CuO nanostructures: facile synthesis and applications for enhanced photodegradation of organic compounds and reduction of p-nitrophenol from aqueous phase

Abstract

1D CuO nanorods and 2D CuO nanosheets were synthesized for the first time using amino acids, namely L-lysine and L-glycine, by a microwave assisted green synthetic route. The amino acids act as a good complexing and capping agent in the synthesis of CuO nanoparticles. L-Lysine mediated synthesis leads to the formation of CuO nanorods having an average diameter of 30 nm, while L-glycine assisted synthesis leads to the formation of CuO nanosheets with dimensions of 300–600 nm length, 200 nm width and 30–60 nm thickness. Hence, the morphology and size of the CuO nanoparticles can be varied by changing the amino acids, with other parameters remaining unaffected. CuO nanorods and CuO nanosheets were characterized by FTIR, XRD, TEM, SAED and UV-visible spectroscopy. XRD, FTIR and SAED reveal the monoclinic phase of CuO nanoparticles. Absorption spectra of CuO nanorods and nanosheets show a broad absorption band around 380 nm and 383 nm, respectively, due to surface plasmon absorbance of CuO. A clear blue shift in the band gap energy of synthesized CuO nanorods was observed from CuO nanosheets, with a decrease in particle dimension due to the quantum effect. The catalytic activity of synthesized CuO nanorods and CuO nanosheets was studied for the reduction of p-nitrophenol to p-aminophenol. The results showed an enhanced catalytic potential of 1D CuO nanorods as compared with that of 2D CuO nanosheets. The photocatalytic activity of synthesized CuO nanoparticles was also evaluated for the degradation of two different toxic dyes, namely methylene blue and eosin Y, under solar irradiation. The degradation products were analyzed and a mechanistic pathway for the degradation is proposed.

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1. Introduction

In the last two decades, much research has been concentrated in the field of nanotechnology because of its advanced applications in various fields. The research thrust in nanotechnology has greatly increased in recent years. Nanoparticles have immense applications in various fields such as synthetic chemistry, catalysis, drug design, medicine, optoelectronic devices, solar cells, sensors, batteries, conducting electrodes, etc. Nanoparticles show unique characteristics and differ to a great extent from their bulk counterparts. Due to the fascinating physical and chemical properties of transition metal oxide nanoparticles, tremendous efforts have been dedicated towards the synthesis of such metal oxides. Among them, CuO, a p-type semiconductor with a narrow band gap of 1.2 eV, is one of the most intensively studied metal oxides because of its potential applications in catalysis, solar cells, magnetic storage media, semiconductors, field transistors, gas sensors, batteries etc.^1–5 The size and morphology of the nanoparticles play a significant role in developing the chemical and physical properties and influence their existing applications largely. Therefore, much effort has been dedicated to the fabrication of CuO nanostructures with different size and morphology. A variety of methods such as hydrothermal, quick precipitation, thermal oxidation, sol–gel, solvothermal etc. have developed for the synthesis of different dimensions of CuO nanostructures having controlled size.^1–8 Nowadays, the microwave assisted route has gained a lot of attention due to various advantages over other methods. It is a faster, energy saving and environmentally benign (green) route for the synthesis of various nanostructures.^9,10

This article describes a facile, green and microwave assisted synthesis of one- and two-dimensional (1D and 2D) CuO nanorods and nanosheets using the amino acids L-lysine and L-glycine in aqueous medium. The amino acids act as a good complexing and capping agent in the synthesis of CuO nanoparticles. It was evident from the literature that size, morphology and properties of nanoparticles were modified by the presence of amino acids. Therefore, a microwave assisted synthetic route was designed using amino acids, namely L-lysine and L-glycine. To the best knowledge of the authors, microwave assisted synthesis of 1D CuO nanorods and 2D CuO nanosheets using L-lysine and L-glycine has not been reported previously in the literature.

Among the various organic compounds, para-nitrophenol is one of the most commonly used chemicals in the manufacture of dyes, pharmaceuticals, pesticides, insecticides, plasticizers and explosives.¹¹ para-Nitrophenols are one of the most hazardous, toxic classes of anthropogenic pollutants used extensively in various industries. para-Nitrophenols (PNP) are highly soluble and stable in water system. Therefore, the discharge of PNP from industries into the ecosystem causes severe health hazards. The high solubility of para-nitrophenol in water hinders the use of traditional water purification methods for their removal from industrial waste water. Hence, various techniques, such as adsorption, photocatalytic degradation, microwave assisted catalytic oxidation, electrochemical treatment, microbial degradation, the electro-Fenton method and electrocoagulation¹² have been developed for the removal of PNP from the water system. The utilization of all these techniques requires high energy, and various organic solvents are also involved. Hence, it is very important to develop a method which involves aqueous phase conversion of PNP to a compound which is non-toxic in nature under mild conditions. Among the various methods, the catalytic reduction of PNP to p-aminophenol (PAP) has gained a lot of attention due to the important applications of PAP. PAP is an important intermediate in the synthesis of various analgesic and antipyretic drugs such as paracetamol, acetanilide and phenacitin. It has enormous applications in the synthesis of various dyes, as a photographic-developer, and as a corrosion inhibitor.^11,13 Hence, synthesis of PAP from PNP by a cheaper and effective method would be appreciated. Herein, we report the catalytic reduction of PNP to PAP in aqueous phase using 1D and 2D CuO nanoparticles as catalysts. The catalytic activities of 1D CuO nanorods and 2D CuO nanosheets were evaluated for the reduction of PNP to PAP and a comparative study is also reported.

