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A novel hydrothermal approach for synthesizing CdS quantum dots (QDs) and nano-heterostructured CdS/ZnS QDs has been reported. This innovative approach proved to reproduce nanoparticles with extraordinary electronic and optical properties. The as-synthesized CdS and CdS/ZnS nano-heterostructured QDs were characterized by X-ray diffraction (XRD), UV-visible spectroscopy (UV-Vis), Transmission electron scanning (TEM), high resolution transmission electron scanning (HR-TEM), X-ray photoelectron scanning (XPS), and photoluminescence (PL) emission spectra. The average particle sizes of CdS and CdS/ZnS QDs as shown by XRD and TEM, were 4~6 nm. The optical band gap of CdS and ZnS/CdS nanoparticles was calculated from Tauc plot using UV-vis spectra. The estimated band gaps are measured to be 2.8 and 3.3 eV for CdS and CdS/ZnS QDs, respectively. The blue shift of the absorption edge compared to that of bulk clearly explained the quantum confinement effect. Such nano-hetrostructures of CdS/ZnS provided enhanced physical properties compared to individual CdS nanoparticles. The photocatalytic activity of the photo-stable CdS/ZnS was found to be superior towards the photodegradation of Methylene Blue, MB, dye than that of CdS QDs under visible light irradiation. The enhanced photodegradation mechanism of the nano-heterostructure was investigated and correlated with the optoelectronic properties.
© 2016 Optical Society of America
1. Introduction
Semiconductor nanoparticles and quantum dots have recently gained popularity because of its vast applications in light emitting diodes (LED), bio-imaging, diode lasers, catalysis, infrared windows and solar cells [1–3]. Further popularity was gained with the II-VI semiconductors for their excellent optical, electronic and magnetic properties. These properties are enhanced due to the nanoparticles quantum confinement effect which in turn provided a wider range of excitation wavelengths and narrower emission spectra. This is rather depicted by the reduction of their particle size to around 10 nm [3]. Through such a transformation, their properties dramatically change from their bulk level to the molecular level that is comparable to the particle’s Bohr diameter [4, 5]. Over nanoparticles, quantum dots reduced size beyond Bohr radius induces a shift of the electronic excitations to higher energies, granting unique quantum-confined photonic and electronic properties. Also QDs show strong resistance to photocorrosion and chemical degradation, and high quantum yields [5]. Such characteristics attracted much attention in the technological applications such as biological fluorescence and imaging where conventional dyes have been replaced because of the inorganic nanoparticle’s high photo-bleaching threshold, brightness during extended periods and ability to track tumors in deep levels [6]. Furthermore, these nanomaterials have been used for photocatalytic degradation and disinfection routines for wastewater treatment and water splitting [7].
CdS and ZnS nanoparticles are typical semiconducting II-VI nanomaterials. There are several methods used to prepare these nanomaterials which include aqueous phase precipitation [8], interphase synthesis [9], polymer stabilized solvothermal reaction [10], microemulsion [11], microwave irradiation [12], electrospinning [13], direct elemental route [14] and many more. The quantum confinement effect restricts the spatial movement of the exciton (electron-hole) of the particle thereby; revealing discrete energy levels (energy sub-bands) with wider band gaps which later on, alter the electronic and optical properties in comparison to their bulk level. Hence, the electron-hole pair is detached from one another, forming free carriers upon photo-excitation. In this case, the ionization energy is enhanced due to the strong Columb electrostatic forces of attraction between the electron-hole pair hence; energy of the photons should equalize the band gap energy in order for excitation to occur from the valence band to the conduction band. Moreover, with the reduced particle sizes, the de Broglie wavelength of the particle (inversely proportional to the frequency) becomes smaller therefore, enhancing the particle’s momentum and kinetic energy. As a result, the particle’s surface energy increases which causes aggregation because of the particle’s high tendency of interacting with neighboring particles. These sites are referred to as surface trap sites where recombination is unabated [15].
The cubic/hexagonal CdS nanoparticles exhibit band gap energy of 2.42 eV (wurtzite structure) [16]. ZnS nanoparticles showed a wider band gap of 3.7 eV compared to CdS nanoparticles generating higher redox potentials and better charge transfer. As a result, ZnS nanoparticles can be used as a capping agent to CdS nanoparticles due to its size being well-matched with CdS where reduction in lattice mismatch and uniform strain distribution allows for the enhancement of surface passivation. This is done by abolishing surface related-defect sites through eradication of surface dangling bonds [17].
