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Effect of annealing time on optical properties of CdS QDs containing glasses and their application for degradation of methyl orange dye
Materials Characterization 152 (2019) 230–238
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Effect of annealing time on optical properties of CdS QDs containing glasses and their application for degradation of methyl orange dye S.Y. Janbandhu, S.R. Munishwar, G.K. Sukhadeve, R.S. Gedam
T
⁎
Department of Physics, Visvesvaraya National Institute of Technology, Nagpur 440010, India
A R T I C LE I N FO
A B S T R A C T
Keywords: CdS QDs HRTEM UV–vis Photoluminescence Methyl orange dye Photodegradation
Borosilicate glass system (SiO2:B2O3:Na2O:ZnO) with the addition of 3 wt% CdS was prepared by conventional melt quench technique. Amorphous nature of glass sample was confirmed from XRD measurement. CdS QDs were grown in the glass matrix using an optimized heat treatment schedule in between glass transition (Tg) and crystallization (Tc) temperature. The phases of CdS QDs were noted by XRD and confirmed by HRTEM. Optical absorption study was carried out to measure the energy band gap and it is found that the energy band gap decreases with an increase in the size of QDs. Size determination was done by optical absorption spectra and match with HRTEM results. This change in the size of QDs was confirmed by PL spectra. The optimized glass sample was used for the degradation of methyl orange (MO) dye.
1. Introduction Nanostructured materials are considered as an important bridge between the molecular level and bulk. The nanocrystal of semiconductors having discrete energy levels it follows the quantum confinement effect are known as semiconductor quantum dots (SQDs) [1]. Semiconductor quantum dots (QDs) have maintained its peak level in several fields of science like optoelectronics [2] photonic device [3] and biomedical applications [4] since the last few decades. Tunable properties of materials by increasing or decreasing the size of QDs provide a strong hope to apply in various fields like in optical communication [5], signal transmission [6] and solar cell industry [7]. The eco-friendly and clean energy source is needed in order to consider environmental pollution as well as the shortcoming of natural energy resources. Photocatalysis is one of the most promising methods which not only removes the environmental pollutants but also a powerful way to tackle solar energy as substitutional natural energy [8]. Semiconductor nanomaterials, specifically TiO2 bears tremendous belief in helping the energy crisis over effective utilization of solar energy by using photovoltaic and water-splitting devices [9]. Nevertheless, its wide band gap (greater than 3 eV) restricts it to use in broad range spectrum. As an optional semiconductor, CdS nanostructure has been broadly investigated [10,11]. Beside the improvement of the properties of CdS at the nano level, it has the challenge of agglomeration and instability. Photocorrosion is one of the biggest issues during photocatalysis [12]. In order to use these QDs in practical applications by
⁎
fixing the challenges mentioned above, some inert and inorganic solid matrix is needed to embed them. In the optical point of view, the glass matrix has been selected for the incorporation of QDs which also serves the thermal and mechanical stability [13,14]. By altering the shape and size, QDs can be manipulated to emit over a broad range of wavelengths. In our earlier reported article, optical properties of glasses containing CdS QDs by varying heat treatment temperature was studied [15]. In that study, it was observed that with an increase in temperature the PL intensity was decreasing due to the less electron-hole recombination. In the photocatalysis process low electron-hole recombination is favorable for better activity [16]. The glass sample heat treated at the highest temperature (low emission intensity) among all the samples was used for degradation of methylene blue (MB) dye and degradation efficiency were found to be 70% for 270 min of natural sunlight irradiation. In this paper, an attempt is made to see the effect of annealing time on the size of CdS QDs and to use these QDs for methyl orange (MO) dye degradation. 2. Experimental section 2.1. Synthesis of CdS embedded borosilicate glass For the synthesis of CdS containing glass, melt quench technique is used. The raw materials were taken in the composition of 40 mol% SiO2, 28 mol% B2O3, 22 mol% Na2O and 10 mol% ZnO (purity greater
Corresponding author. E-mail address: rupesh_gedam@rediffmail.com (R.S. Gedam).
https://doi.org/10.1016/j.matchar.2019.04.027 Received 15 December 2018; Received in revised form 9 March 2019; Accepted 22 April 2019 Available online 24 April 2019 1044-5803/ © 2019 Elsevier Inc. All rights reserved.
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average energy density of solar radiation about 1100 W/m2. The catalyst added to the MO dye solution and stirred in dark for 1 h before the irradiation. 4 ml dispersions were taken at the equal interval of time and centrifuged to analyze remaining dye concentration by JASCO V670 spectrophotometer before and after irradiation at λmax = 463 nm. The degradation efficiency was calculated by using Eq. (1):
than 99%, Sigma Aldrich, Merk). A dry powder of these raw materials was thoroughly mixed by agate mortar and pestle to get a homogeneous mixture and fine powder. After that, Cadmium sulfide (CdS 3 wt%, 99.999% purity, alfa aesar) was added in glass composition and ground up to 2 h in order to enhance the homogeneity of the mixture further. This mixture was taken in a platinum crucible and transferred into the high-temperature furnace with silicon carbide heating elements. The melt was kept for soaking for 1 h at around 1050 °C. The glasses were produced by quenching the hot melt on aluminium mould in open atmosphere at room temperature and transferred in annealing furnace which was maintained at 320 °C for 2.30 h to remove thermal stresses and allow to cool up to room temperature.
C DE% = ⎛1‐ ⎞ × 100 C o⎠ ⎝ ⎜
⎟
(1)
where Co and C are the concentrations of MO dye at time 0 and t respectively and t is the irradiation time.
2.2. Heat treatment process
3. Results and discussion
The glass transition temperature (Tg) and crystallization temperature (Tc) were noted from the DTA curve which was carried out by differential thermal analysis and found to be 534 °C and 615 °C respectively [15]. The prepared glass was divided into five small pieces by cutting with a diamond saw. Keeping in mind Tg and Tc, the heat treatment schedule was optimized for the nucleation and crystal growth. The glasses were named as G0, G1, G2, G3, and G4 (Table 1) and the photograph of the glasses are shown in (Fig. 1). These glasses were optically polished for further optical characterizations.
