Synthesis, characterization and photocatalytic degradation efficiency of CdS quantum dots embedded in sodium borosilicate glasses

Accepted Manuscript Title: Synthesis, characterization and photocatalytic degradation efficiency of CdS quantum dots embedded in sodium borosilicate glasses Authors: S.Y. Janbandhu, S.R. Munishwar, R.S. Gedam PII: DOI: Reference:

S0169-4332(18)30409-4 https://doi.org/10.1016/j.apsusc.2018.02.065 APSUSC 38523

To appear in:

APSUSC

Received date: Revised date: Accepted date:

23-9-2017 2-2-2018 6-2-2018

Please cite this article as: S.Y.Janbandhu, S.R.Munishwar, R.S.Gedam, Synthesis, characterization and photocatalytic degradation efficiency of CdS quantum dots embedded in sodium borosilicate glasses, Applied Surface Science https://doi.org/10.1016/j.apsusc.2018.02.065 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Synthesis, characterization and photocatalytic degradation efficiency of CdS quantum dots embedded in sodium borosilicate glasses

S.Y. Janbandhu, S. R. Munishwar, R. S. Gedam* Department of Physics, Visvesvaraya National Institute of Technology, Nagpur-440010,

Corresponding author:- [email protected]

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India

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Graphical abstract

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Highlights 

CdS embedded simple borosilicate glass system were prepared by conventional melt quench technique.

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Growth of CdS QDs was controlled by optimized heat treatment schedule and confirmed by various characterization techniques.

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The size of CdS QDs increases and energy band gap decreases with increasing heat treatment temperature.

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Optical properties were tuned by controlling size of CdS QDs.

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Prepared CdS embedded glasses shows good consistency for dye degradation of MB

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dye.

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ABSTRACT:

Borosilicate glass system (SiO2-B2O3-Na2O-ZnO) with the addition of 3 wt% CdS was

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prepared by conventional melt quenching method. Amorphous nature of glass sample was

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confirmed by XRD measurement. Glass transition temperature and crystallization

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temperature was determined from DTA curve. CdS quantum dots (QDs) were grown in this

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glass matrix by optimized heat treatment schedule which was confirmed by XRD measurement. The growth of CdS QDs was also confirmed by optical absorption spectra and

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photoluminescence (PL) measurement. Energy band gap was calculated from the absorption

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spectra and found to decrease with increase in size of QDs. The size of QDs measured by optical absorption and it was also confirmed by HR-TEM results. The increase in size of QDs

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is also noticed from emission spectra which show red shift in PL intensity. The glass embedded with CdS QDs were tested for photocatalysis study and it shows 70.6% and 68.0%

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photodegradation efficiency for 5 ppm and 10 ppm respectively. Keywords: QDs; XRD; HR-TEM; UV-vis; PL; Photocatalysis

1.

Introduction

In last decade, most of the single-compound semiconductor quantum dot materials have been found out for their optical properties such as ZnO, CdS, CdSe, ZnSe, and ZnS [15]. Detailed studies on the optical properties of these II–VI semiconductors in bulk form have been reported [6, 7] in the literature. When the size of these semiconductor nanocrystals (NCs) is less than the exciton Bohr radius, which is the characteristic distance between the

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excited electron and the hole in the material, the electronic energy levels turn into discrete levels. This effect is known as the quantum confinement effect and the NCs are typically

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termed as quantum dots (QDs) [8]. Generally, quantum dots are unstable and hence difficult

to use for practical applications such as optoelectronic devices. Glass matrix provides the best

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option to incorporate the QDs because of its good optical and mechanical properties also

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glass provides chemical and thermal stability [9, 10]. The semiconductor like TiO2QDs is

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used as a catalyst for water treatment. These TiO2 QDs have a large band gap (3.2 eV) which

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needs UV light to excite the electrons from valence band to conduction band. So there is need of material having absorption in the visible range. With this background materials with

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narrow band gaps capable of wide absorption of visible solar light stand up advantageous and superior. Of particular interest CdS that has been extensively studied [11-13] because of its