Dyes constitute a major class of organic compound having huge applications in our daily life. They are used in the textile industries, dyeing, printing, cosmetics etc. However, most dyes are toxic and carcinogenic in nature. Moreover, dyes from the textile and other industries get discharged into the nearby water sources and contaminate water, thereby causing water pollution. These organic dyes can be degraded photochemically by the use of nanostructured semiconductor oxides which act as excellent photocatalysts in the degradation process. In this paper, we report the degradation of two different toxic dyes, namely methylene blue and eosin Y dye, under direct sunlight using CuO nanoparticles as photocatalyst. Methylene blue is a heteropolyaromatic dye. It is a water soluble dye used in pharmaceutical drugs and as a colorant in textile industries. It is toxic and causes anemia, bladder irritation and gastrointestinal problems. Eosin Y is a heterocyclic, water soluble, acid dye containing bromine atoms and is highly toxic in nature. It has been proved to be harmful for living systems and causes serious environmental problems. Therefore, these two dyes cause adverse health effects and are a real threat to human, animal and aquatic life. Herein, the degradation of two different dyes, namely methylene blue and eosin Y, was carried out under solar irradiation using CuO nanoparticles as photocatalyst. The degradation products were identified and the mechanism pathway for the degradation of dyes is also suggested in this paper.

2. Experimental

2.1. Materials

The reagents copper sulfate pentahydrate (CuSO₄·5H₂O), sodium hydroxide, L-lysine, L-glycine, p-nitrophenol, sodium borohydride, methylene blue and eosin Y were procured from Merck and were of analytical grade (AR). The reaction was carried out in a domestic microwave oven.

2.2. Synthesis of 1D CuO nanorods and 2D CuO nanosheets

The synthesis of 1D CuO nanorods was carried out by treating 50 ml of 0.01 M CuSO₄·5H₂O solution with 50 ml of 0.01 M L-lysine solution and 50 ml of 0.05 M NaOH solution under constant stirring. The reaction mixture was then kept in a microwave oven and irradiated with thirty 10 s shots. This resulted in the formation of a black precipitate which was centrifuged and washed five times with distilled water. The final product was dried at 100 °C and collected for characterization. The sample was marked S1.

A similar procedure was followed for the synthesis of 2D CuO nanosheets using CuSO₄·5H₂O, L-glycine and NaOH. The final product obtained was characterized and marked S2.

To check the reproducibility of the results, both the experiments were repeated twice under similar conditions. The XRD pattern, TEM and SAED (selected area electron diffraction) results were recorded, and similar results were obtained by each method. The TEM images, XRD and SAED patterns are provided in the ESI (Fig. S1 and S2†). Table S1† presents a summary of results obtained from TEM images, XRD and SAED patterns for the synthesized CuO nanorods (S1) and CuO nanosheets (S2).

2.3. Characterization

CuO nanoparticles were characterized by powder XRD using a Phillips X'Pert PRO diffractometer with CuKα radiation of wavelength 1.5418 Å. The size, morphology and diffracted ring pattern of CuO nanoparticles were determined by using a JEM-2100 transmission electron microscope. Infrared spectra were recorded with a Bruker Hyperion 3000 FTIR spectrometer. Absorption spectra were recorded on a Cary 100 BIO UV-visible spectrophotometer. The degradation products were identified by LC-MS (410 Prostar Binary LC with 500 MS IT PDA detector).

2.4. Catalytic activity of 1D and 2D CuO nanoparticles

Reduction of p-nitrophenol (PNP) at room temperature using an aqueous solution of sodium borohydride was performed in order to examine the catalytic activity of the synthesized CuO nanoparticles (nanorods and nanosheets). A comparative analysis of the catalytic activity of CuO nanorods and CuO nanosheets was carried out in the light of reduction of p-nitrophenol (PNP) to p-aminophenol (PAP). The progress of the reaction was monitored by recording UV-visible spectra at regular intervals of time. The reaction was performed inside a quartz cuvette initially containing 2.6 ml of water or 60 μl of 0.006 M PNP; the absorbance of each was recorded using a UV-visible spectrometer. This was followed by the addition of 300 μl of 0.1 M aqueous solution of NaBH₄ and the absorbance was recorded immediately. Afterwards, 300 μl of an aqueous solution of CuO nanoparticles (0.001 g) was added to the reaction mixture and absorption spectra were recorded until the peak due to the presence of the nitro group disappeared completely.

