pH-Dependent Single-Step Rapid Synthesis of CuO and Cu2O

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pH-Dependent Single-Step Rapid Synthesis of CuO and Cu2O Nanoparticles from the Same Precursor Arun V. Nikam,†,‡ Arulraj Arulkashmir,§ Kothandam Krishnamoorthy,§ Amol A. Kulkarni,*,‡ and B. L. V. Prasad*,† †

Physical/Materials Chemistry Division, ‡Chemical Engineering & Process Development Division and §Polymer Science and Engineering Division CSIR-National Chemical Laboratory, Dr. HomiBhabha Road, Pune 411 008, India S Supporting Information *

ABSTRACT: A single-step protocol to prepare Cu2O and CuO nanocrystalline particles from the same precursor by microwave irradiation has been developed using the pH of the solution as the only variable parameter. The utility of different bivalent Cuprecursors for synthesis of CuO and Cu2O nanoparticles was also investigated. The morphology, phase purity, and optical properties of these nanoparticles were analyzed using TEM, SEM, XRD, and optical spectroscopy. The band edges were determined using cyclic voltammetry. Field effect transistors based on CuO nanoparticles showed a hole mobility of 3.5 × 10−2 cm2 V−1 s−1, making them a suitable candidate for energy-related applications. The effect of hydrazine vapor exposure on the IV-characteristics of CuO nanoparticles was also investigated. This revealed a decrease in source current with respect to time.

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INTRODUCTION Rapid synthesis of metal and metal oxide nanoparticles within short time with good control over their crystalline phase, size, shape, and the properties1 is one of the very actively pursued areas of research in recent times. It is well-known that transition metals due to their multiple oxidation states can form oxides with different stoichiometry. The properties of these oxides vary, sometimes drastically, depending on the oxidation state and the metal-to-oxygen stoichiometric ratio. One of the wellknown examples of such oxide systems is copper oxides. Copper forms three main types of oxides, viz. Cu4O3, Cu2O, and CuO.2 With the advent of nanoscience and technology, there has been an upsurge in the preparation of nanoscale particles of these copper oxides. Among these, CuO and Cu2O nanoparticles find application in the field of catalysis, sensors, and as an electrode in Li-ion batteries.3−5 However, synthetically it is challenging to prepare these oxides in pure phases. Especially the synthesis of Cu2O without any impurities is regarded as a hard task due to the better stability of copper in +2 state as compared to +1.6 Though there are a few reported procedures on the synthesis of pure Cu2O nanoparticles, they are mostly based on the hydrothermal synthesis which is more energy intensive and time-consuming. 7 Murphy et al. synthesized Cu2O nanocubes by reducing Cu2+ with sodium ascorbate and using CTAB as a protecting agent.8 Yang et al. prepared Cu2O NPs and transformed them into CuO nanoparticles by gas-phase oxidation at 200 °C over a period of two hrs.9 On the other hand, nanocrystalline CuO with different morphology was prepared by adjusting the hydrolysis rate with a different concentration of precipitating agent under hydrothermal conditions for 20 h.10 CuO nanocrystalline © XXXX American Chemical Society

particles were prepared in N,N-dimethylforamide and converted to Cu2O by reductive transformation in 8−42 h at 180 °C.11 Klabunde et al. synthesized CuO nanorods using the hydrothermal method and reduced the CuO into Cu2O and Cu metal in 4% hydrogen.12 Generally, to obtain single phase Cu2O from a bivalent precursor, many reducing agents such as hydrazine,13,14 titanium tetrachloride,15 sodium tartrate,16 and hydroxylamine,17 and so on are added to the reaction mixture. Zheng et al. synthesized Cu2O nanoparticles with various shapes such as nanocube, truncated nanocubes, cuboctehedra, and octehedra by using ascorbic acid as a reducing agent and polyvinylpyrrolidone as a shape-directing agent.18 Wang et al. reported that the control over the shape of Cu, Cu2O nanotubes/nanorods can be exercised by reducing Cu(OH)4−2 with hydrazine and glucose, respectively, using CTAB as a shape-directing agent at room temperature. Hydrothermal treatment of the Cu(OH)4−2 precursor at 120 °C resulted in nanotubes and nanorods of monoclinic CuO.19 Song et al. demonstrated the conversion of Cu2O to CuO by an increase in the pH of solution by adding aqueous ammonia solution.20 Qian et al. reported the synthesis of uniform crystalline Cu2O cubes by reducing the copper−citrate complex solution with glucose and observed that the evolution of Cu2O cubes takes place through intermediate steps that gives multipods and starshaped particles.21 One of the efficient ways for the rapid synthesis of nanomaterials is the use of the microwave. Microwave-based syntheses are attractive because of the faster Received: March 20, 2014 Revised: July 23, 2014

