Aqueous synthesis and characterization of CdS quantum dots capped with some amino acids and investigations of their photocatalytic activities

Colloids and Surfaces A: Physicochem. Eng. Aspects 447 (2014) 111–119

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Aqueous synthesis and characterization of CdS quantum dots capped with some amino acids and investigations of their photocatalytic activities Abdolraouf Samadi-maybodi ∗ , Fatemeh Abbasi, Reza Akhoondi Analytical Division, Faculty of Chemistry, University of Mazandaran, Babolsar, Iran

h i g h l i g h t s

g r a p h i c a l

• CdS QDs were synthesized with stabi-

SEM images of CdS QDs: (a) Ser-CdS, (b) His-CdS, (c) Ala-CdS. It was found that the average diameter of the Ser-CdS is 8.35 nm, smaller than Ala-CdS (11.5 nm) and His-CdS (9.54 nm).

lizers such as L-serine and L-histidine. • The synthesized CdS QDs exhibited significant optical properties. • The CdS QDs were utilized as photocatalyst in photodegradation under sunlight. • The photodegradation efficiency for Ser-CdS QDs was 98%.

a r t i c l e

i n f o

Article history: Received 13 September 2013 Received in revised form 14 January 2014 Accepted 17 January 2014 Available online 25 January 2014 Keywords: Quantum dots Photocatalysis Cadmium sulfide Capping agent

a b s t r a c t

a b s t r a c t The current study examines the influence of some amino acids such as alanine, histidine, and serine as stabilizers in the synthesis of CdS quantum dots. The CdS quantum dots (QDs) exhibited strong absorption and photoluminescence properties upon UV–vis wavelength in the region from 200 nm to 800 nm. Structural and spectroscopic properties of the synthesized CdS QDs were characterized by absorption and fluorescence spectroscopy, X-ray diffraction, field emission scanning electron microscopy, and Fourier transform infrared (FT-IR) spectroscopy. Influence of some parameters on fluorescence intensity of CdS QDs including the precursor ratio, type of modifier, temperature, and initial pH of the reaction was thoroughly studied. Results indicated that the pH and the type of modifier that used played crucial roles in determining luminescence properties of the synthesized CdS nanoparticles. Generally, the luminescence intensity can be enhanced significantly when the CdS QDs are illuminated by room light or sunlight. Also, results specified that the synthesized CdS QDs were capable of effectively degrading organic dyes such as alizarin under visible light irradiation and exhibiting good recycling stability during photocatalytic experiments. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Semiconductor nanocrystals (NCs) as quantum dots (QDs) are the important materials for both fundamental research and technical applications as a consequence of the large ratio of surface atoms and quantum confinement of excitons [1,2]. Among various

∗ Corresponding author. Tel.: +98 1125342350. E-mail address: [email protected] (A. Samadi-maybodi). 0927-7757/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2014.01.036

semiconductor materials, CdS NCs have attracted much attention due to their size-dependent photoluminescence (PL) tunable across the visible spectrum, and also due to the advances in their preparation method [3–10]. In fact, the main interest in studying CdS QDs is related to their preparation and PL properties which make them suitable for the applications such as biomedical labeling [10], solar energy conversion [11], optoelectronic [12], and ion probes [13]. During the past two decades, many studies in this area have been focused on the development of new methods to synthesize high-quality QDs with a high luminescence quantum yield,

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Fig. 1. Fluorescence emission spectra of Ser-CdS synthesized at different pH values. The size and shape of nano particles are decreased at higher pH values.

