Sulfuric disazo dye stabilized copper nanoparticle composite mixture: synthesis and characterization

Abstract

A copper nanoparticle–sulfuric disazo dye (Cu–SD1) composite was synthesized using the sol–gel method. Cu–SD1 nanocomposite formation was monitored by ultraviolet-visible spectroscopy (UV-vis). The acquired experimental results suggested that 8 h of reaction is needed for the synthesis Cu⁰ nanoparticles. Transmission electron microcopy (TEM) and atomic force microscopy (AFM) were employed to elucidate the morphology of the Cu–SD1 nanocomposite. It was found that the diameter of particle sizes were in the range of 2–4 nm. The interaction of SD1 with copper was confirmed by Fourier transform infrared spectroscopy (FTIR). The peak shift of O–H and C–OH functional groups indicated the interaction between SD1 and copper nanoparticles. Moreover, the azo group (N=N) peaks were suppressed after the formation of the nanocomposite, suggesting that a strong linkage was formed between the functional groups and the copper nanoparticles. The surface composition and chemical states of the as-synthesized copper nanoparticles were elucidated by X-ray photoelectron spectroscopy (XPS). In addition, photo-switching of the composites was elucidated in the solution state. It was found that the Cu–SD1 nanocomposite has a faster switching response compared to the parent, SD1, in a solution.

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Introduction

The synthesis of metal nanoparticles has been studied extensively in the past few decades because of their unique properties and potential applications in physical, chemical, electronic, sensing, energy storage and resonance imaging fields.^1–4 Among them, copper nanoparticles have attracted enormous attention due to their catalytic, optical, electrical and antifungal applications and low costs.^5–8 Different wet chemical methods have been developed for the synthesis of metal nanoparticles such as chemical reduction, sol–gel, photochemical, hydrothermal, solvothermal and electrochemical.^9–12 Sol–gel method is one of the most useful methods for the synthesis of copper nanoparticles,¹³ due to its simplicity and ease in controlling particle sizes, its low synthesis temperature and low cost.^6,14,15 In most of the cases, a stabilizer was used to prevent particle growth and agglomeration such as polyvinylpyrrolidone (PVP) or polyvinyl alcohol (PVA). Huang and Kim¹⁶ reported shape control synthesis of gold nanoparticles using a photo-responsive material as a stabilizer via a modified seed growth method. To the best of our knowledge, the use of azobenzene as a stabilizer in the synthesis of copper nanoparticles and its fast photo-switching behavior has never been reported. There has been some previous work done with nanoparticles and their effect on the photo-conversion of photochromes.^17,18 Nishi et al.¹⁷ synthesized variously sized gold nanoparticles covered with photochromic polymers consisting of diarylethenes with various structures. It was found that the gold nanoparticles covered with photochromic polymers enhanced the rate of the photo-cycloreversion reaction of the chromophores through reversible changes in localized surface plasmon resonance (LSPR) absorption around the particle. It was shown by Díaz et al.¹⁸ that the incorporation of multiple photochromic diheteroarylethene groups around the semiconductor core can be used to devise photo-switchable quantum dots (psQDs). Azobenzene, a photochromic dye, can convert from the trans to the cis form by short wavelength light (e.g., 380 nm), and from the cis to the trans state by longer wavelength light (e.g., 450 nm) or by thermal relaxation. Azobenzene is very important in photonic fields as it has a fast switching property.^19–21 Copper nanoparticles modified with azobenzene might enhance the switching behavior of the composite, which could be used in fast switching devices.

In this context, the aim of this work is to study the formation of copper nanoparticles incorporating azo composites via an in situ sol–gel method at ambient conditions. The formation of copper nanoparticles with azo dye was studied using UV-vis spectroscopy. The composite was characterized using, TEM, AFM, FTIR and XPS. The effects of nanoparticles on the composite and on their optical properties were elucidated.

Experimental section

Materials

Copper(II) chloride salt (CuCl₂·2H₂O), hydrazine monohydrate (N₂H₄·H₂O), potassium bromide (KBr) and ethanol (99.9%) were obtained from Sigma-Alridch (Malaysia) Sdn. Bhd. and used without further purification. Azo dye material, sulfuric disazo dye (SD1), was obtained from Di-Nippon Ink Japan. The molecular structure of SD1 is demonstrated in Scheme 1.

Scheme 1 Molecular structure of SD1.

