Photo-catalytic activity of Zn1-x Mn x S nanocrystals synthesized by wet chemical technique
© Chitkara et al; licensee Springer. 2011
Received: 23 April 2010
Accepted: 24 June 2011
Published: 24 June 2011
Polyvinyl pyrrolidone capped Zn1-x Mn x S (0 ≤ x ≤ 0.1) nanocrystals have been synthesized using wet chemical co-precipitation method. Crystallographic and morphological characterization of the synthesized materials have been done using X-ray diffraction and transmission electron microscope. Crystallographic studies show the zinc blende crystals having average crystallite size approx. 3 nm, which is almost similar to the average particle size calculated from electron micrographs. Atomic absorption spectrometer has been used for qualitative and quantitative analysis of synthesized nanomaterials. Photo-catalytic activity has been studied using methylene blue dye as a test contaminant. Energy resolved luminescence spectra have been recorded for the detailed description of radiative and non-radiative recombination mechanisms. Photo-catalytic activity dependence on dopant concentration and luminescence quantum yield has been studied in detail.
Environmental pollution, toxic water pollutants, and industrialization on a global scale have drawn attention for sustained fundamental and applied research in the area of environmental remediation. The increased public concern with environmental pollutants has prompted the need to develop novel treatment methods  where photo-catalysis is gaining a lot of attention in the field of pollutant degradation. Semiconductor photo-catalysts offer the potential for complete elimination of toxic chemicals through their efficiency and potentially broad applicability [2, 3]. Recently, semiconductor nanocrystals have attracted great attention due to their size tunable physical and chemical properties. Transition from bulk to nanoparticles lead to the display of quantum mechanical properties and an increased dominance of surface atoms, which give rise to unique photo-physical and photo-catalytic properties of nanomaterials, for example, with the decrease of particle size, extremely high surface to volume ratio is obtained leading to an increase in surface specific active sites for chemical reactions and photon absorption to enhance the reaction and absorption efficiency. The enhanced surface to volume ratio causes increase of surface states, which changes the activity of electrons and holes, affecting the chemical reaction dynamics. The size quantization increases the bandgap of photo-catalysts to enhance the redox potential of conduction band electrons and valence band holes .
Various new compounds and materials for photo-catalysis have been synthesized in the past few decades [5–13]. Semiconductor photo-catalysts, with a primary focus on TiO2[14–17], have been applied to variety of problems of environmental interest in addition to water and air purification. The application of illuminated semiconductors for degrading undesirable organics dissolved in air or water is well documented and has been successful for a wide variety of compounds . Transition-metal sulphides, in particular ZnS [18, 19], have unique catalytic functions as a result of the rapid generation of electron-hole pairs by photo-excitation and the highly negative reduction potentials of excited electrons. Moreover, incorporation of metal ion dopants in these semiconductor nanoparticles can influence their photo-catalytic performance. Doping of Mn2+ ions in ZnS lengthens the lifetime of generated charge carriers, resulting in an enhancement in the photo-activity. Hence, ZnS:Mn2+ nanocrystals can be efficiently used for environmental cleaning, H2 production, and water purification. This article reports photo-catalytic activity of Zn1-x Mn x S nanocrystals. Photo-catalytic activity has been well correlated with the luminescence quantum yield. Moreover, photo-catalytic and luminescence efficiency dependence on the Mn2+ concentration have been described in detail.
Zn1-x Mn x S (0 ≤ x ≤ 0.1) nanocrystals have been synthesized using wet chemical co-precipitation method already opted by Singh et al.  for the synthesis of Eu3+ doped Cd1-x Zn x S quantum dots. All synthesis was carried out at room temperature under ambient conditions in aqueous media for its inherent advantages of being simple and environmental friendly. Analytical reagent grade chemicals: zinc acetate (C4H6O4Zn · 2H2O), manganese acetate (C4H6MnO4 · 4H2O), sodium sulphide (Na2S · H2O), and polyvinyl pyrrolidone (PVP) [(C6H9NO) n ] were used without further purification. Solutions of 0.5 M zinc acetate, 0.5 M sodium sulphide, and 1 M manganese acetate were prepared in separate beakers. Then zinc and manganese precursor solutions were mixed in the stoichiometric proportion under vigorous stirring, 4 ml of 2% PVP solution was added to total 50 ml volume, before drop wise addition of sulfur precursor. PVP will act as the capping agent to avoid the agglomeration of nanocrystals. The resulting precipitates were centrifuged and dried in vacuum oven for 10 to 12 h continuously.