excellent photostability, and good biocompatibility [14–16]. One of the main concerns in the synthesis of nanoparticles involves controlling the size and stability of these materials [17–19]. Sizeselective precipitations [20–22], cages of molecular sieves [23], different starting pH [24], and exclusion chromatography [25] have been used to obtain nanoparticles within narrow size ranges. However, owning to the ultra-small size and high surface energy, QDs tend to agglomerate seriously, leading to the significant difficulties in the preparation of the stable QDs. CdS QDs have been generally prepared using surfactants and polymers as stabilizers [26,27]. Besides, due to toxicity issues and complicated procedures, their applications are considerably limited. Therefore, it is of interest to construct a facile method to prepare stable and well-dispersed QDs. Because of the coordinating interactions between amino acids and metal ions [28], amino acids might be used as stabilizers to prepare stable QDs [29,30]. However, hitherto little on this has been reported. Currently, QDs are widely used in photocatalytic degradation of biological non-degradable organic pollutants [31,32]. Most of the common photocatalysts, such as TiO2 , can only function under UV light irradiation, which seriously limits their application. Thus, it is still a great challenge to find an effective photocatalyst to treat organic pollutants under visible light irradiation. To the best of our knowledge, there have not been any reports hitherto on the synthesis of stable CdS QDs using hydrophobic amino acids as stabilizers. Therefore, two hydrophobic amino acids, that is, D-alanine (Ala), L-histidine (His) and one hydrophilic amino acid, that is, L-serine (Ser) were used as stabilizers. The stable CdS QDs were successfully prepared through a facile one-pot method which exhibited special optical properties upon UV-vis wavelength region. Moreover, the CdS QDs showed strong photocatalytic degradation activities for organic dyes such as alizarin under visible light irradiation. This suggested that the as-synthesized CdS QDs might be used as photocatalyst to treat the organic pollutants under visible light irradiation.

2. Materials and methods

2.2. Methods 2.2.1. Synthesis of CdS QDs To synthesis CdS QDs, 0.12 mmol amino acids and 0.06 mmol CdCl2 were dissolved into 20 mL of double distilled water and incubated for 12 h at 30 ◦ C. Then, 20 mL TAA aqueous solution (6 mM) was added resulting in a yellow colloidal solution. To precipitate the particles, NaCl solution was slowly added to the colloidal solution. Ultimately, these contents were decanted, washed, and dried at 25 ◦ C in vacuum oven. The bulk-CdS was also prepared in the absence of amino acids for comparison. 2.2.2. Photocatalytic dye degradation To examine the photocatalytic activity of CdS QDs, 30 mg CdS QDs were suspended in 24 mL double distilled water by sonicating and then 6 mL of alizarin aqueous solution (250 ppm) was added and stirred in the dark for 60 min to attain an adsorption/desorption equilibrium. Afterward, the suspension was irradiated by sunlight for the photocatalytic experiment under moderate stirring. At specified time intervals, the suspension was moved and centrifuged. The concentration of alizarin in the supernatant was determined by UV–vis absorption spectroscopy. As the comparison, the photocatalytic activity of bulk-CdS was also determined under same conditions. 2.2.3. Characterization The size and morphology of the samples were determined through field emission scanning electron microscopy (FE-SEM), Hitachi (s-4160). The UV–vis absorption and PL spectra were recorded at room temperature on CECIL (CE5501) UV–vis spectrophotometer and Perkin-Elmer LS-3B Fluorescence spectrophotometer, respectively. The Fourier transform infrared (FT-IR) spectra were recorded on TENSOR 27 infrared spectrometer. The X-ray diffraction (XRD) patterns were recorded on an Advance Bruker D8 X-ray diffractometer (Germany) with Cu K˛ radiation (=1.5418Å).

2.1. Materials

3. Results and discussion

All chemical compounds were pure and used without further purification. Herein, CdCl2 (98%, Merck), the amino acids such as serine/alanine (99%, Fluka) and histidine (99.9%, Merck), as well as thioacetamide (TAA; 97%, Fluka) as source of sulfide were used. Double distilled water was used throughout the experiments.

