Biogenic synthesis and photocatalytic activity of CdS nanoparticles

Abstract

The present work demonstrates the photocatalytic activity of biologically synthesized cadmium sulfide (CdS) nanoparticles, using the fungal biomass of Trichoderma harzianum. The photocatalytic activity of the nanoparticles was determined upon the degradation of methylene blue (M.B.) in a photocatalytic reactor and the residual concentration was monitored using UV-Vis spectroscopy. The mechanism behind the photocatalytic activity was discussed with a diagrammatic representation. The synthesized CdS nanoparticles were characterized by UV-Vis spectroscopy, Transmission electron microscopy (TEM), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). The UV-Vis spectrum of the CdS nanoparticles suspension showed an absorption peak at 332 nm. The photocatalytic activity of the CdS nanoparticles was evaluated using reaction kinetics. The energy band gap of the synthesized CdS nanoparticles was estimated by absorbance spectra. TEM analysis showed that the particles are formed on the nano scale and have a spherical morphology. X-ray diffraction analysis showed intense peaks which correspond to the crystalline nature of CdS. Fourier transform infrared spectroscopy provided clear evidence of the presence of proteins as probable biomolecules responsible for the stabilization of the CdS nanoparticles.

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1. Introduction

In recent years, semiconductor materials in nanocrystalline form and their synthesis, have gained huge interest due to their unique optical and spectroscopic properties.¹ A number of articles describing the synthesis and characterization of semiconductor nanoparticles such as CdSe,² CdS,³ ZnS⁴ and ZnO,⁵ have already been reported in the literature. Cadmium sulfide (CdS) nanoparticles are the most widely studied binary chalcogenide material belonging to the II–VI group. CdS is a wide band gap material having an energy band gap of 2.42 eV.⁶ The use of CdS nanoparticles has proven to be very efficient in comparison to the bulk material. This is because of their size dependent properties due to the high surface to volume ratio.⁷ CdS nanoparticles are also utilized an ideal materials to investigate the optical and electronic properties of quantum confined semiconductors.⁸ Due to such unique properties and a large number of technical applications, a number of routes have been utilized for the synthesis of CdS nanoparticles such as solvothermal⁹ and hydrothermal¹⁰ methods.

In order to meet the exponentially growing technological demand of nanomaterials, there is a need to evolve new cost effective, sustainable and eco-friendly approaches for the synthesis of nanomaterials. Therefore, researchers are paying attention towards biological routes for synthesis of nanomaterials. Synthesis of various metal/semiconductor nanoparticles using various plants^11–16 and microorganisms^17,18 has been reported. Studies have shown that microorganisms can synthesize nanoparticles of tailored size and shape.¹⁹

Cadmium sulfide (CdS) nanoparticles have been of huge interest because of their potential technological applications in the field of solar cells,²⁰ photo-electronic devices,²¹ sensors,²² cell imaging²³ and photocatalysts.²⁴ Photocatalysis is the process of charge separation due to the absorbance of light energy greater than or equal to the band gap of the semiconductor material. This charge separation creates electron–hole pairs which lead to the generation of free radicals in the system.²⁵ These generated radicals are good oxidizers and degrade pollutants.^26,27 It is an effective technique for waste water treatment containing organic dyes.

We herein report the biological synthesis, characterization and photocatalytic activity of CdS nanoparticles using T. harzianum. The biosynthesis of CdS nanoparticles has been already reported in the literature by filamentous fungi like Fusarium Sp.²⁸ But these are pathogenic to both plants²⁹ and animals.³⁰ This inspires us to use T. harzianum for the synthesis of CdS nanoparticles as it is non-pathogenic and has the ability to antagonize plant pathogens.³¹ This property of Trichoderma Sp. prevents the contamination of unwanted microorganisms, this is because different types and species of microorganism synthesize nanomaterials with different morphology and size.¹⁹ The CdS nanoparticles prepared using T. harzianum biomass were highly stable as no agglomeration was recorded after 2 months by UV-Vis spectroscopy. The photocatalytic activity of the synthesized CdS nanoparticles was analyzed with the help of M.B. reduction using UV-Vis spectroscopy.