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band gap in visible (2.420 eV) region. The glasses containing CdS QDs have been used for the optical filter. These glasses are colourless when made by conventional melt quenching

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method and obtain colours only after heat treatment due to the growth of CdS nanocrystals [14]. This optimum band gap and its thermodynamic as well as electrochemical properties

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play a very important role in photocatalysis. Again it is noticed that naked QDs suffer from photo-corrosion issue in photocatalysis which can be solved by embedding them into glass matrix [15]. In this article we have successively grown CdS QDs in very simple borosilicate glass system (SiO2-B2O3-Na2O-ZnO) by the single step heat treatment process. The growth of QDs

is confirmed by obtained colour to the glass. By controlling the size of QDs by heat treatment we have tuned the optical properties. These glasses have been characterized by XRD, HRTEM, UV-vis and PL. Prepared glasses have been used for photocatalysis study.

2.1.

Preparation of CdS containing borosilicate glass system

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2. Experimental

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The starting chemicals for glass preparation by melt quench technique were 40 SiO2, 28 B2O3, 22 Na2O and 10 ZnO (all in mole%,  99% purity, Sigma Aldrich). These raw

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materials were thoroughly mixed by agate mortar and pestle to get a homogeneous mixture.

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Cadmium sulfide (CdS 3 wt%, 99.999% purity) was added in above glass composition and

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mixed up to 2hrs in order to enhance homogeneous mixture. After mixing, the mixture was

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taken in a platinum crucible and transferred into high-temperature furnace. The melt was kept for homogenized for 1 hr at around 1050 0C. After refining, the glasses were air quenched on

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aluminium mould at room temperature and processed immediately for annealing at 320 0C for 2.30 hrs to remove thermal stresses in the glass and cooled up to room temperature. These

Growth of CdS in Glass by heat treatment

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2.2.

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cooled transparent glasses are ready for the further procedure.

The differential thermal analysis (DTA) measurement was carried out by using

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HITACHI STA7200. The glass transition temperature (Tg = 5340C) and crystallization temperature (Tc = 6150C) were found by DTA curve (Fig. 1). The prepared glass was cut into five small pieces. The glass piece namely GQ0 was as made glass (uncoloured) without heat treated. Remaining glass pieces namely GQ1, GQ2, GQ3 and GQ4 were heat treated at 550

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C, 570 0C, 590 0C, and 610 0C for 12 hrs respectively (Table 1). The visual image of glass

sample is depicted in Fig. 2.

Characterization

GQ0

GQ3

GQ2

GQ1

GQ4

0

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The powder X-ray diffraction pattern were recorded by PAnalytical X’pert Pro.

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Diffractometer with CuK α (λ=1.5406 A) . Determination of size of CdS QDs and lattice fringe

observation were performed by High resolution transmission electron microscope (HR-TEM, 300 kV). Optical characterizations of all the glasses were studied by using UV-Vis-NIR

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2.4. Photocatalytic degradation of Methylene blue (MB)

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Spectrophotometer (JASCO V-670) and spectrofluorometer (JASCO FP-8200).

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The photocatalytic activity of CdS QDs embedded in the glass was carried out. The glass

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powder was used for this study. The photocatalysis study was done using methylene blue

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(MB) as an organic pollutant under irradiation of sunlight. For this 100 ml aqueous solution of MB of different concentrations was taken with the immersion of 0.1 g of glass powder as a

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catalyst. The suspension was magnetically stirred in absence of light exposure to get perfect adsorption/desorption in catalyst and dye. After that, photocatalysis reaction was started

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when the suspension exposed in sunlight. In successive time interval, 3 ml solution was taken out and centrifuged. By using UV-vis-NIR spectrophotometer (JASCO V-670) the

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concentration of MB was checked at 663.0 nm.