Preparation of the Cu–SD1 nanocomposite

Cu–SD1 nanocomposite synthesis was performed in a two-necked round bottom flask (100 mL capacity) with magnetic stirring at room temperature. The required amount of CuCl₂·2H₂O and SD1 were dissolved in 45 mL deionized (DI) water. Subsequently, 5 mL of hydrazine monohydrate was injected dropwise into the mixture, and the concentration of CuCl₂·2H₂O and SD1 were controlled at 200 mg L⁻¹ and 2000 mg L⁻¹, respectively. The sol was continuously stirred for 12 h and samples were withdrawn at different time intervals. The samples were transferred into a 1 cm optical length quartz cuvette and analysed by UV-vis (Hitachi U 1800 UV-visible spectrophotometer). The spectrum was recorded in the range of 400–800 nm.

Characterization of the Cu–SD1 nanocomposite

FTIR analysis. To obtain a dried nanocomposite, 50 mL sol was centrifuged at 10 000 rpm for 30 min and the precipitated solid was collected and dried in a vacuum dryer at 60 °C for 4 h. The KBr pellet method was used to prepare SD1 and Cu–SD1 nanocomposite samples prior to spectrum recording. FTIR (Model: Spectrum 100, Brand: PerkinElmer) was used to analyze SD1 and the Cu–SD1 nanocomposite. The spectrum was recorded in the range of 400–4000 cm⁻¹.

XPS analysis. The dried nanocomposite powder was compressed in a cylindrical mold with a pressure of 1 MPa to obtain a disc specimen. XPS data were acquired using a PerkinElmer PHI 5400 X-ray photoelectron spectrometer. This system was equipped with a Mg X-ray source operated at 300 W (15 kV, 20 mA). The carbon (C 1s) line at 285.0 eV was used as the reference line.

TEM and AFM analysis. Particle sizes were determined by TEM using a LEO 912 AB EFTEM operating at 100 kV. The sample for TEM analysis was obtained by dispersing dried nanocomposite into 99.9% ethanol and then placing a drop of the ethanol solution on a copper grid and evaporating it in air at room temperature. Furthermore, AFM analysis of the nanocomposite was conducted using a NT-MDT AFM in close contact mode.

Photo-switching analysis. The dried nanocomposite powder was dissolved in DI water and the liquid was transferred into a 1 cm optical length quartz cuvette for UV-vis spectroscopy. The UV-vis absorption spectrum and photo-switching times were measured using an Ocean Optics (HR2000⁺) spectrophotometer equipped with a 365 nm filter.

Results and discussion

Cu–SD1 nanocomposite formation

The formation of SD1 stabilized Cu nanoparticles was monitored using UV-vis spectroscopy. Fig. 1a presents the UV-vis absorption spectra of an aqueous solution of SD1 and CuCl₂–SD1. SD1 showed the maximum absorbance at 475 nm that was shifted to 487 nm after CuCl₂ was dissolved into the SD1 solution. The shift indicates the formation of a Cu(II)–SD1 complex.

Fig. 1 (a) UV-vis absorbance spectra of SD1 (i) and CuCl₂ in SD1 (ii). (b) UV-vis absorbance spectra of the Cu–SD1 nanocomposite in a solution at various reaction times. (c) Normalized absorbance of peak at ■ 520 nm.

Fig. 1b demonstrates the UV-visible spectrum of the sol at different times during its formation. The 0 h time point refers to the addition of hydrazine in the precursor solution and a pale yellowish solution was observed. It can be seen that the intensity of the Cu(II)–SD1 complex peak (487 nm) reduces as the reaction progressed and it diminishes after 2 h of reaction. A new peak at 520 nm appears after 3 h of reaction time and the color of the sol turned red, corresponding to the formation of copper nanoparticles.^6,8,22,23 The characteristic Cu⁰ plasmon peak intensity increased as the reaction time was extended. The absorbance at 520 nm for all reaction times was normalized with respect to the absorbance at 8 h and plotted against time, as shown in Fig. 1c. A sharp increase in the rate of copper nanoparticle formation was observed after 3 h of reaction and reached a plateau after 7 h of reaction. The concentration of copper nanoparticles slightly dropped after 10 h of reaction and the sol color turned a deep red. This might be due to the formation of CuO. The chemical transformation of Cu²⁺ to Cu⁰ appears to be completed after 8 h of reaction time.

