A facile low temperature (350 °C) synthesis of Cu₂O nanoparticles and their electrocatalytic and photocatalytic properties

Abstract

We have synthesized Cu₂O nanoparticles (∼25 nm) starting from CuO (∼107 nm) and copper oxalate nanorods in an inert atmosphere (Ar) at a very low temperature of 350 °C. The process in the absence of the oxalate nanorods yields Cu₂O at a much higher temperature of 850 °C. The Cu₂O synthesized at lower temperature (350 °C) has smaller particles than Cu₂O synthesized at 850 °C. We explored these nanomaterials as electrocatalysts for hydrogen and oxygen evolution which are highly desirable for renewable and clean energy applications. Electrochemical studies such as the hydrogen evolution reaction and oxygen evolution reaction (HER & OER) were carried out on glassy carbon as well as on platinum as the working electrode in KOH solution. Cu₂O synthesized at lower temperature (350 °C) has 14 times higher current density during HER and 2 times higher current density during OER. These electrocatalysts were stable for 50 cycles. However, the Cu₂O synthesized at higher temperature (850 °C) showed very efficient (∼98%) degradation of methylene blue (MB) in 120 min and the catalyst is stable up to the 4^th cycle whereas Cu₂O (350 °C) shows only 40% degradation.

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Introduction

Energy and the environment (water, soil and air pollution) are the two key issues of concern across the world and thus, a significant amount of research has been dedicated to this sector. Novel materials are being designed for better efficiency and low cost. Successful splitting of water into H₂ and O₂, (clean fuel) is the best method (if any efficient route is found) for clean and sustainable energy. To produce H₂ and O₂ researchers need to develop cheap and efficient electrocatalysts. Nanosized materials are being pursued for electrochemical and photocatalytic application due to their geometrical, electronic and surface properties which are different from bulk materials.¹

Cuprous oxide (Cu₂O) having a direct band gap of 1.9–2.2 eV is a p-type semiconductor which absorbs light in the visible range of the solar spectrum² and the band gap can be easily tuned by changing the particle size.³ Cu₂O is an important material as catalyst due to its high stability, low toxicity, low cost and availability. It has importance in gas sensing,⁴ CO oxidation,⁵ photocatalysis,^6–9 photocurrent generation,¹⁰ organic synthesis,^11–13 photochemical evolution of H₂,¹⁴ solar cells¹⁵ and electrochemical¹⁶ applications. There are some reports for producing H₂ and O₂ by nano-size metal and alloys.^17–20 Cu₂O in various shapes such as flowers,²¹ spheres,²² cubes,²³ wires²⁴, rods,²⁵ octahedra,^26,27 tubes²⁸ and hollow structures²⁹ has been synthesized by hydrothermal,³⁰ microwave irradiation,³¹ electrochemical deposition,³² wet chemical,^33–35 oxidative etching,³⁶ solvothermal,³⁷ and reverse micellar³⁸ route.

To the best of our knowledge, HER & OER studies have not been carried out using Cu₂O as electrocatalysts. However, reports on hydrogen evolution from the hydrolysis of ammonia borane using Cu₂O–Co₃O₄³⁹) composites are known. Solar hydrogen generation⁴⁰ is carried out using Cu₂O nano/micro sphere. Photocatalytic degradation of MB (90–95%) has been studied using Cu₂O catalyst in presence of H₂O₂.

In this paper, we discuss the low temperature synthesis of Cu₂O nanoparticles (∼20–30 nm) from CuO by mixing with copper oxalate nanorods. These Cu₂O nanoparticles are efficient electrocatalysts though the high temperature (850 °C) particles (Cu₂O synthesized from CuO without oxalate nanorods) show better photocatalytic properties. Such differences in the properties show competing effects of crystallinity and surface morphology in the two sets of nanostructured cuprous oxide and hence such studies gives inputs to design of inorganic materials with appropriate functions.