3. Results and discussion 3.1. XRD analysis

The formation of quantum dots of CdS in borosilicate glass matrix could be investigated by XRD pattern. Figure 3 shows the XRD pattern of without heat treated glass and glass heat treated at 590 0C for 12 h respectively. XRD pattern for glass GQ0 confirms the amorphous nature of glass in which no diffraction peaks were observed. But due to controlled heat treatment, some diffraction peak appeared in glass sample GQ3. The diffraction peaks were

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observed at diffracting angle 2𝜃= 24.800, 26.590, 28.330 and 43.840 which could be indexed as (100), (002), (101) and (110) planes of hexagonal phases of CdS QDs respectively (reference

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code 01–075-1545). Since the amount of CdS is very less and amorphous nature of glass dominates the crystallinity of CdS peak, the diffraction peak intensity is less [14, 16-19].

HR-TEM analysis

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3.2.

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High resolution transmission electron micrographs (HR-TEM, 300kV) were taken to

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identify the size and shape of QDs formation. Fig. 4 shows the HR-TEM pictures of

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ground fine powder of CdS containing glass heat treated at 590 0C for 12 hrs. (GQ3). The observed black dots in Fig. 4(a) and 4(b) (dash-dot-dot circles) confirm the growth of

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CdS QDs which are spherical in shape having an average diameter in the range of 5-8 nm.

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HR-TEM image 4(c) reveals lattice fringes (in a white dashed circle) which confirm CdS QDs formation in glass medium. The distance between two (110) lattice planes is

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evaluated of 0.207 nm. Diffraction pattern for respected diffraction peak is shown in the inset of 4(c). Very few bright spots were seen in SEAD as glassy nature dominates the crystallization of CdS QDs. The SAED pattern very well matched with reference code

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01– 075-1545 indicating the uniformity with XRD [14, 15, 20].

3.3. Absorption spectra

Fig. 2 shows the photograph of GQ0, GQ1, GQ2, GQ3 and GQ4 glass sample. From this photograph, it is seen that as made glass sample (GQ0) is transparent and the glass sample become dark yellow with an increase in annealing temperature from 550 0C to 610 0C (GQ1 to GQ4) for constant time 12 hrs. This change in colour may be due to increase in the size of CdS QDs in the glass matrix. This change in colour is also confirmed by UV-vis and PL

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study.

The absorption spectrum of CdS containing glasses with different heat-treatment schedule is

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shown in Fig. 5. It is observed from these spectra that cut off wavelength shift towards larger

wavelength with an increase in temperature as compared to as made glass sample (GQ0).

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This red shift in absorption spectra confirm the increase in the size of QDs. Grown CdS QDs

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showed the clear shift of band edge from QDs (467.5 nm) to bulk CdS (512.0 nm) due to

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quantum confinement effect [21].

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This red shift of wavelength (i.e. blue shift in energy) governs the decrease in band gap with

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respect to bulk CdS [22] and indicates the increase in the size of QDs results in a change in colour of glasses from transparent to dark yellow.

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The effective energy band gap E opt was calculated by standard equation ( Egopt  g

hc



) and

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shown in Table 2.

The direct energy band gap E gdirect was also calculated by equation (1) using Tau’s plot (inset

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of Fig. 5) and these values are depicted in Table 2[16]. n αhν = A(hν-E opt g )

--(1)

Where, α is the absorption coefficient, hν is photon energy, E opt g is optical energy band gap and A is the constant which is depend on transitions and n = 2 and 1/2 are indirect allowed transition and direct allowed transition respectively.

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direct It is observed from Table 2 that E opt and E g are decrease with increase in heat treatment. g

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The observed E opt suggest that these QDs can be used in visible transmitting applications g since the range of band gap for visible transmission is 1.500–3.000 eV [23].

h2 1 1 1.8e2 + [ + ]-[ ] 8m0 r 2 m*e m*h 4πrεε 0

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=

E gbulk

--(2)

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E opt g

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The size of quantum dots was calculated from Brus's equation (2) [24]:

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bulk Where, E opt is band gap energy of bulk of CdS (2.420eV), g is the optical band gap energy, E g

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m 0 is mass of electron, r is radius of CdS QDs, h is Planck’s constant, e is charge on electron, m*e and m*h are the effective masses of electron and hole and which have the values 0.19 and

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respectively.

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0.8 respectively. Again  and  0 are dielectric coefficient and permittivity of free space

The calculated value of QDs is depicted in Table 2. From Table 2 it is observed that size of CdS QDs increases with heat treatment temperature. This increase in the size of QDs results

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in a decrease in the energy band gap which confirms the quantum confinement effect.

