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Silver modified cadmium oxide – A novel material for enhanced photodegradation of malachite green
Accepted Manuscript Title: Silver modified cadmium oxide - A novel material for enhanced photodegradation of malachite green Authors: R K Mandal, M D Purkayastha, T Pal Majumder PII: DOI: Reference:
S0030-4026(18)31821-7 https://doi.org/10.1016/j.ijleo.2018.11.066 IJLEO 61914
To appear in: Received date: Accepted date:
13 October 2018 19 November 2018
Please cite this article as: Mandal RK, Purkayastha MD, Pal Majumder T, Silver modified cadmium oxide - A novel material for enhanced photodegradation of malachite green, Optik (2018), https://doi.org/10.1016/j.ijleo.2018.11.066 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.
Silver modified cadmium oxide - A novel material for enhanced photodegradation of malachite green R K Mandal1, M D Purkayastha1, T Pal Majumder1* Department of Physics, University of Kalyani, Kalyani-741235, West Bengal, India
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*Corresponding author Email id: [email protected] Tel: 9432358393 Fax: 03325828282. Abstract
An enhancement of Cadmium oxide (CdO) based catalyst activity was observed on modification
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with silver (Ag) by hydrothermal route in a short time and at low temperature. The products were
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characterized by XRD, Infrared spectroscopy (FTIR) and scanning electron microscopy (SEM).
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XRD confirming the formation of CdO showed an intense diffraction peak at 32.73° with lattice
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plane (111). The IR spectrum shows characteristic peaks at 607, 1387 cm-1 corresponding to Cd-
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O bonds. Both CdO and Ag modified CdO (Ag-CdO) showed band gaps of 2.76 eV and 2.56 eV
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respectively stretching into the red portion of the solar spectrum. On irradiation with Ultraviolet (Uv) light CdO-Ag showed 78.02 % degradation of malachite green oxalate (MG), a hazardous
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dye within 175 minutes in comparison to native CdO (75.85 % in 175 minutes). Photoluminescence (PL) studies demonstrated better emission intensity quenching in case of Ag-
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CdO which is particularly promising for photocatalytic applications.
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Keywords: semiconductor; nanostructure; luminescence; catalytic activity
1. Introduction The history of preparation and use of dyes dates back to the early centuries. In the past, natural dyes were used which got biodegraded. But today with the growing demand of dyes in textile,
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paper, paint, leather and other industries, a number of artificial dyes are being synthesized which are chemically as well as ultraviolet light resistant and biologically stable. This poses serious threat to environment particularly the aquatic ecosystem. Malachite Green dye is one such water pollutant with carcinogenic and mutagenic effects on living organisms [1, 2]. Its salt form is malachite green oxalate (MG) whose chemical structure is given in Figure 1. Keeping aside the
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various conventional methods for organic pollutant removal, advanced oxidation process (AOP)
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has evolved as the most cost effective, quick and environment friendly [3-6]. Among the various
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semiconducting materials, CdO, an n-type semiconductor, appears as a promising candidate for
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optoelectronic, solar cells, transparent electrodes, gas sensors and photocatalytic applications [7]. But the major setback in using this semiconductor is the rapid recombination of photo-induced
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electron-hole pairs that tends to reduce photoactivity [8, 9]. Many strategies were adopted to
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remove this problem out of which composites with noble metals has attracted significant attention.
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Compared to other noble metals, silver (Ag) is relatively less expensive and has tendency to form a schottky barrier at the interface of the semiconductor to which it is bound [10-14]. This in turn
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promotes the charge transfer between the two and accordingly suppresses electron-hole
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recombination.
Figure 1: Chemical structure of malachite green oxalate (MG) In the present study, pure CdO and Ag-CdO nanocomposite were synthesized using hydrothermal method and their photocatalytic performances were investigated. The dye
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degradation mechanism was also discussed in detail. The phase purity and morphology were characterized by spectroscopic techniques. 2. Materials and methods 2.1.Materials for synthesis
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Anal grade chemicals were used as received commercially without further purification. Cadmium
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nitrate (Cd(NO3)2,4H2O), ammonia, silver nitrate (AgNO3), sodium hydroxide (NaOH) were
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purchased from Merck, India. Doubly distilled water was used in all experiments.