3.1. Phase structure characterization (PXRD) Examination of the phases of CdS quantum dots formed in the heattreated glass is done by the powder X-ray diffraction (PXRD) analysis. Fig. 3 represents powder diffraction pattern of as-prepared glass (G0) sample and glass sample heat treated at 600 °C for 4 h, 8 h, 12 h and 16 h (G1, G2, G3 and G4). The XRD pattern for glass G0 clearly indicates an amorphous nature as no diffraction peak is observed in it. The XRD patterns of glasses G1, G2, G3 and G4 display small diffraction peaks of CdS nanocrystals. It is observed that the intensity of the diffraction peak is very low for G1, G2, G3 and it is more observable for G4. The peak intensity of diffraction peaks is poor in all glasses because of the very low quantity of CdS semiconductor and hence the influence of amorphous nature of glass matrix on crystalline phases is more, therefore, the ordinate data in Fig. 3 is overlapped [17–19]. Diffraction peaks were observed at diffracting angle 2θ= 24.73o, 26.61o, 28.31o, 37.03o, 43.80o and 47.86o which are corresponding to the planes (100), (002), (101), (102), (110) and (103) respectively [20,21]. By using the JCPDS cards, the observed XRD peaks were assigned to the hexagonal wurtzite phases of CdS (reference code 01–075-1545).
2.3. Characterization of glasses The X-ray diffraction (XRD) were reported by PAnalytical X'pert o
Pro. Diffractometer using Cu target with CuK α ⎛⎜λ = 1.5406 A) at 40 mA ⎝ and 45 kV to see the crystalline structure. The glass powder sample was used for XRD measurement with a step size of 0.0170 and 2θ scan range of 20 to 65 degree. The surface morphologies and elemental composition of prepared samples were conducted on field emission scanning electron microscope (FESEM) and energy dispersive spectroscopy (EDS) respectively on a JEOL microscope (JSM-7610F). High resolution transmission electron microscope (HR-TEM, TECNAI G2, 300 kV) is used to investigate the shape and size of CdS QDs and lattice fringe observation. The infrared spectra of prepared glasses were inspected at room temperature by FTIR spectrometer (SHIMADZU IR Affinity-1) in the range of 500–3500 cm− 1. Absorption spectra of prepared glass samples and also methyl orange dye during photocatalytic activity was recorded using a double beam UV–Vis-NIR spectrophotometer (JASCO V-670). A JASCO FP-8200 spectrofluorometer was used to measure the photoluminescence (PL) spectra.
3.2. Morphological study Surface morphology of glass sample G4 was examined by Scanning electron microscope (SEM). Fig. 4 (a) shows an SEM image of a G4 sample in powder form and (b) shows its EDS pattern. Elements obtained in the EDS spectra were well balanced with chemical composition used during synthesis of glass. Fig. 4 (c) and (d) represent FESEM images of CdS quantum dots containing glass. It is observed from this image that the cluster of CdS in the interstitial position is formed in the glass sample. This cluster formation is due to Ostwald ripening process due to heat treatment [22,23].
2.4. Degradation of methyl orange (MO) dye The glass powder of the prepared glass sample was used as a catalyst to study the photocatalytic activity. The structural formula of methyl orange is shown in Fig. 2. For the sunlight/QDs based degradation process, an aqueous QDs containing glass powder dispersion was prepared by adding 0.2 g of glass powder (G0/G4) in a 200 ml, 1.5275* 10−5 mol/L (5 ppm) methyl orange (MO) dye solution. The initial pH of aqueous dye solution was found to nearly 7. The degradation was carried out in sunlight at an
3.3. HRTEM study To obtain the size and shape of grown CdS quantum dot in the glass matrix, High Resolution Transmission Electron Microscopy (HRTEM) study has been performed. Fig. 5 shows that HRTEM micrograph of glass sample heat treated at 600 °C for 16 h (G4). It is observed from this figure that due to the heat treatment spherical black dots of CdS QDs grown in the glass matrix. The size distribution in the range of in 4–16 nm acknowledged (Fig. 5 (c)) in the style of the histogram and mean diameter of CdS QDs was monitored to 9.4 nm [24]. Fig. 5 (b) depicted inverse fast Fourier transform (iFFT) in white (dash) circle that confirms the formation of CdS QDs in which the lattice spacing between
Table 1 Identification of glasses. Sample code
Description
G0 G1 G2 G3 G4
Prepared glass (without heat treatment) Glass heat-treated at 600 °C for 4 h Glass heat-treated at 600 °C for 8 h Glass heat-treated at 600 °C for 12 h Glass heat-treated at 600 °C for 16 h
0
lattice planes (d110) is 2.065 A which is also supported by observed XRD data of the sample. Selected area electron diffraction (SAED) pattern (Fig. 5 (d)) further gave the back up for X-ray diffraction peaks and equated with reference code (01–075-1545) indicating the consistency with data of XRD [25]. 231
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Fig. 1. Glass samples heat treated for different time schedule.
O S O O Na
Eg =
N N
N
hc λ
(2)
where, λ is cut off wavelength, h is Planck's constant and c is velocity of light. The direct energy band gap Egdirect was also calculated by Eq. (3) using Tauc's plot (Fig. 8) and presented in Table 3 [35,36].
αhν = A(hν‐Eg ) n
(3)
where, α is the absorption coefficient, hν is the photon energy, Eg is the energy band gap (direct if n = ½ and indirect if n = 2) and A is a constant depends on the material. It is observed from Table 3 that the energy band gap decreases with annealing time. The decrease in energy band gap could be understood on the basis of the size of QDs. With an increase in annealing time, the size of QDs become widen and hence the band gap decreases and found to be 2.427 eV which is nearly equal to that of bulk one (2.420 eV). The size of CdS quantum dots was calculated from Brus's Eq. (4) and depicted in Table 3 [37]:
Fig. 2. Structural formula of methyl orange (MO) dye.
3.4. FTIR study FT-IR study is carried out to find the structural changes occur in glass structure due to heat treatment. The FTIR spectra were recorded for all the glass in the mid-IR region 500–3500 cm−1 (Fig. 6). The spectra expose the information about the different vibration bands present in glasses. In FTIR spectra, no intrinsic vibrational bands are observed in the range from 1600 to 3500 cm−1. The FTIR spectra show the band at the energy position ~706 cm−1 ~953 cm−1 ~1356 cm−1 , , −1 and ~1514 cm . The various bands position appeared due to heat treatment are depicted in Table 2. It is observed from the table that the transmission peak present at 706 cm−1 is due to the bending of the B-OB bond linkage. Because of applied heat treatment, this band is slightly shifted towards the lower energy value (665 cm−1) [26,27] (Table 2). The formation of BO4 structural unit is the result of alkali oxide added in the glass matrix. The band present at ~953 cm−1 is attributed to SiO-B stretching bond vibrations because of B2O3-SiO2 binary system present in the glass [28]. With an increase in heat treatment schedule, this band is shifting towards lower energy (Fig. 6). BeO vibration of various borate rings is detected at ~1350 cm−1 [29] and stretching of BeO bond in BO3 unit is observed at ~1500 cm−1 [30]. It is observed from this study that FTIR spectra confirm the formation, shift in the band position due to heat treatment, and do not confirm the formation of CdS QDs.