3.4.

Photoluminescence study

Photoluminescence spectra of the CdS QDs embedded in glass sample for four types of heat treatments are shown in Fig. 6 for excitation wavelength 350 nm which provide further proof of quantum confinement behaviour. From the spectra, it is observed that emission peak appears at 416.0 nm for all glasses (GQ0, GQ1, GQ2, GQ3, and GQ4) due to compositional impurities or lattice defects in glasses [14]. PL spectra of GQ0 does not show any emission

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peak due to no formation of QDs which was also confirmed by XRD results. For GQ1

emission peak is observed at 451.0 nm due to band edge emission of exciton and it shifts

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towards longer wavelength 468.0 nm, 475.0 nm and 500.0 nm for GQ2, GQ3 and GQ4 respectively. Glass sample GQ1 shows high intense emission peak at 564.5 nm due to trap

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states [20] and this shift towards longer wavelength 573.5 nm, 576.5 nm and 635.0 nm for

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GQ2, GQ3 and GQ4 respectively. The shift in emission peak towards longer wavelength also

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confirmed the increase in the size of QDs. It is interesting to observe that intensity of the

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emission peak decreases due to decrease in electron-hole recombination and it is very less for

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GQ4 (635.0 nm).

Fig. 7 shows the CIE chromaticity diagram for all glasses. Glass GQ1 emits high intense

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green light (0.41, 0.46) because of high recombination rate of electrons and holes as shown in CIE chromaticity. The CIE chromaticity coordinates (x, y) for each glass sample are depicted

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in Table 3 which confirm that the emission peaks are shifting towards longer wavelength [12,

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14].

3.5. Spectral overlapping In order to get an the relationship between optical absorption and PL of glass sample, the PL and absorption spectra of GQ1 were plotted as shown in fig. 8. From this figure it is confirmed that when the glass sample GQ1 was excited by energy (i.e. Eexe =3.542 eV) which

is greater than the band gap, the first PL emission peak appears at band edge (2.749 eV) which is close to the absorption band edge of UV-vis spectra (2.652 eV). The second highintensity peak in PL spectra at 2.197 eV is due to trap state generated in the glass sample during heat treatment rather than band edge emission. The transitions of an electron between trap state and the conduction/valence band is called trap-state emission. Trap state emission

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generally arises at smaller energy (2.197 eV) than the emission due to band edge (2.749 eV).

This indicates that various trap state emissions are correlated with different emission

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energies. Emission of glass sample GQ1 shows green color light due to size distributions and broadening of QDs. Thus the observed optical absorption and photoluminescence spectra

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supports that electronic transition at the band edge in QDs as a result of quantum

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confinement. [14, 23].

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4. Photocatalytic activity of CdS QDs on Methylene Blue (MB)

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The photocatalytic activity of prepared CdS QDs containing glass samples of methylene blue (MB) as an organic pollutant was performed. In a photocatalytic process, the photocatalytic

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activity is more effective for less recombination of photogenerated electron-hole pairs. It is generally believed that a lower recombination rate of electron-hole pairs under light

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irradiation cause lower PL intensity [25]. From photoluminescence study (Fig. 6) very less electron-hole recombination was found for GQ4 glass sample, therefore GQ4 is used for the

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photocatalytic activity.

4.1. Mechanism of photocatalysis on MB dye The performance of prepared photocatalysts could be explained below [26-28] and depicted in Fig. 9:

When the suspension exposed in sunlight by energy having greater value than the energy band gap of the semiconductor, the electrons from the valence band jump to the conduction band creating holes in the valence band. This is given by (3) hν + CdS → h+ (CdS)+ e- (CdS)

--(3)

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Where e- and h+ are electrons and holes in the conduction band and valence band respectively.

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As discussed the earlier photocatalytic efficiency of semiconductor can improve for low PL intensity (less electron-hole recombination). By using electron-hole pairs, the catalyst can

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perform redox reaction by accepting electrons (reduction) into its valence band and donating

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the excited electrons to the conduction band (oxidation). The surface area of nanoparticles

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(especially for QDs) is high and large charge carriers generated by sunlight sit on

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photocatalyst.