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2.2. Sample preparation
In a typical synthesis of CdO, 0.308 mg of cadmium nitrate was added to 200 mL doubly distilled
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water and subjected to vigorous magnetic stirring at the rate of 500 rev/sec for 2 hours. Next 10
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mL ammonia was added to it and after stirring for another 30 min the contents were transferred to an autoclave and kept at 200 °C for 3 hours. The precipitate was then washed with water thrice
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and dried in an oven overnight. The residue was then powdered to get nano CdO. To synthesize
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Ag modified CdO, 1.33 g cadmium acetate was mixed with 5 mL AgNO 3 and the solution was added to 50 mL water and subjected to vigorous magnetic stirring. In another conical flask, 0.8 g
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NaOH was added to 20 mL water and stirred. This solution was added to the cadmium acetate solution drop wise under continuous stirring. After 2 hours, the solution was filtered and the precipitate was taken in a crucible and kept in a furnace at 200 ºC for 3 hours. The resultant residue was nano Ag- CdO. 2.3. Sample characterization
The as-synthesized product was characterized by an X’Pert PRO PANalytical X-ray diffract meter (XRD) using CuKα radiation (wavelength λ= 0.15418 nm) at an accelerating voltage and applied current of 30 kV and 20 mA respectively to identify the resultant phase and crystal structure of the
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synthesized powder. The data was collected with a step of 0.02° in the range of 2θ = 10°–80°. The chemical identity of the product was determined by comparing the experimental X-ray powder patterns to standards compiled by the Joint Committee on Powder Diffraction and Standards (JCPDS). Uv-vis absorption spectra were obtained with a UV 3600 spectrophotometer in the region 200-800 nm. Identification of functional groups in the sample was studied with a FT-IR
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spectrophotometer Perkin Elmer (Spectrum one L120-000A). PL emission spectrum was obtained
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by employing an excitation wavelength of 280 nm using PTI QM 40 spectrophotometer.
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Morphology, elemental analysis of the synthesized nanomaterial was investigated by a SEM
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device coupled with EDX (Energy dispersive X-ray spectroscopy). 2.4. Photo catalytic degradation measurements
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The Uv experimental set up for the photo degradation of malachite green oxalate includes a glass
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photo-reactor (volume = 500 mL) , a sampling port to take out the sample solutions at
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predetermined intervals, four Uv lamps fitted at equal distances, a water circulating system and a magnetic stirrer. The Uv lamps (Philips TUV 30 Watt T8) producing a 253.7 nm light wave is
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ideal for water and air purification. The reactor temperature was maintained at 25 ± 0.2 °C through the circulating water and a fan. The whole set-up was well insulated from the surroundings within
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a wooden chamber. Photo catalytic degradation kinetic tests were carried out for both the CdO and CdO-Ag samples under the effect of Uv light. Specifically, 10 mg/L aqueous solution of MG powder and 10 mg of the photocatalyst were loaded in the photo-reactor. Prior to irradiation, the system was dispersed by ultrasonic waves for 5 min and then stirred for 60 min in the dark to
ensure an adsorption-desorption equilibrium. Afterwards, the reactor was subjected to the Uv irradiation. Water sample (5 mL each) was taken at predetermined intervals and immediately centrifuged at 8000 rpm for 10 min. The supernatant was filtered and analyzed using Uv-vis
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spectrophotometer. 3. Result and discussion 3.1. XRD and EDS
The diffraction pattern of both CdO and Ag-CdO [Figure 2(a) _ 2(b)] well coincided with the cubic
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structure of cadmium oxide corresponding to the diffraction planes (1 1 1), (2 0 0), (2 2 0), (3 1 1)
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and (2 2 2) supported by JCPDS card no.05-0640. The nano powder exhibited creamish hue and
K Cos
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the crystallite size was calculated from the peak widths using the Scherer equation (1) [15]
(1)
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where K is a dimensionless shape factor that varies with the actual shape of the crystallite, λ is the
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X-ray wavelength λ= 1.5418 Å, β is the line broadening at half the maximum intensity (FWHM)
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and θ is Bragg’s angle. It was observed that the crystallite size of synthesized CdO was 23 nm while with the incorporation of Ag the crystallite size decreased to 15.5 nm. This could be
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due to the relative decrease in concentration of CdO with the addition of Ag. Interestingly, no peak corresponding to Ag nor any peak shift was observed. This could be attributed to very low
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concentration of Ag or the uniform distribution of Ag into CdO.
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Figure 2: XRD pattern of (a) CdO (b) Ag-CdO.