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Figure 1. XRD patterns of (A) Cu2O nanoparticles prepared at pH 4 from Cu−AC (curve 1) and CuO nanoparticles prepared from the same precursor at pH >12 (curve 2). (B) Cu2O nanoparticles prepared from Cu−ACAC at pH 4 (curve 1) and a mixture of Cu2O and CuO nanoparticles prepared from Cu−ACAC at pH > 12. In (B), the peaks denoted by (*) correspond to the CuO phase. min. This small variation in reaction procedure leads to the formation of a black product in short time. This was identified as CuO nanoparticles (vide infra). The amount of NaOH was slightly varied to change the pH of solution. We also studied the synthesis of CuO and Cu2O nanoparticles by changing the metal precursor, wherein, all other conditions remained the same as described above except the time for the reaction. The details of the precursor dependence on the type of product obtained are discussed in detail in the section below.

reaction kinetics due to significant enhancement in the local temperature that leads to an increase in the yield and less time consumption.22 Preparation of metal oxides such as ZnO, CoO, BaTiO3, Cu2O, MnO2, Fe2O3, etc. using the microwave has been reported in the literature.22−24 However, a rapid one-step synthesis of CuO and Cu2O nanoparticles from the same precursor using microwave methods has not been reported as far as we are aware. In this paper, we report the controlled synthesis of Cu2O and CuO using a microwave-mediated synthetic strategy. We show that both Cu2O and CuO could be obtained from Cu(II) acetate as the metal precursor and benzyl alcohol as the solvent, which also acts as a reducing agent25 under the microwave condition. Phase pure nanoparticles of Cu2O were formed when Cu(II) acetate was heated using the microwave for 10 min in benzyl alcohol. On the other hand, CuO nanoparticles formed when small amounts of base were added by keeping all other conditions similar to those used for Cu2O nanoparticle synthesis. We demonstrate that changing the metal precursor and pH has tremendous influence on the Cu2O/CuO nanostructure formed. The optical properties and hole mobilities of these nanostructures were determined. We also show that exposing the CuO nanostructure to hydrazine vapor leads to interesting IV features.

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CHARACTERIZATION The shape and size of the nanoparticles were analyzed by TEM. Samples were made by putting a 7 µL drop of as-prepared solution of Cu2O and CuO nanoparticles on a carbon coated TEM grid and the images were obtained from a FEITechnaiTE-20 transmission electron microscope which was operated at 200 kV. Morphology of nanoparticles were analyzed using field emission scanning electron microscope (NOVA NANO FE-SEM operated at 15 kV). XRD pattern of CuO and Cu2O were obtained with a Panalytical Xpert Pro PXRD operated at 40 kV and 30 mA using Cu Kα radiation. Optical properties were measured using a UV−visible spectrophotometer (JASCO V-570) in the reflectance mode. FET characteristics were obtained using a semiconductor analysis instrument. Thin film UV−visible measurements were performed on Specord 200 by coating the sample on quartz glass, which was dried under an IR lamp for 15 min. The electrochemical band gaps were determined by cyclic voltammetry. The FET devices were fabricated using prefabricated substrates. The gate was heavily doped silicon, the dielectric substrate was SiO2, and the source and drain electrodes were of gold. The IV characteristics were recorded using a Keithley 4200-SCS semiconductor characterization system.