Reaction between water and photogenerated holes produce OH• (given by (4)) and

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photogenerated electrons react with dissolved oxygen in water which form superoxide (given

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by (5)). h+ + H2O → OH• + H+

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--(4)

h+ + OH-→ OH• O2+ e- → O2•-

--(5)

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Proton generated in above reaction reacts with superoxide which form hydroperoxyl radical (given by (6)) and then finally the formation of hydrogen peroxide (H2O2) (given by (7)). O2•- + H+ → HO2•

--(6)

2HO2• → H2O2 + O2

--(7)

Now photogenerated electrons attack on H2O2to form hydroxyl radical (given by (8)).

H2O2 + e- → OH• + OH-

--(8)

These generated radicals further attack on MB dye to oxidise it and final product was (CO2, H2O) formed (given by (9)). OH• + MB (dye) → CO2 + H2O

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--(9)

4.2. Effect of dye concentration

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Two concentrations were taken to check the effect of photocatalytic activity. By varying the

concentration of MB dye from 5 mg/L (5 ppm) to 10 mg/L (10 ppm) (pH for both was 7) at constant catalyst addition 0.1g (GQ4), absorption spectra of degraded MB were checked and

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depicted in Fig. 10 (a) and 10 (b) respectively. It is observed from the absorption spectra that

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there is no degradation of dye without catalyst even though exposed in sunlight for a long

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time (Blank). On the other hand, very small degradation observed after loading catalyst even

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in dark condition (Dark). As soon as the suspension was brought in sunlight with the addition

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of catalyst, the colour of MB dye became faint and peak maximum of absorption spectra at

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663.0 nm was decreasing gradually with increasing irradiation time (Fig. 10 (a)and (b)) [29].

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In this study, the adsorption/desorption time for GQ4 catalyst kept constant about 30 minutes. Degradation was tested for maximum wavelength of MB in the absorption spectra at 663.0 nm and almost kept the same.

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The concentration of MB dye was found to decrease in dark and sunlight for both type of solutions (Fig. 11 (a)). Rate of degradation with respect to irradiation time could be determined from equation (3) and shown Fig. 11 (b). Fig. 11 (b) revealed that as concentration of dye increased, the degradation efficiency decreased. The decreased in the degradation efficiency could be understood on the basis of

concentration of dye. As concentration of dye raised the adsorption of dye on surface of catalyst increases and hence OH - adsorbed on that site is decreased results less formation of OH• radical. Again from Beer-Lambert law, as the concentration of dye increases, the path

length of photons entering the dye solution decreases, resulting in less photon absorption on

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surface of catalyst particles and cause a lower photo degradation rate [29].

% degradation =

C0 - C C0

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The degradation efficiency of MB was calculated with following equation (10): ×100

--(11)

Where, C0 and C are the concentration of MB dye solution at time 0 (after dark adsorption)

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and t respectively. The photocatalytic degradation of MB contains catalyst obeys pseudo-

dC = k app C dt

--(12)

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-

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first-order kinetics with regard to the concentration of MB which follow the equation (12):

C = - k app t C0

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ln

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By integrating above equation at limit C = C0 for t = 0 results equation (13):

--(13)

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Where, t is the irradiation time, kapp is apparent reaction rate constant. The apparent rate of reaction (k) was calculated from the slope of ln (C/C0) versus irradiation time (t) and was

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found to 0.0041 min-1 and 0.0034 min-1 for 5 ppm and 10 ppm respectively [17, 19, 30]. Degradation rate of MB (for 5 ppm and 10 ppm) is shown in Fig. 12. As the suspension irradiate under sunlight decomposition of MB increases with increase of time. The glass sample GQ4 shows 70.6% and 68% photodegradation for 5 ppm and 10 ppm MB solutions over a period of 270 min irradiation respectively and depicted in Table 4 [7].