Energy dispersive X ray spectroscopy (EDS) analysis of the synthesized CdO [Figure 3(a)]
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and Ag-CdO [Figure 3(b)] obtained from different peak areas of the sample shows the presence of
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cadmium and silver in the range 3-3.5 KeV and oxygen in the range 0-1 KeV [16]. Furthermore,
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neither N nor C signals were detected in the EDS spectrum which means that the product is pure
Table 1: EDS: composition data Element Cd O Cd O Ag
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Sample CdO
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Ag-CdO
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and free of any surfactant or impurity. The composition data (at % and wt %) is given in table 1.
At % 25.90 74.10 30.05 67.33 2.62
Wt % 71.06 28.94 73.96 22.02 4.02
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Figure 3: EDS spectra of (a) CdO and (b) Ag-CdO.
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3.2. Morphology
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The morphology of CdO and Ag-CdO were investigated at a magnification of 20 µm using SEM
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[Figure 4]. The SEM images of CdO [Figure 4(a)] were foam like structures. However, as soon as
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Ag was incorporated [Figure 4(b)] leaf like structures with pores were observed in the pattern. In
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the Ag-CdO nanocomposite, the leaf like structures could result from the overlap of smaller particles and depict porous nature with larger surface area. This could have led to greater
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photoactivity. The particle size in the composite was however seen to vary from 1µm to 200 nm. Thus there arises a discrepancy in the particle sizes as measured by XRD and SEM which implies
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that the particles must have coalesced. This tends to reduce the benefits of large interfacial area
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between the particles to some extent [17].
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Figure 4: SEM image of (a) CdO at magnification 1.82 K X and (b) Ag-CdO at magnification 1.91 K X.
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3.3.Uv-vis spectra
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Light absorbing properties are crucial in interpreting the behavior of the nanoparticles. Figure 5(a)
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shows the Uv-vis absorption spectra of CdO with a sharp peak at 497 nm. The peak appeared red
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shifted to 504 nm with the incorporation of Ag [Figure 5(b)]. These values correspond to the visible region of the electromagnetic spectrum [18]. The optical band gap was calculated using the Tauc
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h C (h Eg )n
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relation (2) from the absorption spectrum [19] (2)
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where C is a constant, α is molar extinction coefficient, Eg is the band gap energy of the material and n determines the type of transition. When n=1/2, Eg will denote the direct allowed band gap.
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The value of Eg was estimated from the intercept of the linear portion of (αhν)2 versus hν plot on the hν axis as in Figure 5 (inset) . The values of band gap was found to decrease in case of Ag-
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CdO (2.56 eV) in comparison to that of CdO (2.76 eV). These values predict semiconducting nature of the samples and may be caused by structural disorder due to oxygen deficiency in the samples [20].
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Figure 5: Uv-visible spectra of (a) CdO and (b) Ag-CdO (inset) Tauc plot indicating band gap. 3.4. FTIR
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The FT-IR spectra of CdO and Ag-CdO have been illustrated in Figure 6. On examining the spectra
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obtained after spectral scanning, and focussing on the frequently used portion between 400 and
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4000 cm-1, the bands emerging in relation to stretching collision of Cd-O were observed in the
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range 600-1000 cm-1 [21]. The metal-oxygen bond at 1387 cm-1 corresponds to the formation of CdO from cadmium acetate [22]. The peak at 1635 cm-1 symbolizes the bending collision of
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adsorbed water. The broad band centred at 3443 cm-1 signifies the stretching collision of H-O-H
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bonds that are chemically associated with CdO [23].
Figure 6: FT-IR spectra of (a) CdO and (b) Ag-CdO. 3.5. PL spectra
The room temperature PL emission spectrum was recorded by employing an excitation wavelength of 470 nm [Figure 7].The spectrum for CdO showed peaks at 543 nm, 604 nm and 730 nm while those for Ag-CdO appeared red shifted by nearly 2 nm. The green (543 nm) and orange (604 nm)
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emissions could have resulted due to the presence of oxygen vacancies [24]. Also interestingly, the emission intensity of Ag-CdO was found to be low in comparison to that of CdO. PL spectrum throws light on the processes of recombination of charge carriers [25-29]. Low emission intensity predicts efficient separation of photo induced charge carriers which is desirable for better photoactivity. Ag nanoparticles probably acted as an electron sink trapping the photogenerated
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electrons [30].
Figure 7: PL spectra (a) CdO and (b) Ag-CdO.