Polymer matrices have attracted much attention due to their interfacial interaction with semiconductor nanomaterials. Polymer blends can be used as capping agents and dispersing agents [18, 19]. Most importantly, these polymeric materials prevent aggregation of nanoparticles and avert non-radiative recombination of the exciton (electron-hole pair) at the surface sites, thereby enhancing the photoluminescence properties due to surface vacancies. This produces small-sized monodispersed nanoparticles that have a narrow size distribution. However, an optimal concentration of the polymer blend should be obtained in order to prevent large aggregated cluster sizes of nanoparticles [20, 21].
In a previous study [22], we reported the synthesis of ZnS QDs via a facile, one pot and reproducible method and studied its optoelectronic properties, both experimentally and theoretically. The aim of this paper is to report the synthesis of nano-sized CdS QDs and nano-heterostructured CdS/ZnS quantum dots through one-pot, reproducible and large scaled hydrothermal method using a blend of PVA/PVP polymeric materials as dispersing agents in an aqueous matrix. The purpose behind this study is to investigate the rigidity and reproducibility of our synthetic route. Also the optoelectronic properties of this nano-heterostructure was investigated and compared with single CdS QDs. The enhanced photodegradation mechanism of the nano-heterostructure was investigated and correlated with the optoelectronic properties.
2. Experimental
2.1 Materials
Reagent grade cadmium nitrate Tetra-hydrate (Fluka, cryst. >98.0%), zinc nitrate, sodium sulphide (AnalaR, cryst. 32-38%), PVP (Sigma, Av. Mol. Wt. 10,000) and PVA (Fluka, Av Mol. Wt. 72,000) were purchased and used without further purification.
2.2 Synthesis of nanoparticles
Synthesis of CdS quantum dots nanoparticles
The CdS QDs nanoparticles were prepared as follows: 0.2 M cadmium nitrate was gradually added to a 5% aqueous solution of 50:50 (%wt) PVA/PVP under continuous stirring in order to obtain a homogeneous solution. After a clear vision of a dispersed homogenous solution, an equal volume of 0.2 M sodium sulphide was gradually added to the solution under continuous stirring to obtain a furthermore well-dissolved solution. After that, the mixture was moved to a 100 mL Teflon lined stainless-steel autoclave and placed in an oven at 110 °C for 5 hours. Finally, the product was washed three times with distilled water using centrifugation at a speed of 16,000 rpm. The resultant yellow precipitate of CdS nanoparticles was dried overnight in a vacuum oven at 40°C.
Synthesis of CdS/ZnS nano-heterostructured nanoparticles
The synthetic procedure for the synthesis of CdS/ZnS Heterostructured nanoparticles follows the aforementioned procedure for the synthesis of CdS QDs with slight difference. In a typical procedure, 0.1 M cadmium nitrate and 0.1 M zinc nitrate were used as the cadmium and zinc precursors, respectively. The zinc precursor was firstly introduced then followed by the cadmium precursor. The end product was washed three times with distilled water using centrifugation at a speed of 16,000 rpm. The resultant pale-yellow precipitate of CdS/ZnS nanoparticles was dried overnight at the vacuum oven at 40 °C.
2.3 Characterization
UV-Visible absorption spectra were measured on a Shimadzu UV-2450 spectrophotometer by diluting the aqueous solution of nanoparticles to 10% its original concentration. The X-ray diffraction (XRD) measurements were carried out by using X’Pert PRO Panalytical diffractometer with a copper target and nickel filter with CuKα radiation (λ = 0.154056 nm) to study the crystallinity and the average crystal size of the nanoparticles. The morphology of the nanoparticles and electron diffraction patterns were obtained by transmission electron microscopy (TEM) using a Jeol JEM 1230 operating at 120 kV. X-ray photoelectron spectroscopy (XPS) was conducted using a model VG Scientific 200 spectrometer (UK) equipped with MgKα radiation (1253 eV) and operated at 23 kV and 13 mA. Laser induced photoluminescence measurements were performed at room temperature using an excitation wavelength 355 nm.
2.4 Photocatalytic reaction
The photocatalytic reactor is a cylindrical Pyrex-glass cell with 1.0 L capacity. A 500-W halogen lamp as the visible light source (emission range of 400–800 nm) was placed in a quartz lamp holder which was immersed in the photoreactor cell. The cell was filled with 0.6 L of 10 mg/L of MB solution and 100 mg/L of nanoparticles as photocatalyst and the pH value was measured to be 5.8. The whole reactor was cooled with an electric fan outside the cell and the temperature was kept at 25°C. Analogous control experiments were performed without the photocatalyst nanoparticles (blank). The degradation of MB was monitored by taking 4 mL of the suspension at the irradiation time intervals (30 minutes). Each time the suspension was centrifuged to separate the photocatalyst particles from the MB solution. Subsequently, the degradation rate was calculated according to the change in absorbance of the dye solution.