FTIR analysis

Fig. 2a illustrates the FTIR spectra of parent SD1 and the Cu–SD1 nanocomposite, recorded as the transmittance (%) versus frequency in the range of 4000–400 cm⁻¹. The spectra in Fig. 2a depict a number of absorption peaks representing the multiple functional groups in the samples. The FTIR spectrum displays broad bands at 3700–3100 cm⁻¹ that correspond to hydrogen bonded O–H groups.^24–32 The absorption band at 1574 cm⁻¹ and 1077 cm⁻¹ corresponds to the phenyl ring^33–35 and S=O²⁷ groups, respectively. Moreover, the azo group (N=N) in SD1 was detected in the ∼1460 cm⁻¹ region^27,29,36 and C–OH group appears in the range of 1300–1000 cm⁻¹.²⁷ The absorption band of the O–H groups of SD1 appears at 3433 cm⁻¹ and 3441 cm⁻¹ before and after nanocomposite formation, respectively, showing 8 red shift units for this polar group. This shift in frequency indicates that SD1 molecules assist in coordination with copper.⁷ In addition, the C–OH of SD1 at 1186 cm⁻¹ was shifted to 1166 cm⁻¹ after the nanocomposite formed. This amount of shift indicates the coordination of copper with SD1. Such coordination might be due to the electrostatic attraction between SD1 and copper.³⁷ In addition, the intensity of the azo group (N=N) in SD1 drastically decreased after formation of the nanocomposite that suggested an interaction of the azo group with the copper nanoparticles.

Fig. 2 (a) FTIR spectra of parent SD1 (i) and the Cu–SD1 nanocomposite (ii). (b) XPS spectra of Cu–SD1 nanocomposite in core level regions: (i) deconvolution of Cu 2p, (ii) Gaussian/Lorentzian fitted profile of Cu 2p_3/2.

XPS analysis

A typical XPS spectrum of the Cu–SD1 nanocomposite was measured and the core level spectrum of Cu 2p is depicted in Fig. 2b(i). XPS is an effective approach for the investigation of the surface composition and chemical states of solid samples,³⁸ including the oxidation states of metals.^8,23,39,40 The core level spectrum and shake-up satellite line of copper (2p_3/2 and 2p_1/2) are illustrated in Fig. 2b(i) where the Cu 2p_3/2 and 2p_1/2 are assigned at ca. 932.0 and 951.9 eV, respectively. These findings were also consistent with the findings of other researchers.^41,42 In order to acquire a more precise knowledge of these environments, the spectrum of Cu 2p_3/2 was fitted using a Gaussian/Lorentzian mixed function and presented in Fig. 2b(ii). The Cu 2p_3/2 peak was fitted to a single peak at 932 eV that related to Cu⁰.⁴³ At the satellite regions, 2 CuO (BE 942.35 and 940.85 eV)⁴⁴ peaks contributed. The binding energy, FWHM and area of Cu 2p_3/2 are summarized in Table 1.

Table 1 The results of deconvolution of XPS Cu 2p_3/2 peaks

Binding energy (eV)	Cu phase	FWHM	Area	Percentage (%)
932.0	Cu	3.40	3083.346	65
940.85	CuO	3.90	1054.291	22
942.35	CuO	2.36	579.304	13

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Morphology analysis

The morphology of the Cu–SD1 nanocomposite was investigated using TEM and AFM techniques. Fig. 3a and b demonstrated the surface roughness and thickness of the Cu–SD1 nanocomposite in 2D and 3D topographs. It can be seen that the Cu–SD1 nanocomposite was homogeneously distributed and the thickness of the sample was 16 nm, indicating the formation of a multi-layer composite. In order to further determine the size of particles, TEM technique was employed. The TEM image of the Cu–SD1 nanocomposite is shown in Fig. 4. In Fig. 4a, the nanocomposite was observed where nanoparticles were agglomerates on SD1 molecules. At high magnification (Fig. 4b), fine and spherical shaped copper nanoparticles were clearly seen and the diameter of particle was in the range of 2–4 nm.

Fig. 3 AFM 2D phase diagram (a) and 3D surface topography (b) of Cu–SD1 nanocomposite.

Fig. 4 TEM micrograph of Cu–SD1 nanocomposite at low magnification (a) and high magnification (b).