EXPERIMENTAL SECTION

Chemicals and Materials. Chemicals and materials were used as received or without further purification. Cu(II) acetate was purchased from LobaChemie. Cu(II) acetylacetonate was purchased from SigmaAldrich, and benzyl alcohol, hydrazine hydrate 80%, NaOH, and 15% HCl were received from Thomas Baker. Synthesis of Cu2O Nanoparticles. Cu (II) acetate (0.199 g, mmol) was mixed with 5 mL of (99%) benzyl alcohol in a 50 mL round-bottom flask at room temperature. This flask containing the reaction mixture was then placed in a microwave oven operating at 700 W with a 2.45 GHz frequency for 10 min with stirring. After microwave irradiation, the reaction flask was removed from the microwave oven and allowed to cool at room temperature. A reddishbrown colored product was observed. It was washed three times using absolute ethanol. This powder was redispersed in water by sonication and centrifuged at 9000 rpm for 20 min. The reddish-brown colored product was dried at 60 °C in an oven for 12 h. This product was identified to be Cu2O nanoparticles (vide infra). The same procedure was followed with addition of a different amount of 15% of HCl to study the pH dependence on the product formed. Synthesis of CuO Nanoparticles. Exactly the same procedure described above was followed except that 100 μL of NaOH (0.1 M) was added to the precursor’s mixture (copper acetate + benzyl alcohol). In this case, the reaction mixture was kept under the microwave condition (2.45 GHz frequency with 700 W power) for 1

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RESULTS AND DISCUSSION The dependence of shape, size, and crystalline phase of the nanoparticles formed under different reaction condition/ parameters was systematically studied. We started our endeavor by the microwave heating of two copper precursors, copper(II) acetate (Cu−AC) and copper acetatylacetonate (Cu−ACAC), at two different pH conditions, viz. pH = 4 and pH > 12. At the acidic condition, a turbid brownish red product formed irrespective of the precursor used. The PXRD profile of product obtained (Figure 1, panels A and B, curve 1) at acidic condition from both the precursors matched with the Cu2O phase (JCPDS card number 65-3288), and no trace of any impurity could be detected under the PXRD condition used. The peaks at 2θ values 29.7°, 36.6°, 42.4°, 61.4°, and 73.8° are B

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assigned to (110), (111), (200), (220), and (311) planes of the Cu2O phase. Interestingly, while the reaction with Cu-AC took 10 min for completion, the reaction with Cu−ACAC as a precursor needed 40 min for completion. PXRD analysis of the reaction products obtained at basic pH conditions (Figure 1B, curve 1) indicated that the product obtained from Cu−AC was purely the CuO phase (JCPDS card number 41-0254), while Cu−ACAC led to the formation of a mixture of Cu2O and CuO phases (Figure 1B, curve 2). Here again the reaction with Cu−Ac needed only 10 min, while continuing the reaction with Cu−ACAC beyond 40 min also did not change the composition of product mixture. Thus, from the above result it can be summarized that when Cu−AC is used as a precursor, we can control the product as Cu2O (at acidic condition) or CuO (at basic condition). On the other hand, changing the precursor to Cu−ACAC led to the formation of Cu2O (at acidic condition) or a mixture of CuO and Cu2O (under basic conditions). Here, the formation of Cu2O where Cu is in the +1 oxidation state was a little surprising because in both precursors used here, Cu is present in the +2 oxidation state. It has been proven that the benzyl alcohol, which is used as a solvent, can act as a reducing agent18 when it is heated above 150 °C. We reckon that under microwave conditions, benzyl alcohol must be acting as a mild reducing agent. This will reduce the Cu2+ ions to Cu1+ ions, ensuring the product formed to be Cu2O. At a basic pH, on the other hand, higher oxygen content may hamper the reducibility of benzyl alcohol.13 Therefore, addition of the base to Cu−AC leads to the formation of CuO nanoparticles. The longer reaction time of 40 min for Cu−ACAC to get converted to Cu2O compared to 10 min when Cu−AC was used as a precursor could be due to the stronger binding between the acetylacetonate ligand with Cu2+ ions as compared to the simple acetate ligand. The stronger binding of the acetylacetonate ligand to Cu(II) could also explain the reason for the formation of mixture of products with Cu−ACAC at basic pH conditions. Since the reactions with Cu−ACAC were taking a significantly longer time and the product formed at basic pH conditions was in the mixed phase, we did not pursue reactions with this precursor further. Next, we have carried out a systematic study of products formed with Cu−AC at different pH conditions (pH = 2, 4, and 6.5). The pH of Cu−AC dissolved in benzyl alcohol was 6.5, and therefore, a pH of 6.5 did not need any addition of acid or base. We achieved an acidic condition such as pHs 2 and 4 by addition of 20 and 10 μL solutions of 15% HCl, respectively. The microwave heating of these Cu−AC solutions at different pH conditions led to a change in the color of the solutions to reddish brown within 10 min of the reaction. Samples obtained at pHs 2, 4, and 6.5 are referred to as samples I, II, and III in this paper, respectively. The PXRD patterns of samples I, II, and III (Figure 2A) matched again with the Cu2O phase without any trace of impurity. Subsequently, the same Cu−AC solution was subjected to microwave heating under basic pH conditions, namely, pH = 8, 10, and >12. These pHs were attained by addition of 30, 70, and 100 μL of 0.1 M of NaOH solution to the Cu−AC solution. The color of the reaction product obtained under the basic condition was black. We refer to the products of the reaction obtained at these basic pH conditions as sample IA (pH = 8), sample IIA (pH = 10), and sample IIIA (pH > 12). The XRD analysis suggested that the pattern of sample IA comprised both phases of CuO and Cu2O (Figure 2B, curve 1). On other hand, the XRD traces of sample IIA (Figure 2B, curve 2) and sample IIIA (Figure 2B, curve 3)