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3.6. Mechanism of photocatalysis When Ag-CdO was irradiated with Uv light, the electrons from the valence band jumped to the
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conduction band while holes were created in the valence band. These holes were adsorbed by the surrounding water molecules to produce highly reactive hydroxyl radicals (OH.). The photogenerated electrons got transferred to the silver nanoparticles which reacted with the
atmospheric oxygen molecules to produce highly reactive superoxide radicals (O2-.). These
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reactive oxidative species are in turn responsible for the degradation of the organic dye pollutants.
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Figure 8: Photocatalysis mechanism of Ag-CdO.
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3.7.Reaction kinetics
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10 mg/L of aqueous solution of MG and 10 mg of the photo catalyst (CdO and Ag-CdO) were
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loaded in the photo-reactor and stirred for 60 min in the dark to ensure an adsorption-desorption equilibrium. After that the mixture was subjected to Uv irradiation and the decrease in absorbance
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as a function of irradiation time was recorded. Figure 9 shows the Uv-vis absorption spectra of (a)
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CdO (b) Ag-CdO. Both the samples showed an appreciable decrease in absorbance with irradiation
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time. Initially the rate of degradation is high which may be attributed to presence of greater active surface area of the photocatalyst but as time progresses the degradation becomes slower since
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intermediates take a longer time to degrade. The greater decrease in absorbance in the case of AgCdO may be attributed to the presence of noble metal Ag which can cause improved trapping of
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electrons during irradiation that tends to inhibit electron-hole recombination better [16].
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Figure 9: Absorbance versus wavelength plot of the photo catalysts (a) CdO and (b) Ag-CdO The photo catalytic degradation efficiency (PD) of the dye was calculated using (3) [31]. ( A0 At ) 100, A0
(3)
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PD
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where A0 is the absorbance of dye solution before the photo irradiation, At is the absorbance of
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solutions in suspension after photo-irradiation for certain time t. It is observed that Ag-CdO
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showed better degradation ability (78.02 %) in comparison to CdO (75.85 %) [Figure 10].
Figure 10: Degradation kinetics PD versus time of (a) CdO and (b) Ag-CdO
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The first-order kinetic model was tested to understand the order of photo catalytic
degradation reaction [32]:
A ln 0 k1t , At
(16)
where A0 and At (mg/L) are the MG concentrations at time zero and t (min), respectively, and k1 (min-1) is the rate constant. In order to calculate rate constant (k1), -ln (At/Ao) was plotted with respect to time [Figure 11] and the data were fitted linearly. The slope of the line was the required
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rate constant (k1) of the reaction which was found to be 0.0112 min-1 for Ag-CdO compared to
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0.0088 min-1 for CdO.
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Figure 11: Degradation kinetics –ln (A/A0) versus time of (a) CdO and (b) Ag-CdO. Compared to CdO, Ag-CdO showed better photoactivity. This could be attributed to the
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lower band gap energy of Ag-CdO which must have rendered more surface active sites for
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photocatalysis. This was also complimented by the higher rate constant shown by Ag-CdO [Figure
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12] which ultimately led to a faster degradation of MG on the catalyst surface.
Figure 12: Rate constant bar diagram showing comparison of CdO and Ag-CdO catalysts 4. Conclusions
In a nutshell, silver incorporated cadmium oxide nanocomposites with high photoactivity and reduced photoluminescence have been successfully synthesized. The ability of the nanocomposite to degrade malachite green dye, which represents a class of organic pollutants in water, has been
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investigated. X- ray diffraction studies confirmed that the synthesized Ag-CdO demonstrated cubic structure with grain size 15.5 nm and a strong (111) orientation. The PL emission peaks at 545 and 606 nm confirm the presence of oxygen vacancies. The change in morphology of the Ag-CdO nanocomposite from foam-like to leaf-like was observed. The Uv-vis spectra showed absorbance in the visible region of the electromagnetic spectrum which makes Ag-CdO suitable for making
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solar cells and transparent electrodes. The band gap studies were also consistent with the Uv-vis
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results. The better photocatalytic activity of Ag-CdO in degrading MG solution may be attributed
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to the efficient separation of photoinduced charge carriers. The mechanism of enhanced
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photoactivity shown by Ag-CdO was also discussed in details predicting that OH. And O2.- radicals play the dominant role in the degradation process. Thus the present study reveals that Ag modified
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CdO nanocomposite may be considered as a potentially promising photocatalyst for degradation
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Acknowledgements
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of organic dyes in waste water.