Panalytical's (Netherlands) X'Pert Pro Powder X-ray diffractometer with Cu Kα radiation (λ = 1.541 Å) was used to record diffraction patterns of the synthesized samples in the 2θ range 20 to 60°. Average crystallite size has been calculated from the line broadening of the X-ray diffraction (XRD) diffractogram using Scherrer formula . Hitachi, [(H-7500), Japan] transmission electron microscope (TEM) was used to record micrographs for average particle size determination. For TEM studies, a drop of well ultrasonicated ethanol dispersed nanocrystals was placed on the carbon coated copper grid.
Atomic absorption spectrometer (Analytic Jena, Germany) has been used for qualitative and quantitative analysis of the synthesized nanomaterials. Sample preparation for the analysis involves dissolution of 0.01 mg of nanocrystals in 10 ml of 0.5% HNO3.
Energy resolved luminescence spectra were recorded using FlouroMax-3 (Jobin-Yvon, Edison, NJ, USA) spectrofluorometer equipped with photomultiplier tube and a xenon lamp.
The photo-catalytic activity of Zn1-x Mn x S nanocrystals was studied by monitoring the degradation of methylene blue (MB) (C16H18ClN3S · 2H2O) dye in an aqueous suspension containing Zn1-x Mn x S nanocrystals under the UV-radiation exposure with continuous magnetic stirring. A 350 ml of aqueous suspension was prepared by completely dissolving 1.1322 mg of the MB dye and then dispersing 140 mg of the Zn1-x Mn x S nanocrystals in the de-ionized water. The resulting suspension was equilibrated by stirring in the dark for 1 h to stabilize the adsorption of MB dye on the surface of nanocrystals. The stable aqueous suspension was then exposed to the UV-radiation with continuous magnetic stirring, using the home made photoreactor containing two 18-W tubes as the UV-source (λ = 200 to 400 nm). Following the UV-radiation exposure, 10 ml sample of aqueous suspension was taken out after every 10-min interval for the total 80 min of the UV-radiation exposure. Suspension sample was centrifuged to filter out the Zn1-x Mn x S nanocrystals, then nanocrystal free aqueous dye solution was examined using UV-vis absorption spectrophotometer (Systronics PC Based Double Beam Spectrophotometer:2202) to study the photo-degradation of the MB dye.
Results and discussion
Atomic absorption spectrometer (AAS) studies show that the actual concentration of manganese doping is approx. 24% of the initial manganese precursor concentration, which was added to the reaction media. So, the value of x in Zn1-x Mn x S corresponds to initial atomic weight concentration of manganese with respect to zinc, which was calculated during stochiometric addition of precursors in the chemical co-precipitation reaction.
Zn1-x Mn x S (0 ≤ x ≤ 0.1) nanocrystals have been successfully synthesized in aqueous media using a simple wet chemical precipitation technique. Crystallographic and morphological studies reveal the zinc blende nanostructures having average crystallite size approx. 3 nm. Energy resolved luminescence spectra report the quenching of host-related 425 nm emission and enhancement in luminescence quantum yield of dopant-related 599 nm emission, with the increasing concentration of Mn2+ ions. Photo-catalytic activity of nanocrystals studied using MB dye as a test contaminant enhances with the addition of Mn2+ ions in ZnS nanocrystals upto to optimal concentration (1 at. wt% of Zn2+), but the further increase of dopant concentration deteriorates photo-catalytic activity of Zn1-x Mn x S nanocrystals as the recombination of trapped carriers dominates the interfacial charge transfer at the higher dopant concentrations. This mechanistic information of photo-catalytic activity dependence on dopant concentration and luminescence quantum yield will significantly contribute to enhance the understanding of photo-initiated processes in semiconductor nanocrystals.
atomic absorption spectrometer
transmission electron microscope
Authors express their gratitude to Dr. Shyam Kumar, Chairman, Department of Physics, Kurukshetra University, Kurukshetra and Dr. Sanjiv Aggarwal, Reader, Department of Physics, Kurukshetra University, Kurukshetra for energy resolved spectroscopic studies. Regional Scientific Instruments Centre (RSIC), Punjab University, Chandigarh is gratefully acknowledged for crystallographic and morphological studies.
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