3. Results and discussions
3.1 XRD of CdS QDs and CdS/ZnS nano-heterostructure
The X-Ray powder diffraction (XRD) patterns of CdS QDs and CdS/ZnS Heterostructured nanoparticles are shown in Fig. 1. The diffraction pattern of CdS showed at 2θ = 26.8°, 43.9° and 52.0° correspond to the crystal planes (111), (220) and (311), respectively. These are assigned to the JCPDS data (No. 10–0454) of the cubic structure of CdS [23]. The diffraction pattern of both CdS QDs and CdS/ZnS Nano-Heterostructure are almost the same. This could be attributed to the fact that the total ionic charges and the radii are very close for both Zn2+ and Cd2+ ions. Consequently, both ions can substitute each other in the crystal lattices [24]. The particle sizes of the synthesized nanoparticles were calculated using the Debye–Scherrer equation [Eq. (1)],
where λ is the wavelength (Cu Kα), β is the full width at half-maximum of the CdS nanoparticles and θ is the diffraction angle. The calculated average crystallite sizes were found to be ~4.0 and ~5.0 nm for CdS and CdS/ZnS, respectively.3.2 TEM and HRTEM of CdS QDs and CdS/ZnS nano-heterostructure
Electron microscopy was used to study the morphology of the synthesized nanoparticles. Figures 2(a) and 2(b) show the TEM images of CdS and CdS/ZnS QDs which confirms the narrow-sized distribution of the nanoparticles. Their sizes were estimated to be ~4.5 ± 0.5 nm and ~5.5 ± 0.7 nm for CdS and CdS/ZnS QDs, respectively. Figure 2(c) and 2(d) show the HRTEM of CdS and nano-heterostructured CdS/ZnS QDs, respectively. The HRTEM image of the CdS QDs is shown in Fig. 2(c), where the lattice fringe is estimated to be 0.33 nm corresponding to the (111) lattice plane of zinc-blend CdS crystal structure [25]. After the introduction of ZnS QDs into the nano-heterostructure, the dark region in Fig. 2(d) revealed interplanar spacings of 0.33 nm and 0.21nm which were close to the interplanar spacings of zinc blende phase with (111) facet and wurtzite with (220) facet of CdS/ZnS nano-heterostructure, respectively [26].
Fig. 2 (a) TEM of CdS QDs; (b) TEM of CdS/ZnS QDs; (c) HRTEM of CdS QDs and (d) HRTEM CdS/ZnS nano-heterostructured nanoparticles.
3.3 XPS Spectra of CdS QDs and CdS/ZnS nano-heterostructure
Surface composition of CdS QDs and nano-heterostructured CdS/ZnS QDs were investigated via XPS. Figure 3 presents the XPS spectra for S, Cd and Zn, respectively. From Fig. 3(a), the asymmetric peak of S2p was deconvoluted to two peaks located at 160.3 eV and 161.5 eV which ascribed for S 2p3/2 and S 2p1/2, respectively. From Fig. 3(b), the two strong peaks located at 404.3 eV and 411.0 eV correspond to Cd3d5/2 and Cd3d3/2, respectively. From Fig. 3(c), two peaks located at 1021.2 eV and 1044.3 eV are assigned to Zn2p3/2 and Zn2p5/2, respectively [27]. The XPS data confirmed the proposed nano-heterostructured CdS/ZnS QDs stoichiometrically as it reported the Cd/Zn/S ratio to be 0.65:0.30:1 which is mainly close to CdxZn1−xS [28]. Notably, the absence of a sulphate peak at 168.3 eV supports that the nano-heterostructure quantum dots are chemically stable without photocorrosion oxidation during the hydrothermal treatment [29].