3.1. Effect of synthetic conditions 3.1.1. Effect of initial pH The pH value of the solution effectively influences on the formation of polynuclear amino acid–cadmium complexes in water [28]. The initial pH levels of 2.5, 7.5, 8.5, 9.5, 10.5, and 11.5 of

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Fig. 2. Fluorescence emission spectra of His-CdS synthesized at different pH values. The fluorescence emission spectra indicated that increasing pH value to basic condition caused to enhancement in intensity of the corresponding spectra.

amino acid–cadmium solutions with 2:1 molar ratio of thioacetamid/cadmium chloride were examined at 30◦ C. 3.1.1.1. CdS QDs capped with L-serine. PL emission spectra of assynthesized CdS QDs capped with L-serine at different initial pH values are exhibited in Fig. 1. Results indicated that the full width at half maximum of the emission is decreased at higher pH values (i.e., 10–10.5), suggesting that the size and shape of nano particles are decreased. Nonetheless, no considerable PL emission was obtained under acidic condition (i.e., pH = 2.5). 3.1.1.2. CdS QDs capped with L-histidine. The CdS QDs capped with L-histidine was synthesized at various pH values and corresponding spectra are exhibited in Fig. 2. The clear solution was obtained at pH of 2.5; as can be seen in Fig. 2, the band spectrum of this solution is asymmetric and also has very low intensity. Results obtained from recording fluorescence emission spectra indicated that increasing pH value in basic condition gave rise to an enhancement in the intensity of the corresponding spectra. The highest binding affinity of amino acids toward Cd2+ occurs in basic solution due to the fact that amino acids is deprotonated and their carboxyl groups become negatively charged thus leading to the reduction in trap states and a higher quantum yield. Results specified that the spectrum corresponding to the green product obtained at pH = 9.5 exhibited the highest PL emission intensity at maximum wavelength of 522 nm. 3.1.1.3. CdS QDs capped with D-alanine. Colloidal synthesis of CdS QDs involves the reaction of TAA with cadmium salt and alanine; two possible binding sites are possible as shown in Scheme 1(ii).

Alanine can act as stabilizer of QDs through one functional group while reserving the other for conjugation with water. The binding of the alanine to nanoparticles is largely depended on the pH of the reaction due to the fact that it can directly affect the electronic characteristics of the molecules (Scheme 1(i)) [33]. Fig. 3 illustrates the corresponding PL emission spectra of assynthesized Ala-CdS. Results indicated that the narrowest band with highest intensity is located at maximum wavelength of 524 nm (ex =343 nm) associating with the synthesized Ala-CdS at pH = 9.5. 3.1.2. Effect of molar ratio of thioacetamide/cadmium chloride The effect of molar ratio of TAA/cadmium chloride on PL peak was studied at various molar ratios of TAA: cadmium chloride such as 1:1, 2:1, and 1:3 (Table 1). Generally, these experiments were performed for each of the as-synthesized CdS QDs under optimal condition. 3.1.3. Effect of temperature The influence of temperature was also studied over amino acid–cadmium complex formation. Herein, the initial temperature was adjusted at three temperatures, that is, 30 ◦ C, 80 ◦ C, and 100 ◦ C. The corresponding PL emission spectra were recorded at the same spectral conditions as shown in Fig. 4. For all complexes, red shifts in PL spectra and decreasing in PL emission intensity as well as reducing in half-widths of the emissions were observed as temperature increased (Fig. 4(a)–(c)). For instance, in case of His-CdS, increasing temperature from 30 ◦ C to 100 ◦ C leads to a shift in wavelength from 518 nm to 532 nm

Scheme 1. Representation of the (i) effect of pH on alanine and (ii) the binding of the alanine to the surface of CdS nanoparticles.

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Fig. 3. Fluorescence emission spectra of Ala-CdS synthesized at different pH values. PL emission spectra indicated that the narrowest band with highest intensity is located at maximum wavelength of 524 nm (ex = 343 nm).

(with ex = 343 nm). Results also specified that by elevating temperature, the corresponding PL spectra of the synthesized particles become narrower (Fig. 4(a)–(c)). It can be deduced that the particles were polydispersed at 30◦ C and 80◦ C while a narrower emission peak at 100 ◦ C implied the monochromatic characteristic of the particles.