2.5. Photocatalytic activity of synthesized CuO nanoparticles

The photocatalytic activity of synthesized CuO nanoparticles was evaluated by the degradation of two different organic dyes, namely methylene blue (MB) and eosin Y (EY). To examine the photocatalytic activity, 10 mg of synthesized CuO photocatalyst was dispersed in 200 ml of 10⁻⁴ M aqueous solution of MB or EY dye. Both the solutions were then exposed to sunlight irradiation. Before exposing to direct sunlight, both the solutions were kept in the dark for the next 30 min to achieve adsorption/desorption equilibrium. The photodegradation of the dyes was carried out on a sunny day between 10 a.m and 2 p.m when there were minimum fluctuations in solar intensity. The experiment was carried out in Silchar city in the month of May (outside temperature 35–30 °C) on a sunny day when the average solar radiation was 4.29 kW h per m per day. At regular intervals of time, 5 ml of the each suspension was withdrawn and centrifuged immediately. The progress of the reaction was monitored by using a UV-visible spectrophotometer. The degradation products were identified using LC-MS analysis.

2.6. Recycling of CuO photocatalyst

The lifetime of the CuO photocatalyst is an important parameter for the photocatalytic process. The use of the catalyst for a longer period of time leads to a significant cost reduction. For this reason, the reusability of CuO nanosheets (S2) was tested. After the first cycle of the degradation of MB and EY dyes, the CuO photocatalyst was recovered by simple filtration using Whatman Grade 42 filter paper. The catalyst was collected and washed thrice with hot water and ethanol to remove any adsorbed products from its surface. The recovered CuO photocatalyst was then added again to MB and EY dye solutions to perform the photocatalytic degradation of the dyes. After each cycle, the photocatalyst was recovered by simple filtration technique using Whatman Grade 42 filter paper and the recovered photocatalyst was washed three times with hot water and alcohol for the removal of adsorbed products. The photocatalyst was reused for the degradation of MB and EY dyes for five cycles. The recyclability of the CuO photocatalyst (S2) showed that the photocatalytic loss totalled about 6–7% after five cycles of operation in both cases, i.e., for MB and EY dyes. Therefore, the catalytic efficiency decreases to a small extent (6–7%) over five cycles, which proved the efficiency of 2D CuO nanosheets as phototcatalyst.

3. Results and discussion

3.1. Characterization

FTIR spectra were recorded in order to detect the formation of CuO and also to investigate the role of amino acids. Fig. 1(a and b) represents the FTIR spectra of synthesized 1D CuO nanorods (S1) and 2D CuO nanosheets (S2), respectively. The assignment of FTIR bands of S1 and S2 nanoparticles is shown in Table 1. The peaks at ∼427–420, ∼528–511 and ∼599–609 cm⁻¹ correspond to the characteristic stretching vibration of the Cu–O bond in monoclinic CuO.^3,4 The bands around 1631–1641 cm⁻¹, 2923 cm⁻¹ and 2854 cm⁻¹ indicate the presence of –COO and –CH₂ groups of amino acids, namely L-lysine and L-glycine.¹⁴ These indicate that the amino acids are adsorbed on the surface of CuO nanoparticles and thereby act as a good capping agent in the synthesis of CuO nanoparticles.

Fig. 1 FTIR spectra of synthesized (a) CuO nanorods (S1) and (b) CuO nanosheets (S2).

Table 1 Assignment of FTIR bands of synthesized CuO nanorods (S1) and CuO nanosheets (S2)

CuO nanoparticles	FTIR bands (cm^−1)
	νC–H	νC=O	νCu–O
CuO nanorods (S1)	2923 (asymmetric stretch), 2854 (symmetric stretch)	1631	599, 528, 427
CuO nanosheets (S2)	2923 (asymmetric stretch), 2854 (symmetric stretch)	1641	609, 511, 420

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The crystal structure and purity of the synthesized CuO nanorods and nanosheets were determined using XRD. Fig. 2(a) represents the XRD pattern of synthesized CuO nanorods (S1). The diffraction peaks obtained at 2θ values correspond to the (110), (002/−111), (111/200), (−112), (−202), (020), (202), (−113), (−311), (113), (311) and (−222) lattice planes. The peaks obtained clearly reflect the monoclinic phase of CuO and correspond to the JCPDS card no. 89-5898 of CuO.¹⁵ The XRD pattern of synthesized CuO nanosheets is shown in Fig. 2(b). All the peaks are well indexed to the monoclinic phase of CuO. The diffraction pattern is also in good agreement with the JCPDS card of CuO (JCPDS 89-2529).

Fig. 2 XRD spectra of synthesized (a) CuO nanorods (S1) and (b) CuO nanosheets (S2).

The XRD pattern of CuO nanorods (S1) and CuO nanosheets (S2) did not show any peak for CuSO₄·5H₂O, Cu(OH)₂ or Cu₂O, which indicates the purity of and complete conversion of the starting material and intermediates into the final product CuO. The XRD pattern also depicts the crystalline nature of CuO nanorods and nanosheets.