Figure 2. XRD patterns of (A) Cu2O nanoparticles prepared from Cu−AC in acidic condition at pH 2 (sample I: curve 1), pH 4 (sample II: curve 2), and at pH 6.5 (sample III: curve 3) and (B) CuO prepared from Cu−AC at pH 8 (sample IA: curve 1), pH 10 (sample IIA: curve 2), and at pH >12 (sample IIIA: curve 3). (+) Indicates Cu2O phase.

correspond to a pure CuO phase (JCPDS card number 410254), and no peak corresponding to the Cu2O phase could be detected in these samples. The size and shape of particles obtained under the different experimental conditions listed above were analyzed by transmission electron and scanning electron microscopes (TEM and SEM, respectively). The image analysis shows that at highly acidic (sample I) conditions, polydisperse quasi spherical particles ranging from 100 to 200 nm (Figure 3 panels

Figure 3. (A and B) FESEM and (C) TEM images of Cu2O nanoparticles (sample I). (D and E) FESEM and (F) TEM images of Cu2O cubes (sample II). (G and H) FESEM and (I) TEM images of Cu2O cubes (sample III). The insets in C, F, and I show the HRTEM images revealing the lattice planes. The distance between the lattice planes in these images correspond to the (111) planes of the Cu2O system.

A, B, and C) are formed. Interestingly, the morphology of the particles obtained in case samples II and III turned out to be cubic (Figure 3, panels D, E, F, G, and H). Cubes formed at pH 4 (sample II) are about 2.9 μm in size, which are bigger in size than those formed at pH 6.5 (sample III). The average size of particles formed at pH 6.5 is 1.25 μm. The inset in Figure 3 (panels C, F, and H) display the HRTEM images revealing space between the lattice planes to be 0.25 nm, corresponding to the (111) plane of the Cu2O cubic phase. The formation of C