3.1.4. Effect of ligand type Usually, the complex stability of Cd2+ ions with various ligands is different, which affect the final properties of CdS QDs. The emission spectrum heavily depends on the particle surface state, size, and surface passivation. The effect of ligand type on the complex equilibrium was studied. The PL spectra of as-prepared CdS QDs

Fig. 4. Fluorescence emission spectra of CdS QDs: a. His-CdS, b. Ser-CdS, and c. Ala-CdS. synthesized at the different initial temperature: 30 ◦ C (A), 80 ◦ C (B), and100 ◦ C (C).

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Fig. 5. (a) Fluorescent emission spectra of the as-synthesized CdS QDs using different amino acids: a. His-CdS, b. Ser-CdS c. Ala-Cds. Experimental conditions: ([CdCl2 ]:[thioacetamide]:[modifier] = 1:2:2, [CdCl2] = 3 mM, pH = 9.5, time of reaction = 12 h and temperature = 30 ◦ C). (b) UV–vis absorption spectra of CdS with different modifiers: (a) Ala-CdS, (b) His-CdS, and (c) Ser-CdS. Regarding Eq. (1) and absorption spectra, it can be estimated that the mean size of Ser-CdS, His-CdS, and Ala-CdS QDs is 3.29 nm, 4.50 nm, and 5.29 nm, respectively.

under same precursor concentration but different type of amino acids as stabilizers, L-His, L-Ser, and D-Ala, are shown in Fig. 5(a). D-alanine exhibits the strongest PL emission intensity compared with L-Ser and L-His. The maximum PL emission wavelength of prepared CdS QDs with L-His, L-Ser, and D-Ala is 522 nm, 518 nm, and 524 nm (ex = 343 nm), respectively. Results indicated that the size of CdS QDs capped with different ligands was different because of the difference in cadmium–ligand complex and growth rate. The ligands can bind to NPs with their carboxylic group through covalent bond with Cd at their surface, leading to the formation of stabilized CdS nanoparticles. The bonding strength of the coordination between capping agents and

Table 1 Effect of molar ratio of CdCl2 :TAA on the maximum wavelength of PL emission spectra (em ) and maximum intensity of PL emission (with ex = 343 nm)a . Type of ligand

Molar ratio of CdCl2 :TAA

em (nm)

L-serine

3:1

Asymmetric PL peak 423 518 532 528 522 564 543 524

L-histidine

D-alanine

a

1:1 1:2 3:1 1:1 1:2 3:1 1:1 1:2

Maximum intensity of PL emission (a.u)

615 700 53 87 395 195 426 831

Optimized value of molar ratio (CdCl2 :TAA)

surface atoms of the NPs increased with decreasing in particle size. Among them, the PL intensity of CdS QDs capped with DAla was higher than that of L-Ser and L-His, due to D-alanine, with smaller stereo hindrance, was favorable to the formation of multi-coordinated complexes and decreases the surface defects of particles. On the other hand, the PL emission wavelength for L-Ser was shorter than that of D-Ala and L-His, because L-serine has two different OH groups which makes it capable of forming the more covalent band than the other ones; therefore, L-serine can successfully control growth rate of particles. On the contrary, the ability of L-serine coordination with surface atoms was decreased due to its excess or just more stereo hindrance than D-alanine. Also, as shown in Fig. 5(b), the CdS QDs exhibits broad absorption band from 200 nm to 800 nm, indicating the effective photoabsorption properties. Moreover, according to the relationship between the absorption edge (em ) and the sizes of QDs (D) (Eq. (1)), the mean size of the particles can be predicted using the following equation [34]:

1:2

2RCdS = 0.1/ (0.1338 − 0.0002345 e )

1:2

Regarding Eq. (1) and absorption spectra (Fig. 5(b)), it can be estimated that the mean size of Ser-CdS, His-CdS, and Ala-CdS QDs are 3.29 nm, 4.50 nm, and 5.29 nm, respectively.