Eg = Egbulk +
π 2ℏ2 ⎡ 1 1 1.8e2 ⎤ + ∗ ⎤‐⎡ ∗ ⎥ ⎢ 2m o r 2 ⎢ m m 4πrεε o⎥ h⎦ ⎣ ⎦ ⎣ e
(4)
Egbulk
is band gap energy of bulk of CdS (2.420 eV), m0 is mass where, of the electron, r is the radius of CdS QDs, ℏ = h 2π here, h is Planck's constant, me∗ and mh∗ are the effective masses of electron and hole and which have the values 0.19 and 0.8 respectively, e is a charge on electron. ε and εo are dielectric coefficient and permittivity of free space respectively. It is observed from Table 3 that the size of CdS QDs increases with increase in annealing time. Thus with an increase in the size of QDs, cut off wavelength shift towards longer wavelengths and hence energy band gap decreases. The average size of CdS QDs is measured by HRTEM and found to be ~9.4 nm (Fig. 5). The size was also calculated (Eq. (4)) using absorption spectra and radius of CdS QDs was found to be 5 nm (diameter - 10 nm) (Table 3). 3.6. QDs size-dependent photoluminescence study
3.5. Optical absorption study The size of grown QDs and quantum confinement is also confirmed by photoluminescence spectra (Fig. 9). When the glass samples excited at Ex = 350 nm, various emission peaks were observed in PL spectra (for G0 to G4). The first peak noticed at wavelength 416 nm for all glasses (G0 to G4) because of compositional impurities or lattice defects present in glass samples [15,27,38]. In addition to this, two extra peaks were observed in all the glasses except G0. The peak assigned at higher energy is due to band edge emission, while the other broad peak originates from trap states at lower energy. From Fig. 9 it is also observed that band edge emission for G1 is 486 nm and it shifts to longer wavelength (497 nm) a with an increase in annealing time. The observed shifts in both absorption and PL bands with heat treatment clearly show that size-tunable optical transitions can be executed in glasses doped with semiconductor QDs. From Fig. 9, it is observed that there is no emission peak appear in G0 glass sample because no QDs grew in the glass sample. For the glass samples G1, G2, G3, and G4 emission peaks were observed at 486 nm, 493 nm, 495 nm, and 497 nm respectively and the intensity of peaks was found to decrease due to increase in size
Change in color of glass samples from transparent to dark yellowish (Fig. 1) may be as a result of nucleation and growth of CdS QDs uniformly distributed in glass matrix [31] with increasing annealing time from 4 h to 16 h, at temperature 600 °C (Table 1). Further confirmation of the growth of CdS QDs is done by absorption and photoluminescence study. Fig. 7 depict the absorption spectra of CdS QDs containing glasses as a function of wavelength with various heat treatment schedule. The absorption band edge for as prepared glass (G0) is found to be at the wavelength of 351.5 nm and with the increase in annealing time the band edge of glasses (G1 to G4) shifted towards longer wavelength. This shift observed for G1 to G4 is very small which is varying from 496.0 nm to 511.0 nm (Table 3). [14,32–34]. This shift in band edge results as an increase in the size of CdS QDs with annealing time and hence decreases the energy band gap of glass samples. Energy band gap (Eg) was calculated by standard Eq. (2): 232
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Fig. 3. PXRD of before (G0) and after heat-treated (G1, G2, G3, G4) glass samples.
peak) at 486 nm (2.511 eV) and another at nearly at 610 nm (trap state emission). From the UV–vis spectrum, an absorption band edge is found at 496 nm (2.500 eV). Interestingly, the energies of band edge emission and absorption band edge are close to each other. Band edge emission shows a blue shift with respect to bulk CdS [42]. The broader and less intense emission peak appears with the larger wavelength shift (Stokes shift) between UV–vis band edge and peak due to trap state at lower energy (2.032 eV), supported by the formation of trap state in QDs. Correlation of transfer of electrons between CB/VB and generated trap states is trap state emission. Broadening in the trap state emission peak
of CdS QDs (inset of Fig. 9) [39,40]. It is observed that the trap state emission (610 nm) is much broader as compared to other emission for the glass sample heat treated for 600 °C similar to our earlier study [15,41]. 3.7. Overlaying of UV–vis and PL of glass G1 Fig. 10 illustrates that the absorption spectrum is ensembled with emission spectrum for the glass sample G1. When glass sample G1 excited at 350 nm (3.543 eV) it gives a band edge emission (excitonic 233
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Fig. 4. (a) SEM image, (b) EDS pattern and (c) and (d) FESEM micrographs of the G4 glass sample.
depends on the size of QDs and on various trap state present in QDs [22,26,43].
ln
C = ‐k appt Co
(5)
where Co and C are the concentrations of dye in solution at time 0 and t respectively and kapp is the apparent first-order reaction rate constant [46]. Fig. 13 shows the plot of ln (C/Co) versus irradiation time for G4 catalyst. The linearity of the plot proposed that the degradation reaction almost follows the pseudo-first-order kinetics having a value of apparent rate reaction is 0.04977 h−1 for the G4 sample in natural sunlight for 6 h [21,44]. The photodegradation efficiency of methyl orange dye was calculated by Eq. (1) and found to be 2.5% in a dark environment and it increases with sunlight irradiation time (Fig. 14). Degradation efficiency was calculated and found to be 29.4% for 6 h sunlight irradiation for glass sample G4 [21]. In order to see the effect of glass without CdS QDs on dye degradation, the experiment was performed for a similar time interval using a glass sample G0. Fig. 15 shows the absorption spectra of MO dye for the glasses with and without CdS QDs (G4 and G0). It is observed from the figure that no degradation is found for the glass sample G0 while G4 shows significant degradation. This study shows that the glasses with CdS QDs will contribute for dye degradation while glass sample without CdS QDs does not play any role in photocatalysis.