Experimental

Copper oxalate nanorods were synthesized by the reverse micellar route.¹⁷ The Cu₂O nanoparticles were obtained by thermal decomposition of CuO. In first case, CuO (Aldrich, 99.99%) was heated in Ar atmosphere, which leads to the formation of pure phase of Cu₂O at 850 °C for 6 h. Below 850 °C, a mixture of CuO and Cu₂O was obtained. In second case, Cu₂O was obtained by mixing CuO (Aldrich, 99.99%) and copper oxalate nanorods in 1 : 1 ratio followed by heating in Ar at 350 °C for 6 h whereas by heating only copper oxalate nanorods in the above conditions, pure copper nanoparticles were obtained.¹⁷

Powder X-ray diffraction studies (PXRD) were carried out using Ni filtered Cu-Kα radiation. Normal scans were recorded with a step size of 0.02° and step time of 1 s. The K_α2 reflections were removed to obtain accurate lattice constants. The crystallite size of the particles were determined from X-ray line broadening studies using Scherrer's formula (t = 0.9λ/B cos θ) where t is crystallite size, λ is the wavelength (for CuKα, λ = 1.5418 Å) and B = √(B_M² − B_S²). B_M is the full width at half maximum for a particular reflection of the sample and B_S is that of standard (with crystallite size of around 2 μm). Quartz was chosen as the standard for the evaluation of B_S. The lattice constant was determined using least-squares fitting procedure on all the observed reflections. Transmission Electron Microscopic (TEM) studies were carried out using a Tecnai G² 20 electron microscope operated at 200 kV. TEM specimens were prepared by dispersing the Cu₂O nanoparticles in ethanol by ultrasonic treatment, dropping onto a porous carbon film supported on a copper grid, and then drying in air. Field emission scanning electron microscopy (FESEM) of the compounds was carried out on FEI quanta 3D FEG-FESEM by coating the powder samples with gold. Diffuse-reflectance (DR) spectra were recorded on Shimadzu UV-2450 spectrophotometer where the baseline was fixed using a barium sulfate reference.

Cyclic voltammetry (CV) was carried out with a computer controlled electrochemical workstation (Autolab PGSTAT 302N). Hydrogen evolution and oxygen evolution reactions were studied by using Ag/AgCl as reference electrode while Pt was used as counter electrode whereas glassy carbon (0.02 cm²) and platinum (0.03 cm²) were used as working electrodes. Working electrode was polished using (0.05 μm) alumina paste, ultrasonicated in distilled water and then in ethanol. 2 mg of Cu₂O nanoparticles was sonicated in 20 μL of isopropanol and then 10 μL of nafion was added. One drop (nearly 10 μL) of this paste was placed on the working electrode and dried for half an hour. All the three electrodes were placed in a freshly prepared solution of 0.5 M KOH. Cyclic voltammetry was carried out at a scan rate of 0.025 V s⁻¹ in the potential range of −1.5 to 0 V in HER studies whereas 0 to 1 V potential was applied to the electrodes for the OER study in case of glassy carbon and 0 to 0.9 V in case of Pt as working electrode. The electroactive surface area of the catalyst on the working electrodes was measured by CV in 0.5 M KOH.

The photocatalytic activity of synthesized Cu₂O powder was investigated by measuring the degradation% of methylene blue (MB) in aqueous solution under visible light. These reactions were carried out in a glass beaker in the presence and absence of H₂O₂. 50 mg of compound was suspended in 100 mL of methylene blue solution of concentration 20 μmol and stirred continuously for half an hour in the presence of small amount of H₂O₂ (10 μL ≈ nearly one small drop) and in the absence of H₂O₂ to ensure the adsorption/desorption equilibrium. After an equilibrium period of 30 min, the sample was illuminated with the help of a tungsten lamp. The concentration of methylene blue was measured by using UV-vis (Shimadzu UV-2450) spectrophotometer at its maximum absorption. The degradation% of MB was determined as follows:

where A₀ and A_t are maximum absorption value at zero minute (start of reaction) and 120 min.

Results and discussion

Powder X-ray diffraction pattern confirm the formation of monophasic Cu₂O synthesized at 850 °C and 350 °C (Fig. 1) in argon gas atmosphere. All the observed reflections were indexed on the basis of simple cubic (Pn3̄m) crystal system (JCPDS: 782076) with refined lattice parameter 4.2682 (3) Å and 4.2474 (16) Å for the oxide synthesized at 850 °C and 350 °C respectively. We have also evaluated the crystallite size for both the samples and it is found to be 27 nm (synthesized at 350 °C) and 48 nm (synthesized at 850 °C). The decrease in the lattice parameter of Cu₂O synthesized at lower (350 °C) temperature as compared to the one obtained at higher temperature synthesized (850 °C) Cu₂O may be due to the decrease in crystallite size⁴¹ and the synthesis temperature.⁴² It is observed that on heating CuO at temperatures lower than 850 °C a mixed phase of CuO and Cu₂O was obtained (Fig. 2). This indicates that CuO does not reduce to Cu₂O before 850 °C. However on addition of copper oxalate to CuO in 1 : 1 ratio, pure phase of Cu₂O can be obtained at 350 °C. Thus, addition of copper oxalate to CuO leads to lowering of the reduction temperature.