1:2

3.2. FT-IR characterization

The PL properties of synthesized CdS QDs are illustrated for comparison.

(1)

Fig. 6 presents the FT-IR spectra of the bulk-CdS and CdS QDs in aqueous solution. Compared with the spectrum of the bulk-CdS,

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Fig. 6. FT-IR spectra of the (a) bulk-CdS, (b) Ala-CdS, (c) His-CdS, and (d) Ser-CdS. Based on these results, the stretching band of the carbonyl group of amino acids located at 1505–1580 cm−1 in basic solution can be found in the spectra of the QDs; this shift of CO band wave number from 1700 Cm−1 to 1505 Cm−1 was caused under resonance of carboxylate group in basic solution. There were certain interactions between the QDs and the carbonyl groups of the amino acids.

Fig. 7. SEM images of CdS QDs: (a) Ser-CdS, (b) His-CdS, (c) Ala-CdS.

the stretching band of the carbonyl group of amino acids located at 1505–1580 cm−1 in basic solution can be found in the spectra of the QDs; this shift of CO band wave number from 1700 Cm−1 to 1505 Cm−1 was caused under resonance of carboxylate group in basic solution. However, the intensities decreased obviously. Based on these results, there was a trace amount of amino acids in the CdS QDs. Furthermore, there were certain interactions between the QDs and the carbonyl groups of the amino acids. It might be these interactions that may contribute to the stability and dispersity of the CdS QDs. The bands located at 3000–3500 cm−1 assigned to O–H and N–H stretching frequencies of amino acids and water. The broad band in 1600 cm−1 is associated with N = H stretching imidazole ring of the His-CdS.

The FE-SEM images illustrated in Fig. 7(a)–(c) reveal the general morphology of prepared Ser-CdS, His-CdS, and Ala-CdS samples using NaCl as a precipitating agent. The FE-SEM images of SerCdS and Ala-CdS indicate aggregation of the particles (Fig. 7(a) and (c)). The FE-SEM of His-CdS (Fig. 7(b)) showed similar spherical morphology. 3.3. XRD characterization Powder XRD patterns of His-CdS and Ala-CdS nanoparticle samples are shown in Fig. 8(a) and (b), respectively. The X-ray diffractograms of His-CdS and Ala-CdS indicated a sharp reflection line in the 2 ranges of 26.6◦ , 44.1◦ , and 51.8◦ corresponding to the (111), (220), and (311) planes of the cubic CdS phase, respectively.

Fig. 8. XRD patterns: (a) His-CdS and (b)Ala-CdS. The X-ray diffractograms of His-CdS and Ala-CdS indicated a sharp reflection line in the 2 ranges of 26.6◦ , 44.1◦ , and 51.8◦ corresponding to the (111), (220), and (311) planes of the cubic CdS phase, respectively.

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Fig. 9. Photocatalytic degradation of alizarin in the presence of the CdS QDs and bulk-CdS under sunlight. The photocatalytic efficiencies of His-CdS, Ala-CdS, and Ser-CdS were calculated to be 96%, 90%, and 98% respectively, which are significantly higher than that of bulk-CdS (12.7%).

Fig. 10. Photocatalytic degradation of alizarin in the presence of the CdS QDs and bulk-CdS under visible light irradiation by 500 W xenon lamp.

3.4. Estimation of band gap by UV–vis absorption studies The band gap energy value of Ser-CdS, Ala-CdS, and His-CdS was estimated using UV–vis absorption spectra according to E = hc/e formula, (where h = Plank constant, c = speed of light, and e is the absorption edge (cutoff wavelength) obtained from absorption spectrum). The band gap for Ser-CdS, His-CdS, and Ala-CdS was evaluated as: 2.81 eV, 2.61 eV, and 2.53 eV1 , respectively, (compared to ∼2.4 eV for bulk-CdS).