2. Materials and methods

2.1. Materials

The strain of T. harzianum was procured from the Department of Microbiology, College of Life Sciences, Gwalior, India. Cadmium chloride (CdCl₂) and sodium sulfide (Na₂S) were acquired from Qualigens Fine Chemicals, Mumbai, India. Potato dextrose agar (PDA), yeast extract, malt extract, peptone and glucose were obtained from Hi-Media, Mumbai, India.

2.2. Fungal culture

The PDA slant was used to maintain the fungal strain of T. harzianum. The fungal biomass was procured by inoculating T. harzianum in 100 ml of MYPG medium (malt extract 0.3%, yeast extract 0.3%, peptone 0.3% and glucose 1%) and maintaining the pH at 5.6 ± 1. The fungal culture was incubated at 28 ± 1 °C with continuous shaking for 120 h at 200 rpm. The biomass was extracted by centrifugation at 8000 rpm for 15 min after 120 h of incubation. Then the biomass was washed with deionized water to remove any media components. The washed biomass was used further for the synthesis of CdS nanoparticles.

2.3. Biosynthesis of CdS nanoparticles

The washed fungal biomass of T. harzianum (5 g) was dispersed in 45 ml of deionized water in a 200 ml Erlenmeyer flask. Cadmium chloride (0.1 mM) was added to this. Subsequently, 5 ml sodium sulfide (0.1 mM) was added dropwise into the solution. Finally, the solution was incubated in dark conditions for 72 h at 28 ± 1 °C with continuous shaking at 200 rpm. A blank solution which does not contain fungal biomass was also run in a similar fashion to understand the function of biomass in the synthesis of CdS nanoparticles. The overall synthesis process can be expressed by the following equation.

2.4. Photocatalytic activity of the CdS nanoparticles

The photocatalytic activity of the biosynthesized CdS nanoparticles was studied by monitoring the degradation of the M.B. solution under UV-irradiation. A solution of 10 mg l⁻¹ of M.B. and 100 mg l⁻¹ of CdS nanoparticles as the photocatalyst was placed in a photoreactor cell. The cell contained a 300 W high-pressure mercury lamp with an emission wavelength of 365 nm (Shanghai Bilon Instruments Co. Ltd) and was surrounded by a circulating water jacket to cool the lamp and minimize the infrared radiation. To ensure the adsorption–desorption equilibrium among the nanoparticles, M.B. and water, the solution was continuously stirred for 30 min. The degradation of M.B. was monitored at regular time intervals using a UV-Vis spectrophotometer (UV-1601 pc Shimadzu).

3. Results and discussion

3.1. Characterization of CdS nanoparticles

UV-Vis spectra of the biosynthesized CdS nanoparticles were recorded using a UV-Vis spectrophotometer (UV-1601 pc Shimadzu) at different time intervals. The reaction mixture was scanned in the wavelength range from 250 nm to 450 nm with a resolution of 1 nm. The spectra show an absorbance peak occurring at 332 nm which is blue shifted compared to bulk CdS (Fig. 1A). The formation of the CdS nanoparticles began 12 h into the incubation period showing an absorbance peak, which is less intense indicating minimal reduction of M.B. ions. The absorbance peak intensity increases as the incubation time increases and the reaction was complete after 72 h of incubation. The cut off wavelength which is the intercept of the linear portion of the spectra was found to be 372 nm from the UV-Vis spectra as illustrated in Fig. 1B. According to the spectra, we estimate the band gap of the biosynthesized CdS nanoparticles using Einstein's energy equation,

E^nano_g = hc/λ

where, E^nano_g is the energy band gap of the nanoparticles, c is the speed of light, h is Planck's constant and λ is the cut off wavelength obtained from the absorption spectra, which is 372 nm.