The morphology, size distribution and crystal structure of synthesized CuO nanoparticles were elucidated using electron microscopic analysis (TEM and HRTEM images, SAED pattern). Fig. 3(a and b) represent the TEM and HRTEM images of CuO nanoparticles (S1) synthesized using lysine. The magnified TEM image (Fig. 3(c)) shows the formation of bundles of 1D CuO nanorods having an average diameter of ∼30 nm. The spacing between adjacent lattice fringes obtained from the HRTEM image (Fig. 3(b)) is 0.132 nm, which reveal the existence of the (311) lattice plane. The SAED pattern of CuO nanorods is depicted in Fig. 3(d). The lattice planes obtained from the SAED pattern clearly indicate the monoclinic crystal structure of CuO nanorods (JCPDS card no. 89-5898). The SAED pattern is in good agreement with the XRD results of CuO nanorods (S1).

Fig. 3 (a) TEM photomicrograph of synthesized 1D CuO nanorods (S1), (b) HRTEM image of synthesized CuO nanorods (S1), (c) magnified TEM image of CuO nanorods (S1), and (d) SAED pattern of synthesized CuO nanorods (S1).

Fig. 4(a and c) represent the TEM and magnified TEM images of synthesized CuO nanoparticles (S2). The magnified TEM image (Fig. 4(c)) reveals the formation of CuO nanosheet-like morphology having an average length, width and thickness of 300–600 nm, 200 nm and 30–60 nm respectively. The microstructure of CuO nanosheets was analyzed using the HRTEM image (Fig. 4(b)). The interplanar spacing of CuO nanosheets is 0.15 nm which corresponds to the (−113) plane. Fig. 4(d) shows the SAED pattern of synthesized CuO nanosheets. The lattice planes obtained from the SAED pattern are in accordance with the monoclinic crystal structure of CuO (JCPDS 89-2529).

Fig. 4 (a) TEM photomicrograph of synthesized 2D CuO nanosheets (S2), (b) HRTEM image of synthesized CuO nanosheets (S2), (c) magnified TEM image of CuO nanosheets (S2), and (d) SAED pattern of synthesized CuO nanosheets (S2).

Therefore, the incorporation of different amino acids gives rise to different morphology of CuO nanoparticles. Hence, the size and morphology of CuO nanoparticles can be varied by changing the amino acids (i.e., lysine and glycine) with other parameters remaining the same. A decrease in the dimensions of CuO nanostructures, i.e., from 2D CuO nanosheets to 1D CuO nanorods, was also observed from the TEM images, which may be attributed to the change in the nature of the functional group associated with the amino acids, namely L-lysine and L-glycine. Amino acids are adsorbed on the surface of CuO nanoparticles and act as a capping agent in the synthesis of CuO nanoparticles. Therefore, amino acids adsorbed on the surface of CuO nanoparticles control and direct the self-assembly and oriented attachment growth mechanism, thereby giving rise to different morphology/dimensions of CuO nanoparticles.

The optical properties of synthesized CuO nanorods and nanosheets were investigated by recording their absorption spectra. Fig. 5(a) and 6(a) represent the absorption spectra of CuO nanorods (S1) and CuO nanosheets (S2), respectively. The absorption spectra of S1 and S2 nanoparticles showed characteristic absorption bands around 380 nm and 383 nm, respectively, which arose due to the surface plasmon absorbance of the metal oxide CuO.^3,16,17 The occurrence of the band around 380–383 nm also confirmed the formation of CuO nanoparticles. The absorption spectra were also recorded in order to investigate the band gap energy of the synthesized CuO nanorods and nanosheets. The band gap energy of the synthesized CuO nanoparticles can be obtained by the following equation:¹⁶

αhν = K(hν − E_g)ⁿ (1)

where K is a constant, E_g is the band gap energy and the exponent n depends on the type of transition, and whose value is 1/2 for allowed direct transition. Therefore, by plotting (αhν)² versus incident photon energy (hν) and by extrapolating the linear portion of the curve to zero absorption coefficient, the band gap energy can be estimated.^1,3,16 Fig. 5(b) and 6(b) show the plot of (αhν)² versus (hν) for the synthesized CuO nanorods (S1) and CuO nanosheets (S2) respectively. The band gap energy of CuO nanorods and CuO nanosheets obtained by plotting (αhν)² versus incident photon energy (hν) was found to be 2.25 eV and 2.02 eV respectively. Hence, a clear blue shift in the band gap energy of the synthesized CuO nanorods and CuO nanosheets was observed relative to the bulk CuO (1.2 eV). Owing to an increase in the dimensions of CuO nanosheets from CuO nanorods, a decrease in the band gap energy of CuO nanosheets was also observed. This was attributed to the quantum effect observed in the semiconductor CuO nanoparticles with a decrease in particle size.

Fig. 5 (a) Absorption spectrum of synthesized CuO nanorods (S1). (b) Plot of (αhν)² versus incident photon energy (hν) for the synthesized CuO nanorods (S1).

Fig. 6 (a) Absorption spectrum of synthesized CuO nanosheets (S2). (b) Plot of (αhν)² versus incident photon energy (hν) for the synthesized CuO nanosheets (S2).