3.4 Optical properties of CdS QDs and CdS/ZnS nano-heterostructure
The UV–visible spectra of the as-prepared CdS and CdS/ZnS QDs nanoparticles were shown in Fig. 4(a).The energy band gaps for CdS and CdS/ZnS QDs were calculated using the Tauc equation:
where Eg is the optical band gap energy, k is a constant and m = 1/2 for a direct energy gap. For this purpose, a plot of (αhν)2 versus hν was constructed and the linear portion of the plot was extrapolated to the ordinate [30]. By using this method, edges of CdS QDs and nano-heterostructured CdS/ZnS QDs have been found at 442 nm and 382 nm, respectively with an optical band gap of 2.8 eV and 3.3 eV, respectively.The excitonic Bohr radius for bulk ZnS and ZnS is 2.5 nm and quasi quantum confinement is typically observed for ZnS and CdS nanoparticles having diameters up to 5 nm [29]. The VB edge potential (EVB) of a ground-state semiconductor can be empirically determined by using the equation, EVB = χSemiconductor − Ee + 0.5Eg, derived using Mulliken electronegativity theory [31], where χSemiconductor is the electronegativity of the semiconductor (5.26 eV for ZnS and 5.18 eV for CdS) [32]), Ee is the energy of free electrons on the hydrogen scale (ca. 4.5 eV), and Eg is the band gap energy of the semiconductor (calculated by the Tauc method). The CB edge potential can be determined using the relationship ECB = EVB – Eg [33]. Accordingly the CB and VB edge potentials of CdS were calculated at −0.72 and 2.08 eV by the above cited theoretical calculations. And the CB and VB edge potentials of ZnS were calculated at −1.59 and 3.11 eV based on our previously published data [22], and for CdS/ZnS nano-heterostructure the CB and VB edge potentials of ZnS were calculated at −0.95 and 2.35 eV.
Also the corresponding room temperature PL spectrum of CdS QDs and CdS/ZnS heterostructured nanoparticles are shown in Fig. 4(b). For CdS QDs, it was reported [34] that the energy emission band at 500–700 nm was attributed to the recombination of charge carriers in deep traps of surface-defect states. As seen in Fig. 4(b), the emission peak of the CdS QDs is located at 540 nm, which presents a blue shift compared to bulk CdS at 650 nm [30]. The PL emission spectra of the nano-heterostructure exhibit two different characteristics other than those of single CdS QDs. Firstly, PL emission spectra of the CdS/ZnS is blue shifted out of the region for the single CdS QD as clearly shown in a single emission peak at 513 nm, This is an evidence for electron transfer pathway from CdS to ZnS on excitation. Secondly, the PL spectrum exhibits a relative increase in the intensity which is a result of the passivation of the surface vacancy and non-radiative recombination sites. As a result, more photogenerated charge carriers confined inside CdS [35, 36].
The particle size can be estimated from the UV-Vis spectra using the following equation derived from the effective mass model [37]:
= band gap energy of the synthesized nanoparticle, = band gap energy of bulk CdS, which has the value of , h = Planck’s Constant, , r = particle radius (m), me = mass of a free electron, , = (effective mass of electron in CdS), = (effective mass of hole in CdS), e = electron charge, , = (permittivity of free space) and = 5.7 (relative permittivity of CdS). The particle size was found to be 3.7 nm and 4.3 nm for CdS QDs and ZnS/CdS nano-heterostructure, respectively. The particle size of 3.7 nm and 4.3 nm for CdS QDs and ZnS/CdS nano-heterostructure are below the Bohr radius for a CdS quantum dot, which is around 5 nm.3.5 Photocatalytic activity of the synthesized nanoparticles
The dye industry has a challenge of disposal of methylene blue (MB) from waste water due to its high solubility in water. Trace amounts of this harmful dye causes skin diseases and intestinal problems. In view of this, MB photodegradation under visible light irradiation was conducted using CdS/ZnS nano-heterostructure.
The photocatalytic activities of CdS QDs and CdS/ZnS heterostructured nanoparticles were tested for the photodegradation of MB dye under visible light irradiation. The photocatalytic efficiency of CdS/ZnS nano-heterostructure was found to be 1.5 fold greater than individual CdS QDs (96% and 63%, respectively).
The photodegradation reaction follows a pseudo-first-order reaction. The degradation rate constant for the photodegradation reaction was determined from the equation:
Where C0 is the initial concentration and C is the concentration at time t and k is the apparent first-order rate constant. A plot of ln Co/C versus time represents a straight line (Fig. 5), where the slope of which upon linear regression equals the apparent first-order rate constant k. The kinetic results revealed degradation rate constant of 0.018 min−1 compared to 0.031 min−1 CdS QDs and CdS/ZnS nano-heterostructure, respectively. This could be explained by saying that, when the nano-heterostructure system is exposed to visible light irradiation, firstly, the electrons in the CdS QDs are excited to the conduction band (CB) of ZnS QDs leaving holes in the valence band (VB). The reason behind the superior photocatalytic activity of CdS/ZnS nano-heterostructure structure compared with individual CdS QDs is most likely related to the tunneling of the electrons generated in the CdS (a quantum well) through ZnS (an energy barrier) as evidenced by PL spectra. This electron transfer channel results in an efficient separation of photogenerated electrons which could be captured by adsorbed oxygen molecules. This leads to the generation of hydrogen peroxide or superoxide radical (O2•-) which are powerful oxidative species needed for the degradation of MB dye [38].Fig. 5 Pseudo-first-order plot for the kinetics of photodegradation of MB dye in the presence of CdS QDs and CdS/ZnS heterostructured nanoparticles under visible light irradiation.