3.5. Photocatalysis studies Because of the unique photoabsorption properties of synthesized CdS QDs, their photocatalytic activities were studied for alizarin degradation under visible light irradiation (sunlight). The plot of degradation efficiency versus time is shown in Fig. 9. Herein, after 12 h visible light irradiation under sunlight and at four intervals of 3 h, the absorbency of alizarin was determined at 423 nm. Under above experimental performance, the photocatalytic efficiencies of His-CdS, Ala-CdS, and Ser-CdS were calculated to be 96%, 90%, and 98%, respectively, which are significantly higher than that of bulk-CdS (12.7%). Meanwhile, the best photocatalytic

decomposition was associated with the Ser-CdS samples, that is, which showed ∼74% degradation within 3 h and ∼98% for 12 h. For further study, all experiments mentioned above were performed by 500 W xenon lamp for photocatalysis under moderate stirring, corresponding results are presented in Fig. 10. Results specified that after 110-min visible light irradiation, the photodegradation efficiencies of the Ser-CdS, His-CdS, Ala-CdS, and bulk-CdS on alizarin are 90.4%, 87%, 83.5%, and 11.4%, respectively. Therefore, it can be concluded that the synthesized samples can be used as an efficient photocatalyst for degradation of the organic pollutants such as dyes under visible light irradiation. Previously, it was shown that the active oxygen species such as •OH and superoxide played key roles in the photocatalytic degradation [35]. The energy of the emitted light promotes the electrons from the valence band (vb) to the conduction band (cb), generating the positive hole (vb + ) at the vb edge and the electron (ecb − ) in the cb (Eq. (2)). Secondly, hvb + and ecb − can react with water or hydroxyl groups to obtain or produce • OH (Eqs. (3) and (4)). Finally, degradation is occurred through interaction of • OH with alizarin (Eq. (5)).



− CdS + hv → CdS ecb + v+ b



•

h+vb + H2 O → H+ + OH •

h+vb + OH− → OH 1

•

Electron volt.

OH + dye → degradation of the dye

(2) (3) (4) (5)

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Fig. 11. Results of the recycling stability of the synthesized CdS quantum dots on the photocatalytic degradation of alizarin. (a) Ala-CdS, (b) His-CdS, and (c) Ser-CdS. The photocatalytic activities did not show significant loss, after five cycles of the photocatalytic reaction under sunlight.

As shown in the above mechanism, the photocatalytic activities of these QDs can be assigned to their • OH production efficiency. Considering the details above, the •OH production efficiencies of the Ser-CdS and His-CdS were higher than that of the Ala-CdS. It is pertinent to point out that the synthesized CdS QDs exhibited good recycling stability during the photocatalytic degradation of alizarin under visible light irradiation (sunlight). The results achieved after five cycles of the photocatalytic reaction (Fig. 11(a)–(c)) demonstrate convincingly that the photocatalytic activities did not show significant loss. These revealed that the CdS QDs can be reused for several times to degrade the organic pollutants under visible light irradiation without a great decrease in photocatalytic activity, exhibiting the prospects for practical and long-term applications.

4. Conclusions In summary, the stable CdS QDs with good optical properties were successfully synthesized using Ser, Ala, and His as the stabilizing agents by simple method with low toxic properties. The CdS QDs showed good photocatalytic activities and recycling stability to degrade the organic dyes (alizarin) under visible light irradiation, suggesting its potential application in the effective treatment of the organic pollutants under visible light irradiation. Moreover, a decrease in the photocatalytic efficiency of the synthesized nanoparticles was observed on degradation of alizarin with an increase in their size; thus follows the order L-serine > Lhistidine > D-alanine. Furthermore, simplicity of the technique of precipitation using NaCl also leads to synthesis of nanoparticles to scale up.

Glossary Quantum dots: They are nanostructure semiconductors that are roughly spherical and their sizes are typically in the range 1–12 nm in diameter. Capping agent: The various ligands bond to QDs that are capable of protecting them from chemical-induced degradation or surface modifications and stabilizers. Photocatalytic dye degradation: The decomposition of dyes using the specified particles as a photocatalyst (under light irradiation).

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