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CuO nanoparticles is a well-observed phenomenon in literature,19 which is a reflection of the monoclinic crystalline nature of CuO. Furthermore, to understand effect of time on the size and morphology of different nanoparticles formed, we performed microwave heating of the Cu−AC sample at optimized pH conditions 6.5 and >12 for different time intervals. The result indicates that there is no significant variation in shape, size, and morphology, with respect to time. TEM and SEM analyses of products obtained at different time intervals are provided in Figure S1 and S2 of the Supporting Information. We also note that these samples were stable for long periods, as evidenced by their XRD patterns recorded after two months of storage (Figure S3 of the Supporting Information). As mentioned previously, among the product obtained, samples I, II, and III appear brown red in color, while samples IA, IIA, and IIIA were black in color. The optical spectra of obtained powders were recorded to determine the optical band gap and transmittance. Figure 5 (panels A and B) displays the representative spectra obtained from samples III and IIIA, respectively. We would like to mention here that optical spectral characteristics of samples I and II resembled that of sample III (Figure S4, panels a and b, of the Supporting Information). On the other hand, the optical spectral characteristics of sample IIA (Figure S4c of the Supporting Information) matched well with those of sample IIIA. Thus, we could conclude that the optical spectra depend more on the crystalline phase of the sample with little or no dependence on the size of the particles. The band gaps of these materials were extracted from Tauc’s26 plot. For sample III (Cu2O nanoparticles), the band gap corresponds to 2.07 eV. The band gap for sample IIIA (CuO nanoparticles) was 1.42 eV. The transmittances of sample IIIA was measured by drop-casting a thin film of these on a quartz plate (Figure S4d of the Supporting Information). We notice that films formed with sample IIIA (CuO nanoparticles) display good transmittance values in the 200−650 nm region, making it a good candidate

cube-shaped Cu2O nanoparticles has been well-reported in the literature.6 The formation of cube morphology is ascribed to the cubic crystal structure of Cu2O and the formation kinetics. It has been established that when the reaction is carried out under slow conditions, the formation of cubic morphology of Cu2O is facilitated.8 On the other hand, the images of samples IIA and IIIA exhibit nanobelt-like morphology (Figure 4, panels

Figure 4. (A and B) TEM images of CuO nanoparticles (sample IIA). (C and D) TEM images of CuO cubes (sample IIIA). The insets in B and D show the HRTEM images revealing the lattice planes. The distance between the lattice planes in these images correspond to the (111) planes of the CuO system.

A and B), which seem to be resulting from the aggregates of small nanoparticles (See Figure S 1, panels d, e, and f, of the Supporting Information). The distance between the lattice planes displayed in inset of images in Figure 4 (panels B and D) correspond to 0.23 nm, which is attributed to 111 planes of the monoclinic CuO phase. Here again, formation of needle-shaped

Figure 5. Tauc’s plots obtained from the DRS spectra of (A) sample III and (B) sample IIIA. The insets in (A) and (B) display the actual DRS spectra. Cyclic voltammograms of (C) sample III and (D) sample IIIA. D

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the linear regime,28 which was found to be 3.57 × 10−2 cm2 V−1 s−1. A similar experiment with nanoparticles of sample III (Cu2O spheres and cubes) did not yield discernible change in the drain current as a function of applied gate voltage. We attributed this to the lower stability and poor film forming ability of Cu2O. It was reported that Cu2O absorbs the oxygen, which leads the surface to be charged with oxygen. These surface-adsorbed O− and O2− species decrease carrier hole density in the surface charge layer and increase resistance.29 Finally, we wanted to examine FET response of CuO nanomaterial (sample IIIA) after exposing it to hydrazine vapor. (The device fabrication is discussed in the Supporting Information). Accordingly, we measured drain-source current (IDS) dependence on the drain-source voltage (VDS) at an operated gate voltage (VG) of 20 V of the CuO FET device by exposing it to hydrazine vapor. As shown in Figure 6B, the exposure of CuO to hydrazine vapor showed a decrease in IDS. This decrease in current was attributed to conversion of CuO to Cu2O on the surface of the material. Indeed, the XRD pattern of sample IIIA exposed to hydrazine indicates a partial conversion of CuO into Cu2O due to the reducing action of hydrazine vapor (Figure S6 of the Supporting Information). On the other hand, the IDS decreases with exposure time due to an increase in the Cu2O phase. The SEM image of CuO exposed to hydrazine vapors at a different time interval reveals a morphological change wherein some discontinuities in the film were seen to develop (Figure S7 of the Supporting Information). This discontinuous nature ultimately decreases electron mobility of the CuO thin film. In addition to that, since the HOMO energy level of CuO is commensurate with the work function of gold source-drain electrodes, injection of charge from gold metal electrodes to the HOMO of CuO nanomaterial is allowed. This kind of charge injection might be unfavorable in the case of the Cu2O system.30