Fig. 1 (A) UV-Vis spectra of the biosynthesized CdS nanoparticles at different time intervals. (B) UV-Vis spectrum of the biosynthesized CdS nanoparticles showing the cut off wavelength at 372 nm. (C) Absorbance vs. time curve illustrating the reaction kinetics using UV-Vis spectroscopic analysis.

The energy band gap of the CdS nanoparticles was found to be 3.31 eV which is higher than the band gap of bulk CdS (2.42 eV). This is due to the presence of quantum size effects in the synthesized CdS nanoparticles.

It can be clearly seen from Fig. 1C that initially at ∼20 h of incubation, the formation of CdS nanoparticles is slow thus showing very little absorbance in the UV-Vis spectra. As the incubation time increases, the growth of the CdS nanoparticles increases, hence obtaining an optimum size. The plot almost resembles a Michaelis–Menten plot and follows zero order kinetics up to 30 h of incubation time giving the rate constant to be 0.04 h⁻¹. This indicates that the dissociation of the complex is a slow process. The biosynthesized CdS nanoparticles are highly stable as the absorption spectra recorded after 2 months shows no agglomeration. This shows the same result as the absorption peak that was obtained after 72 h of the incubation period.

A drop coated thin film of the biosynthesized CdS nanoparticles over a glass substrate was prepared to analyze the crystal structure of the particles using XRD (X'Pert PRO PANanalytical X-ray Diffractometer). Fig. 2 shows the intense peaks occurring at 2θ = 28.33°, 53.08° and 67.16° corresponding to (101), (201) and (203) sets of lattice planes respectively. These planes correspond to the Bragg’s reflections which are in good agreement with the reference to the primitive hexagonal structure of crystalline CdS (JCPDS File no. 80-0006). The broadening of the peaks clearly indicates that the particles formed are in a nano regime and are crystalline in nature.

Fig. 2 X-ray diffraction pattern of the biosynthesized CdS nanoparticles.

The TEM analysis was done by making a drop coated film of the biosynthesized CdS nanoparticles over a carbon-coated copper grid. This technique helps to analyze the size and morphology of nanoparticles. The TEM analysis was performed at an accelerating voltage of 200.0 kV with 20000× magnification (Philips CM-10). The TEM micrograph shows the almost uniform distribution of the biosynthesized CdS nanoparticles which are formed in the size range of 3–8 nm and have a spherical morphology (Fig. 3A). The particles are well separated from each other showing no agglomeration. Using ImageJ 1.49 software,³² the particle size distribution was estimated from the TEM image considering 100 particles, shown in Fig. 3B. Furthermore the HRTEM image (Fig. 3C) shows that the particles have a d-spacing of 3.14 Å.

Fig. 3 (A) TEM micrograph of the biosynthesized CdS nanoparticles. (B) Particle size distribution histogram. (C) High resolution image of a single nanoparticle.

Earlier researchers have reported the synthesis of CdS nanoparticles via a biological route using Serratia nematodiphila bacteria³³ and also by a chemical route using a composite-molten-salt method.³⁴ The average size of the synthesized CdS nanoparticles obtained was 12 nm and 14 nm respectively. However, the bacteria used caused amber disease.³⁵ In the present study, we are using non-pathogenic and agriculturally important fungi T. harzianum for the synthesis of CdS nanoparticles. The average size of the synthesized CdS nanoparticles was found to be 4 nm, which is extremely small compared to the two methods discussed above.

The elemental composition of the sample was analyzed by energy dispersive X-ray spectroscopy (Sigma). The EDX plot (Fig. 4) between X-ray counts and energy (keV) shows the peaks for the elemental signals of Cd and S from the emission of energies. There are two further peaks for Cu which were obtained from the copper grid used for the sample preparation.

Fig. 4 EDX analysis of biosynthesized CdS nanoparticles.