3.2. Role of amino acid in the synthesis of CuO nanoparticles

In this particular experiment, the amino acids, namely lysine and glycine, act as a complexing as well as capping agent in the synthesis of CuO nanoparticles. Initially, the amino acids form a complex with Cu²⁺ ions. On treatment with NaOH, the complex breaks down to form Cu(OH)₂, which on further heat treatment (100 °C) decomposes to form CuO nanoparticles. After the decomposition of the Cu²⁺–amino acid complex, some molecules of the amino acids lysine and glycine are adsorbed on the surface of CuO nanoparticles and thereby act as a capping agent.^2,14,18

However, for further confirmation of the role of amino acids as capping agent in the synthesis of CuO nanoparticles, a control experiment was carried out for the synthesis of CuO nanoparticles in the absence of the amino acids glycine and lysine, and the corresponding TEM image was recorded. The TEM image (Fig. S3†) showed the formation of agglomerated CuO nanoparticles, whereas, on addition of amino acids, CuO nanorod- and nanosheet-like structures were formed. Hence, the involvement of the amino acids in the synthesis of CuO nanoparticles leads to the formation of non-agglomerated CuO nanostructures. The adsorption of amino acids on the surface of CuO nanoparticles controls the agglomeration and directs the self-assembly and oriented attachment growth mechanism, thereby giving rise to different morphology/dimensions of CuO nanoparticles. This confirmed the role of amino acids as capping agent.

The schematic representation (Scheme 1) for the formation of 1D CuO nanorods and 2D CuO nanosheets using amino acids (namely lysine and glycine) and NaOH can be visualized as follows:

Scheme 1 Formation of bundles of 1D CuO nanorods and 2D CuO nanosheets using amino acids and NaOH.

The probable growth mechanism involves three steps. Firstly, CuO nuclei are assumed to be formed from Cu(OH)₂ nuclei by the “Ostwald ripening” process,¹⁹ wherein larger particles grow at the expense of smaller particles. It is a thermodynamically driven process since larger particles are more energetically stable than the smaller ones.

However, Penn and Banfield proposed an oriented aggregation and oriented attachment mechanism by which CuO nanorods were assumed to be formed. By the oriented attachment mechanism (second step), secondary crystalline nanoparticles are obtained from primary nanoparticles via self-organization and self-attachment in a highly ordered and irreversible manner, sharing the same crystallographic orientation in order to reduce the overall energy of the system.²⁰

In the third step, the synthesized CuO nanorods self-aggregate in an ordered fashion to form bundles of CuO nanorods and the adjacent nanorods share a common crystallographic orientation. Similarly, 1D CuO nanorods are the building blocks for the formation of 2D CuO nanosheets. The 1D CuO nanorods assemble via an oriented attachment growth mechanism, leading to the formation of 2D CuO nanosheets, wherein the primary nanoparticles share the same crystallographic orientation.

The amino acids play an important role in determining the different morphologies of CuO nanoparticles. For crystal growth, the small sized nanoparticles are not thermodynamically stable. Therefore, in order to form stable nanoparticles they must be arrested during the reaction by the addition of surface protective agents, viz., inorganic capping agents, organic ligands, etc.²¹ Herein, CuO nanoparticles were synthesized using amino acids as capping agent. Therefore, amino acids play an important role in controlling the morphology of CuO nanoparticles. The amino acids adsorbed on the surface of CuO nanoparticles can suppress or modify the oriented attachment growth mechanism by preventing contact between the faces on which adsorption has selectively occurred. The surface modification with amino acids predetermines the assembly behavior of CuO nanoparticles. Therefore, amino acids adsorbed on the surface of CuO nanoparticles control and direct the self-assembly and oriented attachment growth mechanism, thereby giving rise to different morphology/dimensions of CuO nanoparticles. Moreover, the change in the functional group of the amino acids also influences the morphology and size of the synthesized CuO nanoparticles. Hence, all these factors lead to the change in the morphology and size of synthesized CuO nanostructures.

3.3. Catalytic activity of synthesized CuO nanorods and nanosheets in the reduction of p-nitrophenol

The catalytic activity of synthesized 1D CuO nanorods and 2D CuO nanosheets was analyzed individually by carrying out the reduction of p-nitrophenol using NaBH₄ in aqueous medium. The UV-visible spectrum of PNP was recorded and it was evident that PNP had an absorbance maximum at 317 nm in aqueous medium (Fig. S4†). The subsequent addition of freshly prepared NaBH₄ solution to PNP leads to a red shift from 317 nm to 401 nm (Fig. S5†). It was also observed that the light yellow color of PNP solution changes to intense yellow due to the formation of p-nitrophenolate ions under alkaline condition.¹³ The peak obtained at 401 nm is unaltered after a couple of days in the absence of any catalyst.

However, after the addition of a 300 μl solution of CuO nanoparticles (0.001 g), the yellow color of PNP solution slowly faded away and finally disappeared on complete reduction of PNP. This decolorization was monitored by UV-visible spectroscopy at regular intervals of time.