Authors highly acknowledge DST-FIST (SR/FST/PSI-175/2012) project and Department of
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Chemistry, University of Kalyani for providing instrumental facility.
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Declaration of interests None
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S0030-4026(18)31821-7 https://doi.org/10.1016/j.ijleo.2018.11.066 IJLEO 61914
To appear in: Received date: Accepted date:
13 October 2018 19 November 2018
Please cite this article as: Mandal RK, Purkayastha MD, Pal Majumder T, Silver modified cadmium oxide - A novel material for enhanced photodegradation of malachite green, Optik (2018), https://doi.org/10.1016/j.ijleo.2018.11.066 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.
Silver modified cadmium oxide - A novel material for enhanced photodegradation of malachite green R K Mandal1, M D Purkayastha1, T Pal Majumder1* Department of Physics, University of Kalyani, Kalyani-741235, West Bengal, India
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1
*Corresponding author Email id: [email protected] Tel: 9432358393 Fax: 03325828282. Abstract
An enhancement of Cadmium oxide (CdO) based catalyst activity was observed on modification
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with silver (Ag) by hydrothermal route in a short time and at low temperature. The products were
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characterized by XRD, Infrared spectroscopy (FTIR) and scanning electron microscopy (SEM).
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XRD confirming the formation of CdO showed an intense diffraction peak at 32.73° with lattice
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plane (111). The IR spectrum shows characteristic peaks at 607, 1387 cm-1 corresponding to Cd-
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O bonds. Both CdO and Ag modified CdO (Ag-CdO) showed band gaps of 2.76 eV and 2.56 eV
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respectively stretching into the red portion of the solar spectrum. On irradiation with Ultraviolet (Uv) light CdO-Ag showed 78.02 % degradation of malachite green oxalate (MG), a hazardous
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dye within 175 minutes in comparison to native CdO (75.85 % in 175 minutes). Photoluminescence (PL) studies demonstrated better emission intensity quenching in case of Ag-
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CdO which is particularly promising for photocatalytic applications.
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Keywords: semiconductor; nanostructure; luminescence; catalytic activity
1. Introduction The history of preparation and use of dyes dates back to the early centuries. In the past, natural dyes were used which got biodegraded. But today with the growing demand of dyes in textile,
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paper, paint, leather and other industries, a number of artificial dyes are being synthesized which are chemically as well as ultraviolet light resistant and biologically stable. This poses serious threat to environment particularly the aquatic ecosystem. Malachite Green dye is one such water pollutant with carcinogenic and mutagenic effects on living organisms [1, 2]. Its salt form is malachite green oxalate (MG) whose chemical structure is given in Figure 1. Keeping aside the
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various conventional methods for organic pollutant removal, advanced oxidation process (AOP)
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has evolved as the most cost effective, quick and environment friendly [3-6]. Among the various
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semiconducting materials, CdO, an n-type semiconductor, appears as a promising candidate for
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optoelectronic, solar cells, transparent electrodes, gas sensors and photocatalytic applications [7]. But the major setback in using this semiconductor is the rapid recombination of photo-induced
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electron-hole pairs that tends to reduce photoactivity [8, 9]. Many strategies were adopted to
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remove this problem out of which composites with noble metals has attracted significant attention.
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Compared to other noble metals, silver (Ag) is relatively less expensive and has tendency to form a schottky barrier at the interface of the semiconductor to which it is bound [10-14]. This in turn
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promotes the charge transfer between the two and accordingly suppresses electron-hole
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recombination.
Figure 1: Chemical structure of malachite green oxalate (MG) In the present study, pure CdO and Ag-CdO nanocomposite were synthesized using hydrothermal method and their photocatalytic performances were investigated. The dye
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degradation mechanism was also discussed in detail. The phase purity and morphology were characterized by spectroscopic techniques. 2. Materials and methods 2.1.Materials for synthesis
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Anal grade chemicals were used as received commercially without further purification. Cadmium
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nitrate (Cd(NO3)2,4H2O), ammonia, silver nitrate (AgNO3), sodium hydroxide (NaOH) were
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purchased from Merck, India. Doubly distilled water was used in all experiments.