Fig. 1 Powder X-ray diffraction pattern of Cu₂O synthesized at 850 °C and 350 °C.

Fig. 2 Powder X-ray diffraction pattern of CuO heated in argon at different temperature.

Table 1 Detailed characterization of Cu₂O nanoparticles

S. no.	Compound	Lattice parameter	Particle size (nm)	Electroactive surface area (cm^2 mg^−1)		Band gap (eV)
				GC	Pt
1	Cu2O (850 °C)	4.2063(4) Å	40–50	0.0207	0.0324	1.96
2	Cu2O (350 °C)	4.1825(7) Å	20–30	0.0235	0.0476	2.06

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Field emission scanning electron microscopy (FESEM) studies were carried out on the compounds which show (Fig. 3 and 4) the formation of dense agglomerated nanoparticles forming layers (Cu₂O obtained at 850 °C) whereas the Cu₂O synthesized at lower temperature forms flake type morphology. Fig. 5 and 6 shows the TEM micrograph of Cu₂O nanoparticles obtained at 850 °C and 350 °C respectively. These particles were monodisperse and spherical in nature. The average diameter of the spherical nanoparticles was found to be 40–50 nm and 20–30 nm for Cu₂O obtained at higher and lower temperature respectively (Table 1).

Fig. 3 FESEM micrograph of Cu₂O synthesized at 850 °C at two magnifications.

Fig. 4 FESEM micrograph of Cu₂O synthesized at 350 °C at two magnifications.

Fig. 5 TEM micrograph of Cu₂O synthesized at 850 °C.

Fig. 6 TEM micrograph of Cu₂O synthesized at 350 °C.

The diffuse reflectance spectroscopy shows a band at 605 nm and 634 nm and the corresponding band gap values were found to be 2.06 eV and 1.96 eV for the oxides synthesized at lower (350 °C) and higher (850 °C) temperature respectively (Fig. 9). The optical properties (band gap) of nanomaterials are size and morphology dependent.^43,44 For several cases, it is observed that a change in morphology can alter the band gap. On changing the morphology from hexagonal sheet like structures to spherical particles for cobalt oxide⁴⁴ nanoparticles, a difference of band gap 0.05–0.2 eV has been reported. The slight change in the band gap observed in our study for these two oxides can be attributed to the change in their morphology (particles vs. layered morphology).

By using cyclic voltammetry, we have investigated the electrochemical properties of the Cu₂O nanoparticles. These nanoparticles act as electrocatalyst for hydrogen evolution reaction when a negative potential is applied from −1.5 to 0 V with glassy carbon electrode (working electrode) and platinum electrode (working electrode) in 0.5 M KOH solution at the scan rate of 0.025 V s⁻¹. The hydrogen generation is according to the equation given below.¹⁷

M + H₂O + e⁻ → MH_ads + OH⁻

MH_ads + H₂O + e⁻ → H₂ + OH⁻ + M

where M = electrocatalyst

The net reaction will be the following,

2H₂O + 2e⁻ → H₂ + 2OH⁻

The cyclic voltammograms of Cu₂O nanoparticles for HER is given in Fig. 7. The two redox peaks observed at −0.64 V and 0.39 V for both the electrode (GC as well as Pt) in case of lower temperature synthesized Cu₂O. These peaks are observed due to the oxidation of Cu⁺ to Cu²⁺. The maximum current was found to be 0.03 and 0.52 mA at an applied potential of −1.5 V for GC electrode whereas in the case of platinum as working electrode the maximum current was 0.38 and 2.52 mA for Cu₂O prepared at higher and lower temperature respectively (Fig. S1†). If we compare the current for both the electrocatalysts, it is clearly observed that Cu₂O synthesized at lower temperature show 17 times (with GC) and 7 times (with Pt) higher current compared to Cu₂O prepared at higher temperature. Current density was calculated by using the electroactive surface area which was determined by extrapolating the curve obtained from CV.¹⁷ It is found to be 0.0207 cm² mg⁻¹ & 0.0235 cm² mg⁻¹ for GC electrode and 0.0324 cm² mg⁻¹ & 0.0476 cm² mg⁻¹ for Pt electrode for Cu₂O obtained at higher and lower temperature respectively. The current density is found to be 14 times higher with GC electrode and 4 times higher with Pt electrode (Fig. 7) for Cu₂O synthesized at lower temperature (350 °C) as compared to that synthesized at higher temperature (850 °C). The current density depends critically on the surface area, morphology and size of the nanoparticles. Since at lower temperature we obtained smaller size nanoparticles therefore the current and current density is higher. The electrocatalyst obtained by us show excellent stability for 50 cycles (recorded continuously).