Fig. 7(a and c) represent the absorption spectra for the reduction of PNP using CuO nanorods and CuO nanosheets, respectively, as catalyst. From Fig. 7(a and c) it is evident that with increasing time the characteristic peak for PNP decreases with simultaneous appearance of a new peak centered at 297 nm. This is due to the reduction of PNP to p-aminophenol (PAP). The peak at 297 nm increases gradually with time due to the formation of PAP.^13,22 The complete reduction of PNP takes place within 6 min using CuO nanorods as catalyst. However, conversion of PNP to PAP takes place in 14 min using CuO nanosheets as catalyst. The kinetics of both the reactions were also studied. The rate constant (k) was determined following first order kinetics from the linear plot of ln A_t versus reduction time, t (Fig. 7(b and d)).¹³ The rate constant (k) for the reduction of PNP using CuO nanorods (Fig. 7(b)) and CuO nanosheets (Fig. 7(d)) as catalyst was found to be 0.403 min⁻¹ and 0.24 min⁻¹, respectively. From the absorption spectra, it was evident that the time required for the reduction of PNP using CuO nanorods as catalyst was less than that of CuO nanosheets. The rate constant (k) obtained for the reduction of PNP using CuO nanorods was also higher than that of CuO nanosheets. Hence, CuO nanorods had enhanced catalytic activity as compared with CuO nanosheets in the reduction of PNP.

Fig. 7 (a) Absorption spectra of conversion of PNP to PAP using 1D CuO nanorods as catalyst in the presence of NaBH₄ in aqueous medium. (b) Plot of ln A_t versus reduction time for the reduction of PNP using CuO nanorods as catalyst. (c) Absorption spectra of conversion of PNP to PAP using 2D CuO nanosheets as catalyst in the presence of NaBH₄ in aqueous medium. (d) Plot of ln A_t versus reduction time for the reduction of PNP using CuO nanosheets as catalyst.

Therefore, from the above studies it was evident that 1D CuO nanorods act as a more efficient catalyst than 2D CuO nanosheets in the reduction of PNP to PAP. This may be attributed to the smaller dimensions of CuO nanorods as compared with CuO nanosheets. The decrease in dimensions leads to an increase in the surface area and this enhances the catalytic properties. The porous surface of CuO nanorods as evident from the TEM image also contributes to enhancing the catalytic activities. Table 2 represents the comparative study of catalytic activity of CuO nanorods and CuO nanosheets in the reduction of PNP to PAP.

Table 2 Comparative study of catalytic activity of synthesized CuO nanorods and CuO nanosheets in the reduction of PNP to PAP

Nanocatalyst	Amount added (g)	Time (min)	Rate (min^−1)
CuO nanorods (S1)	0.001	6	0.403
CuO nanosheets (S2)	0.001	14	0.24

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3.4. Mechanism of reduction of p-nitrophenol to p-aminophenol

In this heterogeneous system, the mechanistic pathway for the hydrogenation of p-nitrophenol involves the following steps: (i) adsorption of hydrogen, (ii) adsorption of p-nitrophenol onto the CuO surface, (iii) electron transfer mediated by the metal-oxide surface from BH₄⁻ ion to p-nitrophenol, and (iv) desorption of p-aminophenol. The electron is transferred from donor BH₄⁻ ion in the reduction of p-nitrophenol. In the presence of CuO nanoparticles (S1 and S2), the BH₄⁻ ion is adsorbed on the CuO surface and discharge of electrons from BH₄⁻ ion takes place through the metal oxide to the acceptor, i.e., p-nitrophenol. The aqueous medium provides the required amount of H⁺ ion for complete reduction of p-nitrophenol into p-aminophenol. The probable mechanism can be schematically represented as follows:

3.5. Evaluation of photocatalytic activity of synthesized CuO nanoparticles for the degradation of MB and EY dyes

The photocatalytic activity of nanoparticles under solar irradiation depends on the band gap energy; the lower the band gap energy, the higher is the activity under visible light. 2D CuO nanosheets (S2), having a band gap energy of 2.02 eV, showed better photocatalytic activity than 1D CuO nanorods (S1; 2.25 eV); therefore, the degradation results of MB and EY obtained using CuO nanosheets (S2) as photocatalyst are reported in this paper.

The photocatalytic activity of synthesized CuO nanosheets (S2) was evaluated by monitoring the changes observed in the absorption spectra of samples of the MB and EY dye solutions withdrawn at regular intervals of time. Fig. 8(a) represents the absorption spectra for the photodegradation of MB dye by solar irradiation using S2 as photocatalyst. It is evident that after the addition of CuO nanosheets, the characteristic absorption peak of MB dye at 663 nm decreased gradually and the color of the solution also became less intense with increasing irradiation time. The dye solution became colorless within 180 min, which implied the complete decomposition of the dye molecules. Similarly, after the addition of CuO nanosheets (S2) to EY dye solution, the characteristic absorption band of EY observed at 517 nm also decreased with increase in irradiation time (Fig. 9(a)) and within 40 min the dye solution became colorless and the absorption band also disappeared. This also indicated the complete degradation of the EY dye. The kinetics of the MB and EY degradation reactions were studied by plotting ln A_t versus irradiation time (t) as shown in Fig. 8(b) and 9(b) respectively. The degradation reactions followed pseudo first order kinetics and the degradation rate constants calculated from the slope of the linear plot were found to be 0.021 min⁻¹ and 0.105 min⁻¹ for MB and EY dye, respectively. The photodegradation of MB and EY dye with time is represented in Fig. 8(c) and 9(c). The graph clearly shows that about 97% of MB was degraded within 180 min, whereas 99.1% of EY dye degraded within 40 min using synthesized 2D CuO nanosheets (S2) as photocatalyst.