Effect of photo-switching

The effect of photo-switching was compared between the composite mixture and the parent SD1 in solution. To our surprise, the inclusion of copper nanoparticles makes the switching faster as compared with the SD1 in solution as shown in Fig. 5a. Initially, the solutions were kept in the dark. When UV light of wavelength 365 nm was passed through the solution, immediately, the molecules in the light sensitive material re-oriented in solution, which can be observed through the sharp change in absorbance leading to a saturation value. When light is turned off, the molecules returned to their original positions faster than expected (see ESI Fig. S1†). The photoconversion lifetime for SD1 and nanoparticle modified SD1 was 31 s and 3 s, respectively. The Cu–SD1 nanocomposite has a 10 times faster response as compared to the parent SD1. This might be due to the hydrogen bonding between the copper nanoparticle and –COOH and –OH (electron donating or withdrawing groups) in the Cu–SD1 nanocomposite that enhanced electron transport, leading faster re-conversion of the molecular structure.⁴⁵

Fig. 5 (a) UV on effect of Cu–SD1 nanocomposites and the parent SD1. (b) Stability of the Cu–SD1 nanocomposite with several on and off cycles. (c) UV on time at different UV intensities. (d) Effect of UV intensity on UV on time as extracted from (c).

To check the stability of the composite mixture, a few cycles were repeated as shown in Fig. 5b. One can see that on and off times remain the same throughout the cycles, thus emphasizing the stability of the system. Fig. 5c shows the UV on times recorded as a function of different intensities. It is obvious that as the intensity increases, UV on times become faster, which makes the nanocomposites suitable candidates for fast photo-switching devices. The grey area corresponds to UV on. One can also see a similar behavior on UV off data where molecules return back to the original configuration faster than expected (see ESI Fig. S2†). Fig. 5d is extracted from Fig. 5c where one can see similar values for UV on with respect to different UV intensities. One can conclude from this graph that Cu–SD1 nanoparticles show faster photoconversion times as we increase the intensity. After saturation, there is no further effect by intensity changes.

Although one cannot predict the exact behavior for this result, we speculate the following, as shown in Scheme 2. It can be observed that after UV irradiation, the SD1 compound was transformed into the cis-isomer (Scheme 2(i)). The shape of the molecule was slightly re-oriented; therefore, after the UV light turns off, it can return to trans-isomer in a short time. As seen in Scheme 2(ii), a nanoparticle is bound to the SD1 compound through hydrogen bonds, as suggested by FTIR results (Fig. 2a). The addition of the nanoparticle around the SD1 structure influences the shape of the molecule. After UV irradiation, the shape was almost unchanged, in other words, the shape is just slightly re-oriented. This phenomenon explained that the fast switching behavior of modified SD1 is shorter than the parent SD1.

Scheme 2 Re-orientation of trans–cis–trans of Cu–SD1 nanocomposite before and after UV irradiation.

The present study elucidates that the nanoparticle-incorporated light sensitive molecules might be used in fast photo-switching applications in the near future due to their advantages in bringing the molecules back to the original configuration (trans–cis–trans) quickly. Studies concerning the surface of the solid are underway and will be reported in due course.

Conclusions

A sulfuric disazo dye stabilized copper nanocomposite was prepared via the sol–gel method. The formation of the nanocomposite was monitored by UV-vis spectroscopy and it was found that 8 h of reaction time is needed to synthesize the Cu⁰ nanocomposite. Copper and copper oxide phases were found in the nanocomposite via XPS techniques. Furthermore, XPS revealed the composition inside the composite powder was found to be 65% copper with the rest as copper oxide. FTIR results indicate that copper nanoparticles have chemical interactions with the surface functional groups of SD1 such as the phenyl ring, S=O, –OH, C–OH and N=N groups. The size of the nanoparticles was visualized by TEM and the diameters of the particles were in the range of 2–4 nm. The on–off effect of the Cu–SD1 nanocomposite was elucidated and the Cu–SD1 nanocomposite has a faster switching performance compared to the parent SD1. A mechanistic model for the trans–cis–trans molecule re-orientation of the Cu–SD1 nanocomposite was proposed.

Acknowledgements

This work was financially supported by research grant from Malaysia government (Project No: FRGS-RDU 150118; Mosti e-Science-RDU 130503; RAGS-RDU 131403) and Universiti Malaysia Pahang (Project No: GRS 130350).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra26492h

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