of CuO nano/microparticles catalyst. The correlation of reaction kinetics of 4-nitroaniline (4-NA) showed similar trend as observed with 4-NP (Fig. 10). The smaller CuO-2 and hydrothermally prepared CuO exhibited strong capability of nitro reduction relative to CuO-1 and CuO-3. 2-Nitroaniline (2-NA) was completely converted to amines with all CuO nano/microparticles except CuO-1 (Fig. S9†). However, all four CuO nano/microparticles exhibited complete conversion of 3-nitroaniline (3-NA) to 1,3-diaminobenzene (Fig. S10†). It is noted that substitution of electron donor group (OH, NH2) at ortho and para position could increase the electron density at nitro group by resonance effect. This might hinders the reduction of nitro group. Hence only smaller CuO-2 nano/microparticles exhibited strong catalytic activity. However, electron donor at deactivating meta position might not increase the electron density on the nitro group and hence all four CuO nano/microparticles exhibited complete reduction. The comparison of nitro-reduction reactions indicated faster reduction for CuO-2 and CuO nano/microparticles obtained by hydrothermal method that could be attributed to the smaller nanoparticles formation.26 Thus simple calcination of copper salts in air can produce CuO nano/microparticles, however, its precursor dependent morphology plays significant role on the resulting property. In contrast, hydrothermal method offers structural control by adjusting the reaction temperature.


image file: c6ra16553b-f10.tif
Fig. 10 Comparing 4-NA reducing capacity of different CuO nano/microparticles.

Conclusion

In conclusion, we have synthesized CuO and Cu2O nano/microparticles with pure phases from same precursor, Cu(OAc)2 by adjusting reaction temperature in the hydrothermal method. Interestingly, hydrothermal reaction of Cu(acac)2 precursor produced only Cu2O nano/microparticles. The structure and phase of CuO and Cu2O nano/microparticles were confirmed using PXRD and XPS analysis. Monoclinic CuO and cubic Cu2O nano/microparticles was obtained from Cu(OAc)2 by hydrothermal heating at 125 and 175 °C, respectively. The mechanistic studies suggest that acetate anion and acetylacetonate ligand played significant role in the formation of CuO/Cu2O nano/microparticles. FE-SEM studies showed microsphere formation of CuO and micro-cup for Cu2O. Cu2O nano/microparticles synthesized from Cu(acac)2 exhibited nanowires along with micron sized elliptical cylinders that were further assembled into square and square pyramidal structure. CuO synthesized by calcination exhibited different size and morphology. Finally, CuO nano/microparticles have been explored for industrially important nitroarene reduction that revealed size dependent catalytic nitro group reduction. Thus, the present study suggests that simple hydrothermal method could be employed to fabricate CuO/Cu2O nano/microparticles from single precursor by controlling temperature.

Acknowledgements

Financial support from the Nanomission, Department of Science and Technology, New Delhi, India (DST-Nanomission scheme no. SR/NM/NS-1053/2015) is acknowledged with gratitude. TMM acknowledges DST, India for financial support by fast track scheme (SB/FT/CS-093/2012). We thank CRF, SASTRA University for providing infrastructural support in the form of UV-Visible spectrophotometer.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: PXRD pattern, FE-SEM images of Cu2O nano/microparticles and UV-Visible spectra of 4-NA, 3-NA and 2-NA compounds reduction by CuO nano/microparticles. See DOI: 10.1039/c6ra16553bs
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