Fig. 8 (a) Photodegradation of methylene blue (MB) dye by solar irradiation using synthesized CuO nanosheets (S2) as photocatalyst. (b) Plot of ln A_t versus irradiation time, t, for photodegradation of MB dye using synthesized CuO nanosheets (S2) as photocatalyst. (c) Percentage efficiency for the photodegradation of MB dye with time.

Fig. 9 (a) Photodegradation of eosin Y (EY) dye by solar irradiation using synthesized CuO nanosheets (S2) as photocatalyst. (b) Plot of ln(A_t) versus irradiation time, t, for photodegradation of EY dye using synthesized CuO nanosheets (S2) as photocatalyst. (c) Percentage efficiency for the photodegradation of EY dye with time.

Simultaneously, in order to check the catalytic activity of CuO nanosheets, control experiments were performed wherein MB and EY dye solutions were irradiated with sunlight in the absence of CuO nanosheets. Neither of the dyes underwent degradation. This showed experimentally the visible light activity of the synthesized CuO nanosheets. Again, both the dyes (MB and EY) were also kept in dark (in the absence of sunlight) in the presence of CuO nanosheets, and it was evident that the dyes underwent negligible degradation in 1 day.

3.6. Probable mechanistic pathway for the degradation of dyes

Solar irradiation mainly consists of 45% visible light (above 400 nm) and 5% UV-light (below 400 nm). Since, the percentage of UV-radiation is much less, so the visible light is mainly responsible for the degradation of dyes. For a semiconductor photocatalyst, when the energy of incident sunlight is equal to or higher than the band gap energy of the photocatalyst, the electrons from the valence band are excited into the conduction band, thereby producing holes in the valence band. These holes act as an oxidizing agent and oxidize the water molecules to form oxidizing radical species, i.e., reactive ˙OH radicals which further oxidize the dye pollutants directly. The electrons in the conduction band act as a reducing agent and reduce the oxygen adsorbed on the surface of the photocatalyst. Herein, the band gap energy of the synthesized CuO nanosheets utilized for the degradation of dyes is about 2.02 eV whereas the energy of the solar irradiation, which consists mainly of visible light, is much greater than the band gap energy of the CuO photocatalyst. Therefore, the degradation pathway mentioned above is followed when CuO nanosheets are used as photocatalyst under solar irradiation. Scheme 2 shows the probable mechanistic pathway followed for the degradation of two different dyes, namely MB and EY, using CuO nanosheets (S2) as photocatalyst under solar irradiation.^23,24

Scheme 2 Probable mechanistic pathway for the degradation of the MB and EY dyes using CuO nanosheets as photocatalyst under direct sunlight.

The probable mechanism for the degradation of MB and EY dyes using CuO nanosheets (S2) as photocatalyst can be visualized as follows:

CuO + hν → e⁻ + h⁺

H₂O + h⁺ → OH⁻ + H⁺

OH⁻ + h⁺→ ˙OH

e⁻ + O₂ → ˙O₂⁻

˙O₂⁻ + H⁺→ ˙OOH

Dye + hν → dye*

Dye* + CuO → dye + CuO (e⁻)

CuO (e⁻) + O₂ → CuO + O₂⁻

CuO (e⁻) + ˙O₂⁻ + H⁺ → CuO + H₂O₂

CuO (e⁻) + H₂O₂ → CuO + ˙OH + OH⁻

h⁺ + dye → degradation products

Dye* + O₂ or ˙OH or ˙O₂⁻ → degradation products

3.7. Identification of degradation products of methylene blue dye

The degradation products generated during the degradation of MB dye were analyzed using LC-MS and from the mass spectra the fragmented ions were identified. Fig. 10(a) represents the mass spectrum of the degradation products obtained during the degradation of MB dye. The fragmented products identified from the mass spectrum are represented in Scheme 3 and the pathway for the formation of the degradation products is also depicted in Scheme 3.

Fig. 10 MS fragmentation pattern of (a) MB and (b) EY dye degraded using CuO nanosheets (S2) as photocatalyst under solar irradiation.

Scheme 3 Photodegradation pathway of MB dye and identification of degradation products.