3.6 Photostability of nano-heterostructure
It is well known that ZnS and CdS undergo photocorrosion upon irradiation because of oxidizing holes which causes semiconductor decomposition into sulfur and metal ions [39]. This photocorrosion effect could be suppressed by raising valence band (VB) position. In this aspect, the surface modification of CdS QDs by ZnS QDs in the nano-heterostructure serves this function. The increase of the VB potential enhanced the photostability of CdS/ZnS heterostructured nanoparticles and in turn results in superior photocatalytic activity (as shown in Fig. 7). To prove this photostability experimentally, the CdS/ZnS heterostructured nanoparticles were collected following each reaction cycle up to 6th cycle by centrifugation washed using doubly distilled water and then dried before being reused. The experimentally determined MB photodegradation rates through the six cycles as shown in Fig. 6 show an apparent gradual deactivation of photocatalyst. However after the 6th cycle, the CdS/ZnS heterostructured nanoparticles maintained relatively its photoactivity (from 0.031 min−1 at 1st cycle to 0.005 min−1 at 6th cycle) which reveals the photostability of CdS/ZnS nano-heterostructure and this result is consistent with XPS data in which the sulphide did not converted to sulphate group upon exposure to atmospheric air. The reason behind the decrease of the photoefficiny during the cycles' progress is most likely due to agglomeration and of the MB dye around the nanoparticles after each cycle. This is a possible reason for the obtained decrease of the degradation rate, as after each cycle more parts of the photocatalyst surface become blocked and unavailable for dye adsorption and thus photon absorption will be diminished, which in turn resulted in reducing the efficiency of the catalytic reaction [40].
Fig. 6 Plots of ln Co/C versus time for photodegradation of MB during 6 cycles at constant time = 80 min.
3.7 Photocatalytic mechanism
The higher photodegradation efficiency suggests the advantage of the CdS/ZnS nano-heterostructure of the photocatalyst over bare CdS QDs. As ZnS QDs cannot be excited directly by visible irradiation due to its wide band gap, the visible-sensitive CdS QDs can be readily excited by visible light irradiation. Upon its excitation, CdS QDs would act as a photosensitizer which induces the excitation of ZnS QDs. Based on the results shown in Fig. 7, we have proposed the following possible mechanism. Under visible light illumination, firstly the electron–hole pairs in CdS QDs are excited, and the electrons are excited from the valence band (VB) to the conduction band (CB), leaving holes in the VB. The potential of ZnS is about −1.59 eV, which is less negative than the CB level of CdS (about −0.72 V) [41]. As the conduction band (CB) edge potential of CdS is more negative than that of ZnS, this result in a difference in band potentials between the two materials, inducing a contact electric field at their interfaces. Driven by the electric field created at their interface, the photoexcited electrons in the CB of CdS could be transferred to the CB of ZnS directly. Consequently, the photogenerated holes at VB of CdS could not transfer from CdS to ZnS, making the lifetimes of photogenerated charges in CdS/ZnS nano-heterostructure to increase [42]. This is how we believe the CdS/ZnS nano-heterostructure exhibited enhanced photocatalytic activity. This pathway of electron transfer is applicable for heterostructured systems, however the prohibited charge transfer pathway in the mechanism is related to the core-shell systems [43].
4. Conclusion
In summary, individual CdS QDs and heterostructured CdS/ZnS QDs nanoparticles were successfully synthesized via hydrothermally modified method. CdxZn1−xS was characterized via different analytical techniques in order to investigate the optoelectronic characteristics and compared with individual CdS QDs. Such nano-hetrostructures of CdS/ZnS provided enhanced optoelectronic properties compared with individual CdS QDs nanoparticles. The estimated band gaps are measured to be 2.8 and 3.3 eV for CdS and CdS/ZnS QDs, respectively. This band gap modulation enables efficient photogenerated electron separation and consequently, higher photocatalytic activity under visible light illumination. The mechanism of charge transfer was investigated and correlated with the optoelectronic characteristics of the nanoparticulated heterojunction which is mainly differs from the core shell system.
Acknowledgment
The authors gratefully acknowledge the support of Kuwait University Research Administration, Project No. (SC 11/13) and SAF Facilities No. (GS 01/01, GS 01/05 and GS 02/08 and GE 03/08). In addition, Nanoscopy Science Centre is highly acknowledged.
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