Research in Chemical activity of Zn1-xSexTe Nano Semiconductor crystals

Abstract: Zn1-xSexTe Nano crystals have been synthesized using wet chemical co-precipitation method. Crystallographic and morphological characterizations 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 Nan materials. 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 do pant concentration and Luminescence quantum yield has been studied in detail.

Keywords: Catalyst, Do pant, Nano Crystals, Synthesization, Ternary Semiconductors,

PACS Codes: 72.30.+q, 73.50.Mx, 81.16.-c

Introduction Introducing catalytic activity reduces 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 [1] 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 nano particles 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 nano materials, 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 band gap of photo-catalysts to enhance the redox potential of conduction band electrons and valence band holes [4]. 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 dis-solved in air or water is well documented and has been successful for a wide variety of compounds [2].

Transition- metal sulphides, in particular ZnSe [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 Se2+ ions in ZnSe lengthens the lifetime of generated charge carriers, resulting in an enhancement in the photo-activity.

Hence, ZnS:Se 2+ nanocrystals can be efficiently used for environmental cleaning, H2 production, and water purification.

This article reports photo-catalytic activity of Zn1-xSexTe nanocrystals. Photo-catalytic activity has been well correlated with the luminescence quantum yield. Moreover, photo-catalytic and luminescence. which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. efficiency dependence on the Se2+ concentration have been described in detail.

Experimental Zn1-xSexTe (0.... x =0.1) nanocrystals have been synthe-sized using wet chemical co-precipitation method already opted by Singh et al. [20] for the synthesis of Eu 3+ doped Zn1-xSexTe 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 ,manganese acetate, 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.

Analytical Expert Pro Powder X-ray diffract meter with Cu Ka radiation (l = 1.541 Å) was used to record diffraction patterns of the synthesized samples in the 2.... range 20 to 60°. Average crystallite size has been calculated from the line broadening of the X-ray diffraction (XRD) diffract gram 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, Ger-many) has been used for qualitative and quantitative analysis of the synthesized nanomaterials. Sample pre-paration 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) spec-trofluorometer equipped with photo multiplier tube and a xenon lamp. The photo-catalytic activity of Zn1-xSexTe nanocrystals was studied by monitoring the degradation of methylene blue (MB) (C16H18ClN3S ·2H2O) dye in an aqueous suspension containing Zn1-xSexTe nanocrystals under the UV-radiation exposure with continuous magnetic stirring. Completely dissolving 1.1322 mg of the MB dye and then dispersing 140 mg of the Zn1-xSexTe nanocrystals in the de-ionized water prepared a 350 ml of aqueous suspension.

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 photo reactor containing two 18-W tubes as the UV-source (l = 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-xSexTe nanocrystals, then nanocrystal free aqueous dye solution was examined using UV-vis absorption spectrophotometer

Results and discussion Broad XRD patterns have been recorded for all the synthesized Zn1-xSexTe samples, Figure1 shows one such X-ray diffract gram recorded. It is clear from the diffract gram that synthesized samples crystallize in zinc blende structure with the planes Recorded diffraction peaks are broadened due to the nanocrystalline nature of particles.

Average crystallite size calculated from the recorded XRD patterns is approx. 3 nm. Average particle size calculated from micrograph is approx. 3 to 4 nm, which is in proximity to the average crystallite size determined by XRD.

So, all the synthesized particles are single nanocrystals. 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-xSexTe corresponds to initial atomic. It is clear from the recorded spectrum that pure ZnSe nanocrystals show only 425-nm emission peak, whereas dichromatic emission (l1 = 425 nm and l2 = 599 nm) has been observed in case of Se 2+ -doped ZnSe nanocrystals. Lumi-nescencequantum yield of l2 emission peak go on increasing with the increase of ‘x’ in Zn1-xSexTe nano-crystals, whereas l1 emission intensity go on decreasing with increasing concentration of Se 2+ ions.

More than six-fold increase and two-fold decrease has been observed in the emission intensities of l2 and l1 peaks, respectively, when the value of ‘x’ changes from 0.01 to 0.1 in Zn1-xSexTe nanocrystals. The Se 2+ ions substitute the Zn 2+ ions in the ZnTe nanocrystal acting as trap sites, where the electrons and holes can be trapped. Electrons after photo-excitation process in the host lattice subsequently decay via non-radiative process to the 4 T1 localized state of manganese.

The l2 (599 nm) emission peak is attributed to the radiative decay between the 4 T1 and 6 A1 localized states of manganese inside the ZnS band gap. The l1 emission (425 nm) peak is assigned to the radiative transition of electrons from shallow trap states (ST) near the conduction band to sulfur vacancies (Vs) residing near the valence band.

The increasing do pant concentration quenches the host related 425 nm emission. Detailed mechanism of various processes involved in Zn1-xSexTe nanocrystals upon excitation. Photo-excited electrons from the con-duction band transit spontaneously to the ST and 4 T1 manganese trap sites via non-radiative processes. These ST electrons can recombine radiatively with Vs holes or further relaxed non-radiatively to the localized do pant trapping states. Radiative recombination of ST electrons and Vs holes is faster than the radiative transition between the 4 T1 and 6 A1 localized states [23].