3.8. Photocatalytic testing of CdS QDs embedded glass Prepared glass sample was used for its photocatalytic activity by choosing methyl orange (MO) dye as a model organic pollutant. It is noted that photocatalytic activity shows more effective for a sample having less photoluminescence intensity peak [15]. With this background glass sample heat treated for 16 h at 600 °C (G4) is selected for photocatalysis study (Fig. 9). For this experiment, 5 ppm of an aqueous solution of MO dye was prepared and stirred in dark environment for 1 h to obtain adsorption/desorption in catalyst and MO dye. Then the whole setup was brought to natural sunlight and after every 1 h, 4 ml solution was centrifuged for the measurement of optical absorption spectra. Fig. 11 illustrate the absorption spectra of MO dye with an increase in time for sunlight irradiation. It is observed from Fig. 11 that the presence of a catalyst in dark condition shows very small degradation [44]. It is also observed that with an increase in irradiation time corresponding concentration of dye was noted in presence of a catalyst and it is noted that absorbance decreases with increase in irradiation time (Fig. 11). The photocatalytic degradation of every organic dye with the addition of photocatalyst can be properly described by the LangmuirHinshelwood kinetics model (L-H model) [45]. To investigate the reaction kinetics of the MO dye degradation in a quantitative manner, the experimental data were fitted by applying a pseudo-first-order model by Eq. (5). The decrease in the concentration of MO dye with respect to sunlight irradiation time is interpreted as shown in Fig. 12.
3.9. Kinetic study of photocatalysis reaction The semiconductor under irradiation with photon energy larger than band gap energy can generate electron-hole pairs. The holes in the valence band of the semiconductor are strongly oxidizing, and the 234
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Fig. 5. HRTEM of G4 glass sample (a) size of QDs, (b) iFFT, (c) size distribution histogram and (d) SAED pattern of CdS QD.
Fig. 6. FTIR spectra showing structural changes in glasses with heat treatment.
Fig. 7. Absorption spectra of CdS QDs containing glasses.
Table 2 FTIR bands of glasses with different heat treatment. Sample code G0
G1
G2
G3
G4
706 953 1356 –
709 958 1342 1514
689 936 1360 1517
686 936 1380 1530
665 934 1350 1489
Table 3 Cut-off wavelength, energy band gap, radius of QDs and emission peaks.
FTIR assignment
Sample code
Cut-off wavelength λa (nm)
Eg (eV)
Egdirect (eV)
Radius (nm)
Emission wavelength λe (nm)
B-O-B bending vibration. Si-O-B stretching vibrations. B-O vibration of various borate rings. B-O bond stretching in BO3.
G0 G1 G2 G3 G4
351.5 496.0 502.3 504.5 511.0
3.530 2.500 2.468 2.458 2.427
3.862 2.518 2.482 2.470 2.453
– 3.4 3.8 4.0 5.0
416 416, 416, 416, 416,
235
486 493 495 497
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Fig. 8. Direct band gap of all the glasses by Tauc's plot.
Fig. 10. Overlaying of absorption and PL spectra of glass G1.
Fig. 11. Time-dependence absorption spectra of MO dye under sunlight irradiation for G4 sample.
Fig. 9. PL spectra of CdS QDs containing glasses.
electrons in the conduction band are strongly reducing. Thus, both electron and hole will take part in a redox reaction with dissolved oxygen (O2), water (H2O) or organic pollutants [47]. The mechanism involved photodegradation of MO dye by CdS QDs embedded in glasses is demonstrated in the following reactions [15,48] and pictured in Fig. 16. When CdS QDs/MO aqueous solution irradiated under natural sunlight having photon energy greater than the band gap of CdS QDs (2.427 eV), electrons from the valence band (VB) excited to the conduction band (CB) and leaving holes in VB (Eq. (6)). Dissolved oxygen O2 acts as an electron acceptor from the conduction band and form a superoxide anion radical O2•‐ (Eq. (7)) which combines with protons and generate HO2• (Eq. (9)) and then forms H2O2. The hydrogen peroxide (H2O2) further react with photoelectron (Eq. (10)) which generates hydroxyl radicals (OH•). At the same time holes from the VB oxidize the water to yield highly reactive hydroxyl radicals (OH•) which is given by Eq. (8). This OH• radical serves extremely strong oxidizing properties to attack the pollutant and able to degrade the MO dye (Eq. (11)). After attacking of OH• radicals on the aromatic group of MO, the
carbon atom bearing the azo bond, followed by the hydroxylated ring is opened to finally yield CO2 gas, H2O and other by-products [49].
CdS + hυ → CdS + e‐ + h+
(6)
O2 + e‐ → O•‐ 2
(7)
h+
+ H2 O →
OH•
+
H+
+
OH‐
• + O•‐ 2 + H → HO2
H2 O2 +
e‐
→
OH•
(8) (9)
OH• + MO (dye) → Degraded product
(10) (11)
4. Conclusion CdS containing sodium borosilicate glasses were synthesized by conventional melt quench technique. QDs of CdS semiconductor were homogeneously grown by giving optimized controlled heat treatment. The formation of hexagonal phases of CdS QDs is confirmed by XRD 236
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Fig. 14. Degradation efficiency of G4 glass sample for MO dye for time 6 h.
Fig. 12. Concentration (C/Co) of methyl orange under sunlight irradiation in the presence of G4.
Fig. 15. Absorption spectra of MO dye for with and without CdS QDs. Fig. 13. The plot of ln (C/Co) versus irradiation time for G4 catalyst.
measurement. The cluster formation of CdS QDs in glass matrix was observed by surface morphology study and elements present in the glass are verified by FESEM-EDS characterization. The structural changes in functional groups due to heat treatment were examined by FTIR characterization. The mean size of CdS QDs is determined from the HR-TEM analysis and found to be 9.4 nm. The small shift in band edge with an increase in the size of CdS QDs is confirmed by the optical study. Size dependency was also confirmed by photoluminescence spectra and shifting of excitonic peak towards bluish green region due to quantum confinement effect. The degradation ability of prepared glass sample heat treated at 600 °C for 16 h (G4) is tested and degradation efficiency was found to be 29.4% for 6 h sunlight irradiation. Fig. 16. Schematic diagram of a possible photodegradation mechanism of MO dye over CdS QDs embedded glass as a photocatalyst natural solar light irradiation.
