Temperature Induced Structural and Photoluminescence Properties

Mechanics, Materials Science & Engineering, April 2017 – ISSN 2412-5954

Temperature Induced Structural and Photoluminescence Properties of Poly Ethylene Glycol (PEG) Capped/Uncapped Cadmium Oxide Nanoparticles (CdO NPs)51 P. Lokanatha Reddy 1, S.K. Khadheer Pasha1, a 1 – Sensor Laboratory, Materials Physics Division, School of Advanced Sciences, VIT University, Vellore, Tamil Nadu, India a – khadheerbasha@gmail.com DOI 10.2412/mmse.31.67.343 provided by Seo4U.link

Keywords: cadmium oxide nanoparticles (CdO NPs), X-ray Diffraction (XRD), photoluminescence (PL).

ABSTRACT. Uncapped and PEG capped CdO NPs were successfully synthesized by precipitation technique. XRD and TEM studies were used to investigate the particles structure, size and shape. As synthesized samples showed major hexagonal Cadmium Hydroxide [Cd (OH)2] phase and were completely transformed into cubic CdO crystalline phase above 400 °C annealing temperature. A good crystallinity was noticed in PEG capped CdO NPs. TEM images brought out the information about the synthesized nanoparticles (NPs) existed spherical in shape. The weight loss from thermogravimetric analysis (TGA) graphs depicted the formation of CdO NPs from Cd (OH)2. PEG on CdO NPs evidently increased the direct band gap emission intensity around 480 nm and while indirect band gap emission intensity of around 620 nm was increased in uncapped CdO NPs. Beside this, PL spectra revealed interesting changes with the effect of PEG, annealing temperature and excitation wavelengths. Hence PL spectra ascertained their possible use for optical and electronic applications.

Introduction. CdO nanostructures have attracted considerable attention to various applications such as solar cells, photocatalytic and photodiodes due to its little electrical resistivity, high carrier concentration and high optical transmittance in the visible region [1]. CdO is an important n-type and II–VI semiconductor material with a direct band gap of 2.5eV and an indirect band gap of 1.98 eV [2, 3]. Due to dual band gap nature, CdO finds its potential applications in the field of optoelectronic devices [4]. Being convenient to synthesis NPs, the chemical precipitation method was selected due to low production cost, higher yield rate, low temperature maintenance and easy performance at any climate [5]. It is known that size of the NPs and crystalline structure can be restricted easily through the use of surfactants in the synthesis process of the system. Various surfactants (CTAB, PVP, PEG) can alter the shape, size and other surface properties to different extent depending upon their molecular structure of NPs. It is reasonable that short chain polymers can also promote the formation structure of nanosystems [6]. In the current study, PEG is found to be selectively promoting the formation of CdO NPs. From literature survey there is no report available on systematic investigation of structural and PL properties of PEG capped CdO NPs. Hence we were reporting the effect of PEG and annealing temperature on optimization of structural and photoluminescence properties of CdO NPs. Experimental Synthesis procedure. The cadmium acetate dihydrate [Cd (CH3COO)2·2H2O], sodium hydroxide (NaOH) and PEG were the starting materials obtained from Sigma-Aldrich and used as received without further purification. First, cadmium acetate dihydrate added in the 100 ml of DI water to form 51

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Mechanics, Materials Science & Engineering, April 2017 – ISSN 2412-5954

0.2 M of acetate aqueous solution under continuous stirring till getting clear solution. Then 2M NaOH solution was slowly added drop wise in order to fix pH = 9. After controlling the pH, using hot plate of the stirrer, the temperature of obtained the solution was raised and maintained at 60 °C under continuous stirring for 6 h. After the completion of reaction, the reaction mixture was allowed to cool at the room-temperature (RT) .White colored precipitate was obtained which was poured and washed with DI water for several times. The obtained material was dried at 60 °C temperatures in the hot air oven for 4 h and after drying, annealed at different ambient temperatures 100, 200, 300, 400 and 500 °C for 2h in the muffle furnace. The same procedure was applied for both PEG capped and uncapped CdO NPs. Result and Discussions. Structural and Morphological Studies. XRD measurements were performed and the spectra of as prepared and annealed uncapped/PEG capped samples were given in Fig. 1 (a) – (b).

Fig. 1. (a) – (b). XRD pattern of (a) uncapped and (b) PEG capped CdO NPs annealed at various temperatures.

Fig. 3. TEM images of (a) uncapped and (b) PEG capped CdO NPs annealed at 400 °C temperatures.

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Mechanics, Materials Science & Engineering, April 2017 – ISSN 2412-5954

As-prepared uncapped sample revealed the presence of many diffraction peaks. The major peaks at (0 0 1), (1 0 0), (0 1 1), (2 0 0), (0 1 2), (1 1 0), (1 1 1), (2 0 1), (2 0 2) revealed the Cd (OH)2 hexagonal crystalline phase [JCPDS No. 73-0969], along with minor amount of peaks Cd (OH)2 monoclinic phase. While PEG capped as prepared sample showed only Cd (OH)2 hexagonal crystalline phase [JCPDS No. 73-0969] due to PEG selective formation. Uncapped annealed sample at 200 °C temperature showed mixed phase of cubic CdO along with hexagonal and monoclinic Cd (OH)2 crystallinity [3, 5]. At 300 °C temperature it showed pure cubic CdO crystalline nature with minor peak of monoclinic Cd (OH)2 crystallinity and a perfect cubic CdO was observed at 400 °C with diffraction peaks (111), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) planes with JCPDS card No: 05-0640 [5].