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2.2. Sample preparation
In a typical synthesis of CdO, 0.308 mg of cadmium nitrate was added to 200 mL doubly distilled
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water and subjected to vigorous magnetic stirring at the rate of 500 rev/sec for 2 hours. Next 10
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mL ammonia was added to it and after stirring for another 30 min the contents were transferred to an autoclave and kept at 200 °C for 3 hours. The precipitate was then washed with water thrice
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and dried in an oven overnight. The residue was then powdered to get nano CdO. To synthesize
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Ag modified CdO, 1.33 g cadmium acetate was mixed with 5 mL AgNO 3 and the solution was added to 50 mL water and subjected to vigorous magnetic stirring. In another conical flask, 0.8 g
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NaOH was added to 20 mL water and stirred. This solution was added to the cadmium acetate solution drop wise under continuous stirring. After 2 hours, the solution was filtered and the precipitate was taken in a crucible and kept in a furnace at 200 ºC for 3 hours. The resultant residue was nano Ag- CdO. 2.3. Sample characterization
The as-synthesized product was characterized by an X’Pert PRO PANalytical X-ray diffract meter (XRD) using CuKα radiation (wavelength λ= 0.15418 nm) at an accelerating voltage and applied current of 30 kV and 20 mA respectively to identify the resultant phase and crystal structure of the
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synthesized powder. The data was collected with a step of 0.02° in the range of 2θ = 10°–80°. The chemical identity of the product was determined by comparing the experimental X-ray powder patterns to standards compiled by the Joint Committee on Powder Diffraction and Standards (JCPDS). Uv-vis absorption spectra were obtained with a UV 3600 spectrophotometer in the region 200-800 nm. Identification of functional groups in the sample was studied with a FT-IR
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spectrophotometer Perkin Elmer (Spectrum one L120-000A). PL emission spectrum was obtained
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by employing an excitation wavelength of 280 nm using PTI QM 40 spectrophotometer.
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Morphology, elemental analysis of the synthesized nanomaterial was investigated by a SEM
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device coupled with EDX (Energy dispersive X-ray spectroscopy). 2.4. Photo catalytic degradation measurements
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The Uv experimental set up for the photo degradation of malachite green oxalate includes a glass
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photo-reactor (volume = 500 mL) , a sampling port to take out the sample solutions at
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predetermined intervals, four Uv lamps fitted at equal distances, a water circulating system and a magnetic stirrer. The Uv lamps (Philips TUV 30 Watt T8) producing a 253.7 nm light wave is
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ideal for water and air purification. The reactor temperature was maintained at 25 ± 0.2 °C through the circulating water and a fan. The whole set-up was well insulated from the surroundings within
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a wooden chamber. Photo catalytic degradation kinetic tests were carried out for both the CdO and CdO-Ag samples under the effect of Uv light. Specifically, 10 mg/L aqueous solution of MG powder and 10 mg of the photocatalyst were loaded in the photo-reactor. Prior to irradiation, the system was dispersed by ultrasonic waves for 5 min and then stirred for 60 min in the dark to
ensure an adsorption-desorption equilibrium. Afterwards, the reactor was subjected to the Uv irradiation. Water sample (5 mL each) was taken at predetermined intervals and immediately centrifuged at 8000 rpm for 10 min. The supernatant was filtered and analyzed using Uv-vis
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spectrophotometer. 3. Result and discussion 3.1. XRD and EDS
The diffraction pattern of both CdO and Ag-CdO [Figure 2(a) _ 2(b)] well coincided with the cubic
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structure of cadmium oxide corresponding to the diffraction planes (1 1 1), (2 0 0), (2 2 0), (3 1 1)
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and (2 2 2) supported by JCPDS card no.05-0640. The nano powder exhibited creamish hue and
K Cos
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the crystallite size was calculated from the peak widths using the Scherer equation (1) [15]
(1)
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where K is a dimensionless shape factor that varies with the actual shape of the crystallite, λ is the
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X-ray wavelength λ= 1.5418 Å, β is the line broadening at half the maximum intensity (FWHM)
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and θ is Bragg’s angle. It was observed that the crystallite size of synthesized CdO was 23 nm while with the incorporation of Ag the crystallite size decreased to 15.5 nm. This could be
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due to the relative decrease in concentration of CdO with the addition of Ag. Interestingly, no peak corresponding to Ag nor any peak shift was observed. This could be attributed to very low
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concentration of Ag or the uniform distribution of Ag into CdO.
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Figure 2: XRD pattern of (a) CdO (b) Ag-CdO.