for different light harvesting and sensor type applications. Unfortunately, sample III (Cu2O nanoparticles/cubes) did not form good films and yielded in poor transmittance spectra despite repeated attempts. We also investigated the energy band structure of samples III and IIIA using cyclic voltammetry in an inert atmosphere. A glassy carbon electrode was used as a working electrode. Dry acetonitrile was used as a solvent and 0.1 M of tert-butyl ammonium perchlorate was used as an electrolyte. 0.01 M of ferrocene was used as an internal standard. The oxidation and reduction peaks centered on 0 V are due to ferrocene (Figure 5C and 5D). The onset of first oxidation with respect to the internal standard was used for the determination of the HOMO level. From the cyclic voltammetry valence band and conduction band edges have been determined using eqs 4 and 5.27 onset E HOMO = −(Eox + 4.8eV)

(4)

onset E LUMO = −(Ered + 4.8eV)

(5)

The onset reduction and oxidation potential of curve I for sample III were −0.88 and 1.17 V, respectively (Figure 5C). This corresponds to an energy band gap of 2.0 eV. The onset reduction potential and oxidation potential of sample IIIA were found to be −0.64 and 1.11 V, respectively (Figure 5D). From these, the energy band gap of CuO was determined to be 1.7 eV. Thus, the band gaps estimated from cyclic voltammetry are in reasonably good agreement with those determined from optical spectra. Like in optical spectral measurement, here again, the band gap of samples I and II matched well with that of sample III, while that of sample IIA matched that of sample IIIA (see Figure S5 of the Supporting Information). This again confirms our assertion that the band gap is mainly dependent on the phase of the material and not on the particle size. We further probed the FET characteristic of representative samples III and IIIA. For this, a thin film of the sample was cast over the FET substrates. The measurement was carried out under ambient condition. The VD to ID curve at different gate voltages for the sample IIIA (CuO wirelike nanoparticles) is displayed in Figure 6A. CuO nanoparticles can be used as a

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CONCLUSIONS We have demonstrated the rapid synthesis of monocrystalline Cu2O and CuO from copper acetate with good control over the oxidation state, using microwave-assisted wet chemical synthesis. The pH of reaction solutions played a vital role and determines the nature of the final product formed. CuO nanoparticles displayed good hole mobility under ambient conditions making it a good candidate to be used in FET devices. In this sample, decrease in current was observed when it was exposed to hydrazine vapors due to the conversion of CuO NPs into Cu2O. Thus, CuO nanoparticle can be used for detecting hydrazine or for energy-related applications.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 6. I−V Curves of sample IIIA (A) before hydrazine vapor exposure under different gate voltages and (B) after hydrazine vapor exposure for different time (the gate voltage used was −20 V).

Device fabrication, TEM and FESEM images, comparative PXRD patterns, Tauc’s plots, cyclic voltammograms, timedependent XRD pattern, and SEM image. This material is available free of charge via the Internet at http://pubs.acs.org.

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hole transport layer in organic and hybrid solar cells. Therefore, field effect transistors with sample IIIA were fabricated to determine its charge carrier mobility. An increase in sourcedrain current with respect to source-drain voltage under negative gate voltage was observed for this sample. The output characteristic curves showed linear regime with small gate modulation (Figure 6A). This is likely due to the bulk conductivity of CuO. Hole carrier mobility was calculated from

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: 91-20-25902153. Fax: 9120-25902621. *E-mail: [email protected]. Tel: 91-20-25902013. Fax: 91-20-25902636. E

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.N. and A.K. acknowledge CSIR New Delhi for a Junior and Senior Research Fellowship, respectively. We thank Dr. Satish Ogale and Mr. Raunak Niphade for helping us record the FESEM images. A.K. and B.L.V.P. acknowledge support from CSIR’s XIIth Five Year Plan Project (Grants CSC0123 and CSC0134). Financial assistance from DST is also acknowledged.

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