Acknowledgments SYJ is very much grateful to VNIT for providing research fellowship. The authors wish to express their gratitude towards SAIF, IIT Bombay for providing characterization facility for this work. 237
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Data availability
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Materials Characterization journal homepage: www.elsevier.com/locate/matchar
Effect of annealing time on optical properties of CdS QDs containing glasses and their application for degradation of methyl orange dye S.Y. Janbandhu, S.R. Munishwar, G.K. Sukhadeve, R.S. Gedam
T
⁎
Department of Physics, Visvesvaraya National Institute of Technology, Nagpur 440010, India
A R T I C LE I N FO
A B S T R A C T
Keywords: CdS QDs HRTEM UV–vis Photoluminescence Methyl orange dye Photodegradation
Borosilicate glass system (SiO2:B2O3:Na2O:ZnO) with the addition of 3 wt% CdS was prepared by conventional melt quench technique. Amorphous nature of glass sample was confirmed from XRD measurement. CdS QDs were grown in the glass matrix using an optimized heat treatment schedule in between glass transition (Tg) and crystallization (Tc) temperature. The phases of CdS QDs were noted by XRD and confirmed by HRTEM. Optical absorption study was carried out to measure the energy band gap and it is found that the energy band gap decreases with an increase in the size of QDs. Size determination was done by optical absorption spectra and match with HRTEM results. This change in the size of QDs was confirmed by PL spectra. The optimized glass sample was used for the degradation of methyl orange (MO) dye.
1. Introduction Nanostructured materials are considered as an important bridge between the molecular level and bulk. The nanocrystal of semiconductors having discrete energy levels it follows the quantum confinement effect are known as semiconductor quantum dots (SQDs) [1]. Semiconductor quantum dots (QDs) have maintained its peak level in several fields of science like optoelectronics [2] photonic device [3] and biomedical applications [4] since the last few decades. Tunable properties of materials by increasing or decreasing the size of QDs provide a strong hope to apply in various fields like in optical communication [5], signal transmission [6] and solar cell industry [7]. The eco-friendly and clean energy source is needed in order to consider environmental pollution as well as the shortcoming of natural energy resources. Photocatalysis is one of the most promising methods which not only removes the environmental pollutants but also a powerful way to tackle solar energy as substitutional natural energy [8]. Semiconductor nanomaterials, specifically TiO2 bears tremendous belief in helping the energy crisis over effective utilization of solar energy by using photovoltaic and water-splitting devices [9]. Nevertheless, its wide band gap (greater than 3 eV) restricts it to use in broad range spectrum. As an optional semiconductor, CdS nanostructure has been broadly investigated [10,11]. Beside the improvement of the properties of CdS at the nano level, it has the challenge of agglomeration and instability. Photocorrosion is one of the biggest issues during photocatalysis [12]. In order to use these QDs in practical applications by
⁎
fixing the challenges mentioned above, some inert and inorganic solid matrix is needed to embed them. In the optical point of view, the glass matrix has been selected for the incorporation of QDs which also serves the thermal and mechanical stability [13,14]. By altering the shape and size, QDs can be manipulated to emit over a broad range of wavelengths. In our earlier reported article, optical properties of glasses containing CdS QDs by varying heat treatment temperature was studied [15]. In that study, it was observed that with an increase in temperature the PL intensity was decreasing due to the less electron-hole recombination. In the photocatalysis process low electron-hole recombination is favorable for better activity [16]. The glass sample heat treated at the highest temperature (low emission intensity) among all the samples was used for degradation of methylene blue (MB) dye and degradation efficiency were found to be 70% for 270 min of natural sunlight irradiation. In this paper, an attempt is made to see the effect of annealing time on the size of CdS QDs and to use these QDs for methyl orange (MO) dye degradation. 2. Experimental section 2.1. Synthesis of CdS embedded borosilicate glass For the synthesis of CdS containing glass, melt quench technique is used. The raw materials were taken in the composition of 40 mol% SiO2, 28 mol% B2O3, 22 mol% Na2O and 10 mol% ZnO (purity greater
Corresponding author. E-mail address: rupesh_gedam@rediffmail.com (R.S. Gedam).
https://doi.org/10.1016/j.matchar.2019.04.027 Received 15 December 2018; Received in revised form 9 March 2019; Accepted 22 April 2019 Available online 24 April 2019 1044-5803/ © 2019 Elsevier Inc. All rights reserved.
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average energy density of solar radiation about 1100 W/m2. The catalyst added to the MO dye solution and stirred in dark for 1 h before the irradiation. 4 ml dispersions were taken at the equal interval of time and centrifuged to analyze remaining dye concentration by JASCO V670 spectrophotometer before and after irradiation at λmax = 463 nm. The degradation efficiency was calculated by using Eq. (1):
than 99%, Sigma Aldrich, Merk). A dry powder of these raw materials was thoroughly mixed by agate mortar and pestle to get a homogeneous mixture and fine powder. After that, Cadmium sulfide (CdS 3 wt%, 99.999% purity, alfa aesar) was added in glass composition and ground up to 2 h in order to enhance the homogeneity of the mixture further. This mixture was taken in a platinum crucible and transferred into the high-temperature furnace with silicon carbide heating elements. The melt was kept for soaking for 1 h at around 1050 °C. The glasses were produced by quenching the hot melt on aluminium mould in open atmosphere at room temperature and transferred in annealing furnace which was maintained at 320 °C for 2.30 h to remove thermal stresses and allow to cool up to room temperature.
C DE% = ⎛1‐ ⎞ × 100 C o⎠ ⎝ ⎜
⎟
(1)
where Co and C are the concentrations of MO dye at time 0 and t respectively and t is the irradiation time.
2.2. Heat treatment process
3. Results and discussion
The glass transition temperature (Tg) and crystallization temperature (Tc) were noted from the DTA curve which was carried out by differential thermal analysis and found to be 534 °C and 615 °C respectively [15]. The prepared glass was divided into five small pieces by cutting with a diamond saw. Keeping in mind Tg and Tc, the heat treatment schedule was optimized for the nucleation and crystal growth. The glasses were named as G0, G1, G2, G3, and G4 (Table 1) and the photograph of the glasses are shown in (Fig. 1). These glasses were optically polished for further optical characterizations.