Energy dispersive X ray spectroscopy (EDS) analysis of the synthesized CdO [Figure 3(a)]
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and Ag-CdO [Figure 3(b)] obtained from different peak areas of the sample shows the presence of
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cadmium and silver in the range 3-3.5 KeV and oxygen in the range 0-1 KeV [16]. Furthermore,
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neither N nor C signals were detected in the EDS spectrum which means that the product is pure
Table 1: EDS: composition data Element Cd O Cd O Ag
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Sample CdO
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Ag-CdO
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and free of any surfactant or impurity. The composition data (at % and wt %) is given in table 1.
At % 25.90 74.10 30.05 67.33 2.62
Wt % 71.06 28.94 73.96 22.02 4.02
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Figure 3: EDS spectra of (a) CdO and (b) Ag-CdO.
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3.2. Morphology
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The morphology of CdO and Ag-CdO were investigated at a magnification of 20 µm using SEM
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[Figure 4]. The SEM images of CdO [Figure 4(a)] were foam like structures. However, as soon as
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Ag was incorporated [Figure 4(b)] leaf like structures with pores were observed in the pattern. In
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the Ag-CdO nanocomposite, the leaf like structures could result from the overlap of smaller particles and depict porous nature with larger surface area. This could have led to greater
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photoactivity. The particle size in the composite was however seen to vary from 1µm to 200 nm. Thus there arises a discrepancy in the particle sizes as measured by XRD and SEM which implies
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that the particles must have coalesced. This tends to reduce the benefits of large interfacial area
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between the particles to some extent [17].
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Figure 4: SEM image of (a) CdO at magnification 1.82 K X and (b) Ag-CdO at magnification 1.91 K X.
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3.3.Uv-vis spectra
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Light absorbing properties are crucial in interpreting the behavior of the nanoparticles. Figure 5(a)
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shows the Uv-vis absorption spectra of CdO with a sharp peak at 497 nm. The peak appeared red
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shifted to 504 nm with the incorporation of Ag [Figure 5(b)]. These values correspond to the visible region of the electromagnetic spectrum [18]. The optical band gap was calculated using the Tauc
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h C (h Eg )n
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relation (2) from the absorption spectrum [19] (2)
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where C is a constant, α is molar extinction coefficient, Eg is the band gap energy of the material and n determines the type of transition. When n=1/2, Eg will denote the direct allowed band gap.
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The value of Eg was estimated from the intercept of the linear portion of (αhν)2 versus hν plot on the hν axis as in Figure 5 (inset) . The values of band gap was found to decrease in case of Ag-
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CdO (2.56 eV) in comparison to that of CdO (2.76 eV). These values predict semiconducting nature of the samples and may be caused by structural disorder due to oxygen deficiency in the samples [20].
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Figure 5: Uv-visible spectra of (a) CdO and (b) Ag-CdO (inset) Tauc plot indicating band gap. 3.4. FTIR
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The FT-IR spectra of CdO and Ag-CdO have been illustrated in Figure 6. On examining the spectra
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obtained after spectral scanning, and focussing on the frequently used portion between 400 and
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4000 cm-1, the bands emerging in relation to stretching collision of Cd-O were observed in the
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range 600-1000 cm-1 [21]. The metal-oxygen bond at 1387 cm-1 corresponds to the formation of CdO from cadmium acetate [22]. The peak at 1635 cm-1 symbolizes the bending collision of
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adsorbed water. The broad band centred at 3443 cm-1 signifies the stretching collision of H-O-H
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bonds that are chemically associated with CdO [23].
Figure 6: FT-IR spectra of (a) CdO and (b) Ag-CdO. 3.5. PL spectra
The room temperature PL emission spectrum was recorded by employing an excitation wavelength of 470 nm [Figure 7].The spectrum for CdO showed peaks at 543 nm, 604 nm and 730 nm while those for Ag-CdO appeared red shifted by nearly 2 nm. The green (543 nm) and orange (604 nm)
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emissions could have resulted due to the presence of oxygen vacancies [24]. Also interestingly, the emission intensity of Ag-CdO was found to be low in comparison to that of CdO. PL spectrum throws light on the processes of recombination of charge carriers [25-29]. Low emission intensity predicts efficient separation of photo induced charge carriers which is desirable for better photoactivity. Ag nanoparticles probably acted as an electron sink trapping the photogenerated
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A
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electrons [30].
Figure 7: PL spectra (a) CdO and (b) Ag-CdO.