PEG capped sample showed major cubic CdO and hexagonal along with minor phase of monoclinic Cd (OH)2 crystalline phase at 200°C and monoclinic Cd (OH)2 crystalline phase was diminished with annealing temperatures 300 °C, 400 °C and perfect cubic crystalline phase was observed above 400 °C without any impurity. The average crystallite size for PEG capped and uncapped samples at 400 °C are calculated to be 40 nm. TEM images depicted pseudo spherical shape of CdO NPs and were showing little agglomeration than PEG capped CdO NPs as shown in Fig.3. The average size of the particles was in the range of 30 nm - 40 nm.

Thermal studies. The thermal behavior of all samples and translation temperature of Cd (OH)2 to CdO NPs were carried out by TGA between RT and 800 °C with a heating rate of 10 °C/min.

Fig. 4. (a) - (f) TGA patterns of uncapped CdO and PEG capped CdO NPs showing weight loss in (%) at various annealing temperatures.

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Mechanics, Materials Science & Engineering, April 2017 – ISSN 2412-5954

Fig. 4 shows the TGA traces of PEG capped and uncapped CdO samples. The overall weight losses were estimated about 15%, 17% in uncapped and PEG capped samples as shown in Fig. 4 (a) - (b).

The total weight loss was 7% and 5% observed in 300 °C and 400 °C annealed samples as shown in Fig.4 (c) - (f). This weight loss was very little as compared with as prepared samples. Hence we may confirmed that Cd (OH)2 and CdO samples consists of only OH− ions and organic residues without any other impurities.

Photoluminescence studies. PL spectra were recorded at different temperatures with different excitation wavelengths (325 nm, 336 nm and 350 nm) as shown in Fig. 5. It was observed that the PL spectra consist of maximum emission intense peaks centered around 480 nm and 620 nm along with multiple subsidiary peaks in blue and red regions for both uncapped and PEG capped samples at annealing temperatures 300 °C, 400 °C and 500 °C. The subsidiary peaks below 480 nm may be attributed to the combination of the holes from the valence band (VB) and electrons from the conduction band (CB) and the major peak at 480 nm was attributed to the excitonic transition of CdO NPs [7] was size-dependent. Another broad intense emission peak at 620 nm was attributed to indirect near band edge emission from CdO or may be due to oxygen vacancies (VO), cadmium interstials (Cdi) exist in the CdO crystal lattice ascribed to the heat treatments or oxidation associated with the process which helps form the recombination centers [8]. The surfactant, PEG quenches the electron - hole recombination in PEG capped samples and controlled the formation of defects by enhancing the crystallinity. This decrease in intensity with PEG indicates the decrease in the proportion of electron-hole recombination, which favored the photocatalytic activity for degradation of pollutants.

Hence it was concluded that emission intensity and direct & indirect band nature were controlled by surfactant PEG and annealing temperature for better applications in photocatalytic activity and other optoelectronic devices.

Fig. 5. (a) – (f). Photoluminescence Spectra of uncapped and PEG capped CdO NPs at different excitation wavelengths and annealing temperatures.

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Summary. Uncapped and PEG capped CdO NPs were successfully synthesized by chemical precipitation method. XRD spectra depicted the formations of cubic crystalline phase of CdO NPs from hexagonal crystalline phase Cd (OH)2 nanoparticles after systematic annealing temperature above 400 °C. The observed weight loss from TGA graphs at different annealing temperatures revealed the systematic transformation Cd (OH)2 phase to CdO NPs. It was noticed that the PL emission intensity was influenced by capping agent as well as annealing temperature. Direct band PL emissions of CdO NPs were enhanced in capped PEG and while indirect band gap emission was dominated in uncapped CdO NPs. References [1] R.K. Gupta, K. Ghosh, R. Patel, S.R. Mishra, P.K. Kahol, Highly conducting and transparent tindoped CdO thin films for optoelectronic applications, Mater. Lett., 62, 2008, 4103-4105, DOI:10.1016/j.matlet.2008.06.008.

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[4] G.R. Khayati, H. Dalvand, E. Darezereshki, A. Irannejad, A facile method to synthesis of CdO Nanoparticles from spent Ni–Cd batteries, Mater. Lett., 115, 2014, 272-274, DOI:10.1016/j.matlet.2013.10.078.

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Cite the paper P. Lokanatha Reddy, S.K. Khadheer Pasha (2017). Temperature Induced Structural and Photoluminescence Properties of Poly Ethylene Glycol (PEG) Capped/Uncapped Cadmium Oxide Nanoparticles (CdO NPs). Mechanics, Materials Science & Engineering, Vol 9. doi:10.2412/mmse.31.67.343

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