Fig. 7 HER of Cu₂O (a) on glassy carbon (b) platinum as working electrode.

Oxygen evolution reaction was also studied for these catalysts on both the electrode (GC & Pt) at the scan rate of 0.025 V s⁻¹ in 0.5 M KOH solution at room temperature in the potential range of 0.0 to 0.9 V for Pt electrode and 0.0 to 1.0 V for glassy carbon electrode. Fig. 8 shows the cyclic voltammograms during the OER study. The GC electrode containing Cu₂O (350 °C) electrocatalyst showed an increase in oxidation current at 0.15 V while using Cu₂O (synthesized at 850 °C) it started at 0.55 V. In case of Pt electrode increase in oxidation current started at 0.35 V for both the cases. The following reaction occurs at the electrode surface during OER.

4OH⁻ → 2H₂O + O₂ + 4e⁻

Fig. 8 OER of Cu₂O (a) on glassy carbon (b) platinum as working electrode.

The peak current is proportional to the amount of oxygen generated during electrolysis and it also depends on surface area and size of nanoparticles. The maximum current at 1.0 V potential is 0.22 mA (Cu₂O 850 °C) and 0.47 mA (Cu₂O 350 °C) using glassy carbon electrode and in case of Pt electrode maximum current at 0.9 V is 0.25 mA and 1.72 mA for two samples of Cu₂O respectively (Fig. S2†). It is clearly seen that Cu₂O synthesized at lower temperature produces twice higher current at GC electrode whereas it is 7 times higher at Pt electrode. The current density was found to be 10.7 mA cm⁻² & 20 mA cm⁻² with GC electrode and 7.7 mA cm⁻² & 35.7 mA cm⁻² with Pt electrode for synthesized Cu₂O (850 °C, 350 °C) respectively (Fig. 8). These electrocatalysts were highly stable over 50 cycles. This is the first study of HER and OER using Cu₂O nanoparticles as electrocatalyst.