3.1. Phase structure characterization (PXRD) Examination of the phases of CdS quantum dots formed in the heattreated glass is done by the powder X-ray diffraction (PXRD) analysis. Fig. 3 represents powder diffraction pattern of as-prepared glass (G0) sample and glass sample heat treated at 600 °C for 4 h, 8 h, 12 h and 16 h (G1, G2, G3 and G4). The XRD pattern for glass G0 clearly indicates an amorphous nature as no diffraction peak is observed in it. The XRD patterns of glasses G1, G2, G3 and G4 display small diffraction peaks of CdS nanocrystals. It is observed that the intensity of the diffraction peak is very low for G1, G2, G3 and it is more observable for G4. The peak intensity of diffraction peaks is poor in all glasses because of the very low quantity of CdS semiconductor and hence the influence of amorphous nature of glass matrix on crystalline phases is more, therefore, the ordinate data in Fig. 3 is overlapped [17–19]. Diffraction peaks were observed at diffracting angle 2θ= 24.73o, 26.61o, 28.31o, 37.03o, 43.80o and 47.86o which are corresponding to the planes (100), (002), (101), (102), (110) and (103) respectively [20,21]. By using the JCPDS cards, the observed XRD peaks were assigned to the hexagonal wurtzite phases of CdS (reference code 01–075-1545).
2.3. Characterization of glasses The X-ray diffraction (XRD) were reported by PAnalytical X'pert o
Pro. Diffractometer using Cu target with CuK α ⎛⎜λ = 1.5406 A) at 40 mA ⎝ and 45 kV to see the crystalline structure. The glass powder sample was used for XRD measurement with a step size of 0.0170 and 2θ scan range of 20 to 65 degree. The surface morphologies and elemental composition of prepared samples were conducted on field emission scanning electron microscope (FESEM) and energy dispersive spectroscopy (EDS) respectively on a JEOL microscope (JSM-7610F). High resolution transmission electron microscope (HR-TEM, TECNAI G2, 300 kV) is used to investigate the shape and size of CdS QDs and lattice fringe observation. The infrared spectra of prepared glasses were inspected at room temperature by FTIR spectrometer (SHIMADZU IR Affinity-1) in the range of 500–3500 cm− 1. Absorption spectra of prepared glass samples and also methyl orange dye during photocatalytic activity was recorded using a double beam UV–Vis-NIR spectrophotometer (JASCO V-670). A JASCO FP-8200 spectrofluorometer was used to measure the photoluminescence (PL) spectra.
3.2. Morphological study Surface morphology of glass sample G4 was examined by Scanning electron microscope (SEM). Fig. 4 (a) shows an SEM image of a G4 sample in powder form and (b) shows its EDS pattern. Elements obtained in the EDS spectra were well balanced with chemical composition used during synthesis of glass. Fig. 4 (c) and (d) represent FESEM images of CdS quantum dots containing glass. It is observed from this image that the cluster of CdS in the interstitial position is formed in the glass sample. This cluster formation is due to Ostwald ripening process due to heat treatment [22,23].
2.4. Degradation of methyl orange (MO) dye The glass powder of the prepared glass sample was used as a catalyst to study the photocatalytic activity. The structural formula of methyl orange is shown in Fig. 2. For the sunlight/QDs based degradation process, an aqueous QDs containing glass powder dispersion was prepared by adding 0.2 g of glass powder (G0/G4) in a 200 ml, 1.5275* 10−5 mol/L (5 ppm) methyl orange (MO) dye solution. The initial pH of aqueous dye solution was found to nearly 7. The degradation was carried out in sunlight at an
3.3. HRTEM study To obtain the size and shape of grown CdS quantum dot in the glass matrix, High Resolution Transmission Electron Microscopy (HRTEM) study has been performed. Fig. 5 shows that HRTEM micrograph of glass sample heat treated at 600 °C for 16 h (G4). It is observed from this figure that due to the heat treatment spherical black dots of CdS QDs grown in the glass matrix. The size distribution in the range of in 4–16 nm acknowledged (Fig. 5 (c)) in the style of the histogram and mean diameter of CdS QDs was monitored to 9.4 nm [24]. Fig. 5 (b) depicted inverse fast Fourier transform (iFFT) in white (dash) circle that confirms the formation of CdS QDs in which the lattice spacing between
Table 1 Identification of glasses. Sample code
Description
G0 G1 G2 G3 G4
Prepared glass (without heat treatment) Glass heat-treated at 600 °C for 4 h Glass heat-treated at 600 °C for 8 h Glass heat-treated at 600 °C for 12 h Glass heat-treated at 600 °C for 16 h
0
lattice planes (d110) is 2.065 A which is also supported by observed XRD data of the sample. Selected area electron diffraction (SAED) pattern (Fig. 5 (d)) further gave the back up for X-ray diffraction peaks and equated with reference code (01–075-1545) indicating the consistency with data of XRD [25]. 231
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Fig. 1. Glass samples heat treated for different time schedule.
O S O O Na
Eg =
N N
N
hc λ
(2)
where, λ is cut off wavelength, h is Planck's constant and c is velocity of light. The direct energy band gap Egdirect was also calculated by Eq. (3) using Tauc's plot (Fig. 8) and presented in Table 3 [35,36].
αhν = A(hν‐Eg ) n
(3)
where, α is the absorption coefficient, hν is the photon energy, Eg is the energy band gap (direct if n = ½ and indirect if n = 2) and A is a constant depends on the material. It is observed from Table 3 that the energy band gap decreases with annealing time. The decrease in energy band gap could be understood on the basis of the size of QDs. With an increase in annealing time, the size of QDs become widen and hence the band gap decreases and found to be 2.427 eV which is nearly equal to that of bulk one (2.420 eV). The size of CdS quantum dots was calculated from Brus's Eq. (4) and depicted in Table 3 [37]:
Fig. 2. Structural formula of methyl orange (MO) dye.
3.4. FTIR study FT-IR study is carried out to find the structural changes occur in glass structure due to heat treatment. The FTIR spectra were recorded for all the glass in the mid-IR region 500–3500 cm−1 (Fig. 6). The spectra expose the information about the different vibration bands present in glasses. In FTIR spectra, no intrinsic vibrational bands are observed in the range from 1600 to 3500 cm−1. The FTIR spectra show the band at the energy position ~706 cm−1 ~953 cm−1 ~1356 cm−1 , , −1 and ~1514 cm . The various bands position appeared due to heat treatment are depicted in Table 2. It is observed from the table that the transmission peak present at 706 cm−1 is due to the bending of the B-OB bond linkage. Because of applied heat treatment, this band is slightly shifted towards the lower energy value (665 cm−1) [26,27] (Table 2). The formation of BO4 structural unit is the result of alkali oxide added in the glass matrix. The band present at ~953 cm−1 is attributed to SiO-B stretching bond vibrations because of B2O3-SiO2 binary system present in the glass [28]. With an increase in heat treatment schedule, this band is shifting towards lower energy (Fig. 6). BeO vibration of various borate rings is detected at ~1350 cm−1 [29] and stretching of BeO bond in BO3 unit is observed at ~1500 cm−1 [30]. It is observed from this study that FTIR spectra confirm the formation, shift in the band position due to heat treatment, and do not confirm the formation of CdS QDs.