During the photodegradation process, the photo-generated holes (h⁺) and the OH˙ radicals act as oxidizing agent. The OH˙ radical generated during the photodegradation process attacks the C–S⁺=C functional group of MB to form C–S(=O)–C, thereby making the oxidation state of the sulfur pass from −2 to 0, which was identified from the MS fragmentation pattern obtained at m/z = 301. The sulfoxide group is again attacked by the OH˙ radical to produce sulfone (not detected in MS), wherein the oxidation state of sulfur increases from 0 to +5, and this also leads to the dissociation of the ring. The sulfone is then attacked by a third OH˙ radical to produce sulfonic acid (oxidation state = +6) which was identified from the MS pattern, showing an m/z value at 201. The attack of the OH˙ radical again leads to the release of SO₄²⁻ ions, which reacts again with the OH˙ radical to form phenol (detected in MS fragmentation pattern).

The methyl group present in the ring is also attacked by the OH˙ radical to form the corresponding aldehyde, which on further oxidation gives rise to the corresponding acid, which further undergoes decarboxylation (detected in MS fragmentation pattern).

The dimethyl phenyl amino group (detected in MS; m/z = 136) is also attacked by the OH˙ radical, producing an aldehyde, which was detected in the MS fragmentation pattern. The aldehyde is further oxidized into the carboxylic acid, which decarboxylates giving CO₂. The amino group present in the aromatic ring can be substituted by the OH˙ radical to form a phenolic compound (detected in MS fragmentation pattern). The amino group released can form ammonium ions which can be further oxidized to nitrate.

3.8. Identification of degradation products of eosin Y dye

A schematic representation of the degradation pathway of EY dye is shown in Scheme 4. The degradation products were analyzed using LC-MS and the fragmented products were identified from the mass fragmentation pattern. Fig. 10(b) represents the MS pattern of the degradation product obtained during the degradation of EY dye. The mechanistic pathway of the degradation process is depicted according to the m/z value obtained from the MS pattern.

Scheme 4 Photodegradation pathway of EY dye and identification of the degradation products.

3.9. Recycling of 2D CuO nanosheets (photocatalyst)

2D CuO nanosheets (S2) acting as a photocatalyst were reused effectively, which made the process much more cost effective. The recycling studies were carried out for CuO nanosheets, acting as photocatalyst in the degradation of MB and EY dyes, and the results are shown in Fig. 11(a and b). The activity of the photocatalyst for the degradation of MB and EY was observed during five cycles after each of which the photocatalyst was recovered by simple filtration and washed with hot water and ethanol to remove any adsorbed products. The photocatalyst retained an efficiency of 93–94% after the five cycles. The recyclability of the CuO photocatalyst confirmed that the efficiency decreased to a small extent (∼6–7%) over the five cycles of use for the degradation of MB and EY dyes (Fig. 11(a and b)). Moreover, at the end of the fifth cycle of operation, the XRD pattern and TEM image of exhausted CuO nanosheets acting as photocatalyst were also recorded [Fig. S6 and S7†]. The TEM image and XRD pattern showed that the morphology, size distribution and crystal structure of the CuO nanosheets remain unchanged after five cycles of operation as photocatalyst in the degradation of MB and EY dyes. This also showed the stability of the synthesized CuO nanosheets as photocatalyst for the degradation of MB and EY dyes.

Fig. 11 (a) Recycling of 2D CuO nanosheets (S2), which act as photocatalyst for the degradation of MB dye. (b) Recycling of 2D CuO nanosheets (S2) acting as photocatalyst for the degradation of EY dye.

4. Conclusion

This communication describes briefly the facile, green synthesis of 1D CuO nanorods and 2D CuO nanosheets using L-lysine and L-glycine. The amino acids play an important role in the synthesis of CuO nanoparticles and act as a good complexing and capping agent. TEM images reveal that, using L-lysine, 1D CuO nanorods possessing an average diameter of 30 nm were formed. L-Glycine mediated synthesis leads to the formation of 2D CuO nanosheets having dimensions of 300–600 nm length, 200 nm width and 30–60 nm thickness. Hence, the morphology of CuO nanoparticles can be varied by changing the amino acid. This may be attributed to the nature of the R-groups associated with the amino acids. The XRD and SAED patterns depict the monoclinic crystal structure of CuO nanoparticles. The absorption spectra of CuO nanorods and nanosheets show a broad band around 380 nm and 383 nm, respectively, due to surface plasmon absorption of the metal oxide (CuO). The catalytic activity of 1D CuO nanorods and 2D CuO nanosheets was also studied for reduction of p-nitrophenol in aqueous medium. A comparative study was carried out and it was evident that 1D CuO nanorods are a more efficient catalyst than 2D CuO nanosheets in the reduction of PNP. However, 2D CuO nanosheets act as an efficient and recyclable photocatalyst in the degradation of two different toxic dyes, namely methylene blue and eosin Y.

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Footnote

† Electronic supplementary information (ESI) available: TEM images, and XRD and SAED patterns of CuO nanorods and CuO nanosheets to prove the reproducibility of results are provided in the ESI. TEM image of CuO nanoparticles synthesized in the absence of amino acid, absorption spectrum of PNP, absorption spectrum of PNP after addition of NaBH₄, XRD pattern and TEM image of exhausted CuO nanosheets after the fifth cycle of operation as photocatalyst are available in the ESI. See DOI: 10.1039/c6ra03624d

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