The photocatalytic efficiency of both the samples of Cu₂O nanoparticles were examined by monitoring the degradation of methylene blue dye (absorption maxima of the dye is at 664 nm) under visible light both in presence and absence of H₂O₂. To check the photostability of the dye, an experiment was performed where the dye solution was exposed to light source without the addition of the catalyst (both in presence and absence of the H₂O₂). It was observed that in both the cases the concentration of the dye remained unchanged in the reaction solution even after 3 h. Also addition of catalyst in dye solution in absence of light do not show any change in the UV spectra recorded which confirms that almost no dye gets adsorbed onto the catalyst surface. UV visible spectra before and after two hours of addition of catalyst has been shown in Fig. S14.† We have done zeta potential on both the samples and found that the cuprous oxide carries a net positive charge (18.9 mV for 350 °C and 8.0 mV for 850 °C). Since we have carried out the degradation for a positively charged dye, owing to electrostatic repulsion there is almost no adsorption on the surface of the material. However, in presence of H₂O₂, hydroxyl radical are generated which gets adsorbed on to the surface of catalyst (positive charged) blocking its active sites thereby reducing its activity. The higher the charge of the material higher the adsorption of dye. For both catalysts three different sets of reactions were performed (a) with light and with H₂O₂ (b) with light and without H₂O₂ (c) without light and with H₂O₂ (Fig. S3–S8†). In all the three set of reactions the concentration of dye and the amount of catalyst was kept constant. The first case i.e. a reaction with light and with H₂O₂ shows very good efficiency towards the degradation of the dye compared to the other two conditions. Here, H₂O₂ acts as an electron and hydroxyl radical (OH˙)^45,46 scavenger which prevents the recombination of electron–hole pair generated during the catalysis even when used in very low concentration. Since H₂O₂ was added in very low amount (10 μL) therefore it does not significantly change the pH of the solution. Hydrogen peroxide is very promising and important oxidant due to its low cost, high efficiency and dissociation into harmless byproduct. The Cu₂O (850 °C) showed ∼98% degradation of the dye whereas Cu₂O (350 °C) showed only ∼40% degradation of the dye after an irradiation of 120 min (Fig. 10). Morphology dependent photocatalytic activity is known for nanocrystalline cuprous oxide.^43,47 The higher photocatalytic activity of the material synthesized at higher temperatures may be attributed to the difference in the morphology of the oxide. Cuprous oxide synthesized at high temperature exhibits the growth of the material in a layered manner. As a result, highly active faces are exposed which are potentially active as a photocatalyst probably because of the planes which are in direct contact with the dye solution. The adsorption energy of molecular water on copper depends on the crystallographic plane and follows the order γ [111] < γ [100] < γ [110].⁴⁸ These planes differ in the surface atom density where [111] is reported to have the highest surface atom density whereas [110] has the lowest. The surfaces with lower atom density are reported to have higher affinity for water molecules. It is also reported that the number and density of “Cu” dangling bonds are higher for [110] plane than [111] plane owing to which photoexcitation for the generation of electrons and holes becomes easier. The presence of (110) plane as observed in the HRTEM studies (Fig. 14) for the sample synthesized at 850 °C thus justifies the higher photocatalytic activity than the oxide at 350 °C which shows (111) plane. Thus, the difference in the activity of the two materials. The recyclability of the Cu₂O (850 °C) catalyst was satisfactory as five continuous cycles (Fig. S9–S13†) were studied and the degradation remained almost constant upto 4^th cycle (Fig. 11 & 12). After the 4^th cycle, the efficiency decreases to 67%. To determine the rate constant of the photodegradation process of methylene blue, pseudo first-order kinetic equation was used which is expressed as ln A = −kt, where “A” is absorbance, “t” is the time, and “k” is reaction rate constant. The rate constant of Cu₂O (850 °C) catalyst was found to be 4.28 × 10⁻² min⁻¹ (2.57 h⁻¹) (Fig. 13). Wu et al.⁴⁹ have studied the degradation of MB by Cu₂O film on tin doped indium oxide as substrate in the presence of large amount of H₂O₂ (0.1 to 10 mL). A degradation of 89–98% was observed as compared to 98% degradation in our study by using a very small amount (10 μL) of H₂O₂. Degradation percentage in our case is higher than other reports such as Cu₂O,^7,50 Cu₂O–G (graphene)⁵¹ and Cu₂O–TiO₂⁵² composites.

Fig. 9 Diffuse reflectance spectra of Cu₂O.

Fig. 10 Photodegradation of methylene blue using (a) Cu₂O – 850 °C and (b) Cu₂O – 350 °C.

Fig. 11 Recyclability of Cu₂O – 850 °C as a photocatalyst (5 cycles).

Fig. 12 Percentage degradation of methylene blue after 120 min of Cu₂O – 850 °C.

Fig. 13 Plot of absorbance vs. time of Cu₂O – 850 °C photocatalyst in case of with light with H₂O₂ (inset shows the plot of ln(Abs.) vs. time).

Fig. 14 HRTEM micrograph of (a) Cu₂O – 850 °C and (b) Cu₂O – 350 °C.

Conclusions

Cu₂O nanoparticles (two sizes) have been synthesized from solid state reactions at different temperature. The average size of nanoparticles was found to be 40–50 nm and 20–30 nm for higher (850 °C) and lower temperature (350 °C) synthesized Cu₂O respectively. The low temperature reaction is facilitated by the use of copper oxalate nanorods as starting materials. The HER efficiency is enhanced by 14 times on GC electrode and 4 times on Pt electrode for low temperature synthesized Cu₂O. The OER is also enhanced by 2 and 5 times on GC and Pt electrodes respectively. The Cu₂O synthesized at higher temperature is more efficient for the photocatalytic degradation of methylene blue. As an e⁻ and OH˙ scavenger, a small drop (10 μL) of H₂O₂ is sufficient to accelerate the photocatalytic process. These catalysts are quite stable for electrocatalytic and photocatalytic processes.

Acknowledgements

BK and SS thank IITD and CSIR respectively, for fellowship. Financial assistance from DeiTY and DST (Nano Mission), Govt. of India is gratefully acknowledged.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra46994h

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