Eg = Egbulk +
π 2ℏ2 ⎡ 1 1 1.8e2 ⎤ + ∗ ⎤‐⎡ ∗ ⎥ ⎢ 2m o r 2 ⎢ m m 4πrεε o⎥ h⎦ ⎣ ⎦ ⎣ e
(4)
Egbulk
is band gap energy of bulk of CdS (2.420 eV), m0 is mass where, of the electron, r is the radius of CdS QDs, ℏ = h 2π here, h is Planck's constant, me∗ and mh∗ are the effective masses of electron and hole and which have the values 0.19 and 0.8 respectively, e is a charge on electron. ε and εo are dielectric coefficient and permittivity of free space respectively. It is observed from Table 3 that the size of CdS QDs increases with increase in annealing time. Thus with an increase in the size of QDs, cut off wavelength shift towards longer wavelengths and hence energy band gap decreases. The average size of CdS QDs is measured by HRTEM and found to be ~9.4 nm (Fig. 5). The size was also calculated (Eq. (4)) using absorption spectra and radius of CdS QDs was found to be 5 nm (diameter - 10 nm) (Table 3). 3.6. QDs size-dependent photoluminescence study
3.5. Optical absorption study The size of grown QDs and quantum confinement is also confirmed by photoluminescence spectra (Fig. 9). When the glass samples excited at Ex = 350 nm, various emission peaks were observed in PL spectra (for G0 to G4). The first peak noticed at wavelength 416 nm for all glasses (G0 to G4) because of compositional impurities or lattice defects present in glass samples [15,27,38]. In addition to this, two extra peaks were observed in all the glasses except G0. The peak assigned at higher energy is due to band edge emission, while the other broad peak originates from trap states at lower energy. From Fig. 9 it is also observed that band edge emission for G1 is 486 nm and it shifts to longer wavelength (497 nm) a with an increase in annealing time. The observed shifts in both absorption and PL bands with heat treatment clearly show that size-tunable optical transitions can be executed in glasses doped with semiconductor QDs. From Fig. 9, it is observed that there is no emission peak appear in G0 glass sample because no QDs grew in the glass sample. For the glass samples G1, G2, G3, and G4 emission peaks were observed at 486 nm, 493 nm, 495 nm, and 497 nm respectively and the intensity of peaks was found to decrease due to increase in size
Change in color of glass samples from transparent to dark yellowish (Fig. 1) may be as a result of nucleation and growth of CdS QDs uniformly distributed in glass matrix [31] with increasing annealing time from 4 h to 16 h, at temperature 600 °C (Table 1). Further confirmation of the growth of CdS QDs is done by absorption and photoluminescence study. Fig. 7 depict the absorption spectra of CdS QDs containing glasses as a function of wavelength with various heat treatment schedule. The absorption band edge for as prepared glass (G0) is found to be at the wavelength of 351.5 nm and with the increase in annealing time the band edge of glasses (G1 to G4) shifted towards longer wavelength. This shift observed for G1 to G4 is very small which is varying from 496.0 nm to 511.0 nm (Table 3). [14,32–34]. This shift in band edge results as an increase in the size of CdS QDs with annealing time and hence decreases the energy band gap of glass samples. Energy band gap (Eg) was calculated by standard Eq. (2): 232
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Fig. 3. PXRD of before (G0) and after heat-treated (G1, G2, G3, G4) glass samples.
peak) at 486 nm (2.511 eV) and another at nearly at 610 nm (trap state emission). From the UV–vis spectrum, an absorption band edge is found at 496 nm (2.500 eV). Interestingly, the energies of band edge emission and absorption band edge are close to each other. Band edge emission shows a blue shift with respect to bulk CdS [42]. The broader and less intense emission peak appears with the larger wavelength shift (Stokes shift) between UV–vis band edge and peak due to trap state at lower energy (2.032 eV), supported by the formation of trap state in QDs. Correlation of transfer of electrons between CB/VB and generated trap states is trap state emission. Broadening in the trap state emission peak
of CdS QDs (inset of Fig. 9) [39,40]. It is observed that the trap state emission (610 nm) is much broader as compared to other emission for the glass sample heat treated for 600 °C similar to our earlier study [15,41]. 3.7. Overlaying of UV–vis and PL of glass G1 Fig. 10 illustrates that the absorption spectrum is ensembled with emission spectrum for the glass sample G1. When glass sample G1 excited at 350 nm (3.543 eV) it gives a band edge emission (excitonic 233
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Fig. 4. (a) SEM image, (b) EDS pattern and (c) and (d) FESEM micrographs of the G4 glass sample.
depends on the size of QDs and on various trap state present in QDs [22,26,43].
ln
C = ‐k appt Co
(5)
where Co and C are the concentrations of dye in solution at time 0 and t respectively and kapp is the apparent first-order reaction rate constant [46]. Fig. 13 shows the plot of ln (C/Co) versus irradiation time for G4 catalyst. The linearity of the plot proposed that the degradation reaction almost follows the pseudo-first-order kinetics having a value of apparent rate reaction is 0.04977 h−1 for the G4 sample in natural sunlight for 6 h [21,44]. The photodegradation efficiency of methyl orange dye was calculated by Eq. (1) and found to be 2.5% in a dark environment and it increases with sunlight irradiation time (Fig. 14). Degradation efficiency was calculated and found to be 29.4% for 6 h sunlight irradiation for glass sample G4 [21]. In order to see the effect of glass without CdS QDs on dye degradation, the experiment was performed for a similar time interval using a glass sample G0. Fig. 15 shows the absorption spectra of MO dye for the glasses with and without CdS QDs (G4 and G0). It is observed from the figure that no degradation is found for the glass sample G0 while G4 shows significant degradation. This study shows that the glasses with CdS QDs will contribute for dye degradation while glass sample without CdS QDs does not play any role in photocatalysis.