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3.6. Mechanism of photocatalysis When Ag-CdO was irradiated with Uv light, the electrons from the valence band jumped to the
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conduction band while holes were created in the valence band. These holes were adsorbed by the surrounding water molecules to produce highly reactive hydroxyl radicals (OH.). The photogenerated electrons got transferred to the silver nanoparticles which reacted with the
atmospheric oxygen molecules to produce highly reactive superoxide radicals (O2-.). These
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reactive oxidative species are in turn responsible for the degradation of the organic dye pollutants.
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Figure 8: Photocatalysis mechanism of Ag-CdO.
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3.7.Reaction kinetics
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10 mg/L of aqueous solution of MG and 10 mg of the photo catalyst (CdO and Ag-CdO) were
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loaded in the photo-reactor and stirred for 60 min in the dark to ensure an adsorption-desorption equilibrium. After that the mixture was subjected to Uv irradiation and the decrease in absorbance
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as a function of irradiation time was recorded. Figure 9 shows the Uv-vis absorption spectra of (a)
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CdO (b) Ag-CdO. Both the samples showed an appreciable decrease in absorbance with irradiation
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time. Initially the rate of degradation is high which may be attributed to presence of greater active surface area of the photocatalyst but as time progresses the degradation becomes slower since
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intermediates take a longer time to degrade. The greater decrease in absorbance in the case of AgCdO may be attributed to the presence of noble metal Ag which can cause improved trapping of
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electrons during irradiation that tends to inhibit electron-hole recombination better [16].
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Figure 9: Absorbance versus wavelength plot of the photo catalysts (a) CdO and (b) Ag-CdO The photo catalytic degradation efficiency (PD) of the dye was calculated using (3) [31]. ( A0 At ) 100, A0
(3)
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PD
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where A0 is the absorbance of dye solution before the photo irradiation, At is the absorbance of
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solutions in suspension after photo-irradiation for certain time t. It is observed that Ag-CdO
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showed better degradation ability (78.02 %) in comparison to CdO (75.85 %) [Figure 10].
Figure 10: Degradation kinetics PD versus time of (a) CdO and (b) Ag-CdO
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The first-order kinetic model was tested to understand the order of photo catalytic
degradation reaction [32]:
A ln 0 k1t , At
(16)
where A0 and At (mg/L) are the MG concentrations at time zero and t (min), respectively, and k1 (min-1) is the rate constant. In order to calculate rate constant (k1), -ln (At/Ao) was plotted with respect to time [Figure 11] and the data were fitted linearly. The slope of the line was the required
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rate constant (k1) of the reaction which was found to be 0.0112 min-1 for Ag-CdO compared to
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0.0088 min-1 for CdO.
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Figure 11: Degradation kinetics –ln (A/A0) versus time of (a) CdO and (b) Ag-CdO. Compared to CdO, Ag-CdO showed better photoactivity. This could be attributed to the
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lower band gap energy of Ag-CdO which must have rendered more surface active sites for
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photocatalysis. This was also complimented by the higher rate constant shown by Ag-CdO [Figure
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12] which ultimately led to a faster degradation of MG on the catalyst surface.
Figure 12: Rate constant bar diagram showing comparison of CdO and Ag-CdO catalysts 4. Conclusions
In a nutshell, silver incorporated cadmium oxide nanocomposites with high photoactivity and reduced photoluminescence have been successfully synthesized. The ability of the nanocomposite to degrade malachite green dye, which represents a class of organic pollutants in water, has been
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investigated. X- ray diffraction studies confirmed that the synthesized Ag-CdO demonstrated cubic structure with grain size 15.5 nm and a strong (111) orientation. The PL emission peaks at 545 and 606 nm confirm the presence of oxygen vacancies. The change in morphology of the Ag-CdO nanocomposite from foam-like to leaf-like was observed. The Uv-vis spectra showed absorbance in the visible region of the electromagnetic spectrum which makes Ag-CdO suitable for making
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solar cells and transparent electrodes. The band gap studies were also consistent with the Uv-vis
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results. The better photocatalytic activity of Ag-CdO in degrading MG solution may be attributed
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to the efficient separation of photoinduced charge carriers. The mechanism of enhanced
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photoactivity shown by Ag-CdO was also discussed in details predicting that OH. And O2.- radicals play the dominant role in the degradation process. Thus the present study reveals that Ag modified
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CdO nanocomposite may be considered as a potentially promising photocatalyst for degradation
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Acknowledgements
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of organic dyes in waste water.
Authors highly acknowledge DST-FIST (SR/FST/PSI-175/2012) project and Department of
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Chemistry, University of Kalyani for providing instrumental facility.
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Declaration of interests None
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