FTIR analysis was carried out to determine the possible biomolecules responsible for the stabilization of the biosynthesized CdS nanoparticles (Fig. 5). A CdS suspension was centrifuged at 10 000 rpm for 30 min to carry out the FTIR analysis. The pellet was washed three times with 15 ml of deionized water to remove the free proteins/enzymes that were not capping the CdS nanoparticles. The sample was analyzed using a Perkin-Elmer instrument. The absorbance bands occurring from 3217.27 cm⁻¹ to 3923.21 cm⁻¹ corresponds to an –OH stretching along with a –NH group stretching which shows the presence of an enzyme/protein in the sample. An intense peak occurs at 2357.01 cm⁻¹ which corresponds to the stretching of a –CH (alkane) group. Another peak was also obtained at 2121.70 cm⁻¹ which can be assigned to a C≡C bond. The peak at 1643.35 cm⁻¹ corresponds to the stretching of a C=C bond. The band with three peaks at 1462.04 cm⁻¹, 1450.47 cm⁻¹ and 1415.75 cm⁻¹ corresponds to the bending of a –CH bond. An intense peak at 1309.67 cm⁻¹ can be assigned to the stretching of a C–N bond. The band with three peaks at 1286.52 cm⁻¹, 1247.94 cm⁻¹ and 1085.92 cm⁻¹ corresponds to the stretching of an ester and ether. The band with two peaks at 692.44 cm⁻¹ and 634.58 cm⁻¹ can be assigned to the stretching of a C–Cl bond. An intense peak at 572.86 cm⁻¹ corresponds to the stretching of a C–Br bond. The peaks occurring from 322.11 cm⁻¹ to 443.63 cm⁻¹ can be assigned to the stretching of a C–I bond. It is reported earlier that proteins can bind to CdS nanoparticles through amine groups.³⁶ The higher level of stability in the nanoparticles may occur due to binding of these amine functional groups³⁷ which are present in the sample. We conclude that these functional residues help in capping of the CdS nanoparticles to provide stability.

Fig. 5 FTIR spectrum of the CdS nanoparticles synthesized using the fungal biomass of T. harzianum.

3.2. Photocatalytic activity of the CdS nanoparticles

The M.B. dye was chosen to investigate the photocatalytic activity of biosynthesized CdS nanoparticles. The characteristic absorbance of M.B. was found at λ = 664 nm which was used as the reference for photocatalytic degradation. The degradation of M.B. was negligible in the absence of CdS nanoparticles which is shown by the control analysis. The degradation of M.B. in the presence of the CdS nanoparticles (Fig. 6A) was measured with a 300 W mercury irradiation lamp (λ_max = 365 nm) at various intervals of time. Fig. 6B shows that the CdS nanoparticles degrade 37.15% of the M.B. after 60 min of degradation time. This is due to their high surface-to-volume ratio which helps to increase the photocatalytic reaction site. The kinetics of the reaction was plotted as ln(C/C₀) against time (t) and the photodegradation constant of M.B. was calculated to be 0.0076 min⁻¹ (Fig. 6C). It is seen that after a prolonged time, the graph deviates from its linearity. Since a very low concentration of M.B. dye has been used and where one reactant is being used in excess, the reaction is likely to follow pseudo first order kinetics.³⁸

Fig. 6 (A) UV-Visible spectral changes of M.B. in the presence of the CdS nanoparticles. (B) Photodegradation of M.B. in water under UV light irradiation in the presence of the catalytic CdS nanoparticles and without catalyst. (C) Selected fitting results using pseudo first order reaction.