Incorporation of Se 2+ in ZnTe nanocrystal lattice significantly influences the photo-cataly-tic activity. Addition of Se 2+ ions lengthens the lifetime of excited charge carriers, which results the enhanced photo-catalytic activity.

Various charge carrier recombination and charge carrier trapping processes. The competition between the charge carrier recombination and charge carrier trapping followed by the competition between recombination of trapped carriers and interfacial charge transfer determine the overall quantum efficiency for interfacial charge transfer.

Doping of Mn 2+ up to opti-mal concentration increases the interfacial charge transfer probability, due to which photo-catalytic activity of ZnS nanocrystals is enhanced. photo-cat-alytic activity enhances with increasing value of ‘x’ only in the range x =0 tox = 0.01, further increase of do pant con-centration, i.e., x = 0.01 to x = 0.1 deteriorates photo-catalytic activity of Zn1-xSexTe nanocrystals.

It is due to the fact that up to optimal Se 2+ concentration (x = 0.01), Se 2+ ions lengthens the charge carrier recombination, but at higher do pant concentrations although the possibility of charge carrier trapping is high, but the charge carriers may recombine through quantum tunneling. Moreover, increasing concentration of Se 2+ ions may cause the increased interaction between neighboring Zn 2+ ions and the Se 2+ luminescence center that enhances the spin-orbit coupling of Se 2+ ions, which leads to the relaxation of the spin selection rules [24].

This lowers the radiative recombination time for 4 T1 ® 6 A1 transitions, so the recombination of trapped carriers dominates interfacial charge transfer at the higher do pant concentrations. Due to enhanced recombination rate luminescence quantum yield increases to large extent as shown absorption spectrum of MB dye solution for different durations of UV-radiation exposure in the presence of nanocrystals Zn1-xSexTe nanocrystal photo-catalyst is efficiently degrading the dye, only negligible amount of dye is present in the solution after 80 min.

There is a concentration dependent slight spectral shift in MB dye absorption spectra as the UV irradiation time changes from 0 to 80 min. Red shift in the absorption peak with increasing dye concentration has been observed due to augmented optical density. Moreover, at higher concentrations, aggregation can take place, which affects the optical behavior. These non-toxic, stable, inexpensive nanocrystalline photo-catalyst having high-redox potentials can be efficiently used for environmental cleaning, water purification, and H2 production.

Moreover, due to non-dissolving nature in aqueous Photo degradation of MB dye with time. Absorption spectrum of dye solution for different trapped charge carriers. Incorporation of Se 2+ in ZnSe nanocrystal lattice significantly influences the photo-cataly-tic activity. Addition of Se 2+ ions lengthens the lifetime of excited charge carriers, which results the enhanced photo-catalytic activity. Various charge carrier recombination and charge carrier trapping processes .

The competition between the charge carrier recombination and charge carrier trapping followed by the competition between recombination of trapped carriers and interfacial charge transfer determine the overall quantum efficiency for interfacial charge transfer.

Doping of Se 2+ up to opti-mal concentration increases the interfacial charge transfer probability, due to which photo-catalytic activity of ZnTe nanocrystals is enhanced. As shown in ,photo-cat-alytic activity enhances with increasing value of ‘x’ only in the range x =0 tox = 0.01, further increase of do pant con-centration, i.e., x = 0.01 to x = 0.1 deteriorates photo-catalytic activity of Zn1-xSexTe nanocrystals. It is due to the fact that up to optimal Se 2+ concentration (x = 0.01), Se 2+ ions lengthens the charge carrier recombination, but at higher do pant concentrations although the possibility of charge carrier trapping is high, but the charge carriers may recombine through quantum tunneling.

Moreover, increasing concentration of Se 2+ ions may cause the increased interaction between neighboring Zn 2+ ions and the Se 2+ luminescence center that enhances the spin-orbit coupling of Se 2+ ions, which leads to the relaxation of the spin selection rules [24].

This lowers the radiative recombination time for 4 T1 ® 6 A1 transitions, so the recombination of trapped carriers dominates interfacial charge transfer at the higher do pant concentrations. Due to enhanced recombination rate luminescence quantum yield increases to large extent shows the Absorption spectrum of MB dye solution for different durations of UV-radiation exposure in the presence of Zn1-xSexTe nanocrystals (optimal do pant concentration).

Zn1-xSexTe nanocrystal photo catalyst is efficiently degrading the dye; only negligible amount of dye is present in the solution after 80 min. There is a concentration dependent slight spectral shift in MB dye absorption spectra as the UV irradiation time changes from 0 to 80 min. Red shift in the absorption peak with increasing dye concentration has been observed due to augmented optical Density.

Moreover, at higher concentrations, aggregation can take place, which affects the optical behavior. These non-toxic, stable, inexpensive Nan crystalline photo-catalyst having high-redox potentials can be efficiently used for environmental cleaning, water purification, and H2 production. Moreover, due to non-dissolving nature in aqueous Photo degradation of MB dye with time. Absorption spectrum of dye solution for different.

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