3.8. Photocatalytic testing of CdS QDs embedded glass Prepared glass sample was used for its photocatalytic activity by choosing methyl orange (MO) dye as a model organic pollutant. It is noted that photocatalytic activity shows more effective for a sample having less photoluminescence intensity peak [15]. With this background glass sample heat treated for 16 h at 600 °C (G4) is selected for photocatalysis study (Fig. 9). For this experiment, 5 ppm of an aqueous solution of MO dye was prepared and stirred in dark environment for 1 h to obtain adsorption/desorption in catalyst and MO dye. Then the whole setup was brought to natural sunlight and after every 1 h, 4 ml solution was centrifuged for the measurement of optical absorption spectra. Fig. 11 illustrate the absorption spectra of MO dye with an increase in time for sunlight irradiation. It is observed from Fig. 11 that the presence of a catalyst in dark condition shows very small degradation [44]. It is also observed that with an increase in irradiation time corresponding concentration of dye was noted in presence of a catalyst and it is noted that absorbance decreases with increase in irradiation time (Fig. 11). The photocatalytic degradation of every organic dye with the addition of photocatalyst can be properly described by the LangmuirHinshelwood kinetics model (L-H model) [45]. To investigate the reaction kinetics of the MO dye degradation in a quantitative manner, the experimental data were fitted by applying a pseudo-first-order model by Eq. (5). The decrease in the concentration of MO dye with respect to sunlight irradiation time is interpreted as shown in Fig. 12.
3.9. Kinetic study of photocatalysis reaction The semiconductor under irradiation with photon energy larger than band gap energy can generate electron-hole pairs. The holes in the valence band of the semiconductor are strongly oxidizing, and the 234
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Fig. 5. HRTEM of G4 glass sample (a) size of QDs, (b) iFFT, (c) size distribution histogram and (d) SAED pattern of CdS QD.
Fig. 6. FTIR spectra showing structural changes in glasses with heat treatment.
Fig. 7. Absorption spectra of CdS QDs containing glasses.
Table 2 FTIR bands of glasses with different heat treatment. Sample code G0
G1
G2
G3
G4
706 953 1356 –
709 958 1342 1514
689 936 1360 1517
686 936 1380 1530
665 934 1350 1489
Table 3 Cut-off wavelength, energy band gap, radius of QDs and emission peaks.
FTIR assignment
Sample code
Cut-off wavelength λa (nm)
Eg (eV)
Egdirect (eV)
Radius (nm)
Emission wavelength λe (nm)
B-O-B bending vibration. Si-O-B stretching vibrations. B-O vibration of various borate rings. B-O bond stretching in BO3.
G0 G1 G2 G3 G4
351.5 496.0 502.3 504.5 511.0
3.530 2.500 2.468 2.458 2.427
3.862 2.518 2.482 2.470 2.453
– 3.4 3.8 4.0 5.0
416 416, 416, 416, 416,
235
486 493 495 497
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Fig. 8. Direct band gap of all the glasses by Tauc's plot.
Fig. 10. Overlaying of absorption and PL spectra of glass G1.
Fig. 11. Time-dependence absorption spectra of MO dye under sunlight irradiation for G4 sample.
Fig. 9. PL spectra of CdS QDs containing glasses.
electrons in the conduction band are strongly reducing. Thus, both electron and hole will take part in a redox reaction with dissolved oxygen (O2), water (H2O) or organic pollutants [47]. The mechanism involved photodegradation of MO dye by CdS QDs embedded in glasses is demonstrated in the following reactions [15,48] and pictured in Fig. 16. When CdS QDs/MO aqueous solution irradiated under natural sunlight having photon energy greater than the band gap of CdS QDs (2.427 eV), electrons from the valence band (VB) excited to the conduction band (CB) and leaving holes in VB (Eq. (6)). Dissolved oxygen O2 acts as an electron acceptor from the conduction band and form a superoxide anion radical O2•‐ (Eq. (7)) which combines with protons and generate HO2• (Eq. (9)) and then forms H2O2. The hydrogen peroxide (H2O2) further react with photoelectron (Eq. (10)) which generates hydroxyl radicals (OH•). At the same time holes from the VB oxidize the water to yield highly reactive hydroxyl radicals (OH•) which is given by Eq. (8). This OH• radical serves extremely strong oxidizing properties to attack the pollutant and able to degrade the MO dye (Eq. (11)). After attacking of OH• radicals on the aromatic group of MO, the
carbon atom bearing the azo bond, followed by the hydroxylated ring is opened to finally yield CO2 gas, H2O and other by-products [49].
CdS + hυ → CdS + e‐ + h+
(6)
O2 + e‐ → O•‐ 2
(7)
h+
+ H2 O →
OH•
+
H+
+
OH‐
• + O•‐ 2 + H → HO2
H2 O2 +
e‐
→
OH•
(8) (9)
OH• + MO (dye) → Degraded product
(10) (11)
4. Conclusion CdS containing sodium borosilicate glasses were synthesized by conventional melt quench technique. QDs of CdS semiconductor were homogeneously grown by giving optimized controlled heat treatment. The formation of hexagonal phases of CdS QDs is confirmed by XRD 236
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Fig. 14. Degradation efficiency of G4 glass sample for MO dye for time 6 h.
Fig. 12. Concentration (C/Co) of methyl orange under sunlight irradiation in the presence of G4.
Fig. 15. Absorption spectra of MO dye for with and without CdS QDs. Fig. 13. The plot of ln (C/Co) versus irradiation time for G4 catalyst.
measurement. The cluster formation of CdS QDs in glass matrix was observed by surface morphology study and elements present in the glass are verified by FESEM-EDS characterization. The structural changes in functional groups due to heat treatment were examined by FTIR characterization. The mean size of CdS QDs is determined from the HR-TEM analysis and found to be 9.4 nm. The small shift in band edge with an increase in the size of CdS QDs is confirmed by the optical study. Size dependency was also confirmed by photoluminescence spectra and shifting of excitonic peak towards bluish green region due to quantum confinement effect. The degradation ability of prepared glass sample heat treated at 600 °C for 16 h (G4) is tested and degradation efficiency was found to be 29.4% for 6 h sunlight irradiation. Fig. 16. Schematic diagram of a possible photodegradation mechanism of MO dye over CdS QDs embedded glass as a photocatalyst natural solar light irradiation.
Acknowledgments SYJ is very much grateful to VNIT for providing research fellowship. The authors wish to express their gratitude towards SAIF, IIT Bombay for providing characterization facility for this work. 237
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Data availability
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