5. Conclusion The CdS semiconductor containing glasses were prepared by conventional melt quench technique. The CdS QDs were successfully grown in the glass matrix with controlled single

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step heat treatment schedule. The colour of glasses changes from transparent to dark yellow with heat treatment revealed the growth of CdS QDs in glass. The XRD analysis confirmed

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the formation of hexagonal phases of CdS. The average size of QDs was determined from

HR-TEM and found to be 5-8 nm and further verified by absorption spectra. As the heat

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treatment temperature increases the optical absorption band edge shift toward red region

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because of increase of QDs size and shows quantum confinement effect which is also

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supported by shift of PL peak intensity. Colour emitted by glass sample in UV light and co-

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ordinates of CIE chromaticity diagram also confirms the growth of CdS QDs with the heat treatment. The photocatalysis study using CdS QDs containing glasses as a photocatalyst for

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MB dye decomposition under sunlight shows consistency. Photodegradation efficiency of glass sample GQ4 was found to be 70.6% and 68.0% for 5 ppm and 10 ppm MB dye

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solutions respectively over a period of 270 min irradiation.

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Acknowledgements

SYJ is very much thankful to VNIT for financial support for scientific research. We would

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also like to express our sincere thanks to Mrs. Bharati Patro, Jr. Technical Superintendent, SAIF, IIT Bombay for providing characterization facility for this work.

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Figures

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Fig. 1: DTA curve of glass sample

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Fig. 2: Glass samples heat treated for different temperature

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Fig. 3: XRD pattern for as made glass (GQ0) and GQ3

Fig.4: (a) and (b) HR-TEM images of glass sample GQ3 (c) IFFT image (d) Histogram of CdS QDs distribution and SAED pattern

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Fig. 5: UV-vis spectra of glasses

Fig. 6: Photoluminescence spectra glass sample containing CdS QDs.

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Fig. 7: CIE Chromaticity diagram of glass sample.

Fig.8: Spectral overlapping of absorption and emission spectra of GQ1.

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Fig. 9: Mechanism of photocatalysis.

Fig. 10(a): UV-Vis absorption spectra of degraded 5 ppm MB solution for GQ4 Glass as a catalyst.

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(b): UV-Vis absorption spectra of degraded 10 ppm MB solution for GQ4 Glass as a catalyst.

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Fig. 11 (a): Degradation of MB in dark and sunlight

(b): Plot of ln( C C0 ) versus irradiation time of degraded MB solution (5 ppm

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and 10 ppm) of GQ4 Glass.

Fig. 12: Influence of dye concentration on photo degradation efficiency for the decomposition of MB in sunlight.

Tables

GQ0

As made Glass

GQ1

Glass heat treated at 5500C for 12 hrs

GQ2

Glass heat treated at 5700C for 12 hrs

GQ3

Glass heat treated at 5900C for 12 hrs

GQ4

Glass heat treated at 6100C for 12 hrs

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Description

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Sample ID

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Table 1: Identification of samples.

Table 2: Optical cut off wavelength, energy band gap and size of QDs Sample ID

λa (nm)

E opt g (eV)from

E gdirect

UV-vis

(eV)

Radius from band gap

GQ0

351.5

3.530

3.576

GQ1

467.5

2.652

2.654

GQ2

496.3

2.498

2.493

GQ3

504.7

2.457

2.446

4.0

GQ4

511.5

2.424

2.426

5.2

1.3 2.4 3.4

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(nm)

Table 3: Emission peaks in photoluminescence study and CIE coordinates.

λe (nm)

Sample ID

CIE Coordinates

416.0

(0.24, 0.16)

GQ1

416.0, 451.0, 564.5

(0.41, 0.46)

GQ2

416.0, 468.0, 573.5

(0.41, 0.39)

GQ3

416.0, 475.0, 576.5

GQ4

416.0, 496.0, 635.0

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GQ0

(0.44, 0.37) (0.54, 0.30)

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(x, y)

Table 4: Apparent reaction rate constant and photodegradation efficiency of different concentration of MB for GQ4 glass powder as a catalyst. kapp

%Degradation

(ppm)

(min-1)

For 270 min

GQ4

5

0.0041

70.6

GQ4

10

0.0034

68

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Concentrations

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Catalyst ID