A 37.15% degradation of M.B. was obtained in 60 min using the biosynthesized CdS nanoparticles. Previous reports on the degradation of M.B. using ZnO nanoparticles showed a successful 18% M.B. reduction in 60 min³⁹ which indicates that CdS nanoparticles act as an efficient photocatalysis material. However TiO₂ nanoparticles also show good photocatalytic properties but due to their wide band gap they cannot absorb the wide absorption spectrum of sunlight (visible light).³⁴ Therefore researchers are focusing more on making composites with TiO₂ nanoparticles which can absorb both the UV as well as the visible spectrum of light⁴⁰ giving enhanced photocatalytic activity. In contrast with TiO₂ nanoparticles, CdS nanoparticles have a narrow band gap and can absorb a wider absorption spectrum of sunlight. In addition, the photogenerated electrons and holes due to the high surface-to-volume ratio can access the surface of the particles more effectively and can be readily caught by redox pairs in the solution with less recombination.⁴¹

The mechanism involved in the photocatalytic degradation using the CdS nanoparticles is diagrammatically represented in Fig. 7. It was found that the band gap of the biosynthesized CdS nanoparticles was 3.31 eV which is larger than the band gap of bulk CdS (2.42 eV). When the UV light with energy of hν matches the energy of the band gap (E_g) of the biosynthesized CdS nanoparticles, it generates electron (e⁻) and hole (h⁺) pairs which are powerful oxidizing and reducing agents:

2CdS + hν → CdS (e⁻) + CdS (h⁺)

Fig. 7 The photodegradation mechanism of M.B. in the presence of the CdS nanoparticles.

The high band gap of the nanoparticles leads to the non-radiative recombination of the electron (e⁻) and hole (h⁺) pairs,⁴² which enhances the photocatalytic activity. The water (H₂O) which is adsorbed on the surface of the CdS nanoparticles traps the hole (h⁺) and is oxidized to give a hydroxyl radical (OH˙) which is expressed as:

H₂O + CdS (h⁺) → ˙OH + H⁺ + CdS

Subsequently, the presence of oxygen (O₂) prevents the electron (e⁻) and hole (h⁺) pair recombination and forms an anion radical (˙O²⁻) by accepting electrons (e⁻) from the conduction band and further combines with a proton to give ˙OOH as shown in the expression:

O₂ + CdS (e⁻) → ˙O²⁻ + CdS

˙O²⁻+ H⁺ → ˙OOH

These radicals such as OH˙ and ˙O²⁻ degrade M.B. by adding to the aromatic ring of the organic compound (M.B.) and opening it at the azo bond and the hydroxylated ring to finally yield CO₂ gas, H₂O, SO₂⁴⁻, NO₃⁻, and NH₄⁺ ions.⁴³

M.B. + ˙OH/˙O²⁻ → products (CO₂ + H₂O + NH₄⁺ + NO₃⁻ + SO₂⁴⁻)

4. Conclusions

In the present study, the fungal biomass of T. harzianum was used for the synthesis of CdS nanoparticles in ambient laboratory conditions. The synthesized nanoparticles were characterized using UV-Vis spectroscopy, TEM, XRD and FTIR. The UV-Vis spectroscopy analysis confirmed the formation of the CdS nanoparticles having an absorbance peak at 332 nm. The energy band gap of the CdS nanoparticles was estimated to be 3.31 eV which is larger than the bulk counterpart due to the quantum confinement effect. Furthermore, the reaction kinetics was studied and the rate constant was found to be 0.04 h⁻¹ which indicates that dissociation of the protein–nanoparticle complex is a slow process. The synthesized CdS nanoparticles were highly stable showing no agglomeration even after 2 months. This indicates that the fungal biomass of T. harzianum is the potential candidate for the stabilization of nanoparticles. This underlines the applicability and significance of the biosynthesis of CdS nanoparticles. The synthesis of the CdS nanoparticles using T. harzianum is an eco-friendly, reliable and lucrative method. The photocatalytic activity of the biosynthesized CdS nanoparticles was studied using M.B. In this experiment, the degradation efficiency of the photocatalytic CdS nanoparticles was found to be 37.15% after 60 min of illumination having a reaction rate of 0.0076 min⁻¹. It was found that the synthesized CdS nanoparticles possess a good capability to degrade M.B.

Acknowledgements

The authors are grateful to Dr. Ashok K. Chauhan, Founder President, Amity University (Noida, India) for his encouragement and providing excellent facilities for the above work.

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