Photo-Catalytic Activity of Ca1−xNixS Nanocrystals

Ca1−xNixS (0 ≤ x ≤ 0.05) nanocrystals have been synthesized by facile solid state reaction method. Synthesized nanocrystals are further etched with mild acid solutions to reduce the particle size, which augments the surface to volume ratio and confinement of carriers. Crystallographic and morphological characterizations of synthesized nanomaterials have been done by X-ray diffraction and electron microscopy, respectively. Comparison of the diffraction and electron microscopy studies reveal the formation of single crystalline nanostructures. Optical characterization of synthesized nanomaterials has been done by UV-vis. absorption spectroscopic studies. The photo-catalytic activity of synthesized nanomaterials under UV irradiation has been tested using methylene blue (MB) dye as a test contaminant in aqueous media. Photo-catalytic behaviour dependence on dopant concentration and etching has been thoroughly studied to explore the potential of synthesized nanomaterials for next era optoelectronic industrial applications as well as polluted water purification.


Introduction
Semiconductor nanostructures due to their size tunable photo-physical and photo-chemical properties caused by enhanced surface to volume ratio and quantum confinement effects on nanoscale, establish as competent materials for smart optoelectronic industrial applications as well as efficient photo-catalyts [1]- [5].Efficiently luminescing intrinsic and extrinsic semiconductor nanocrystals have received considerable attention during the last three decades [6]- [10].The changes in the electronic structure as a function of particle size due to quantum size effects are intriguing and have stimulated fundamental research on various types of nanocrystalline semiconductors.Intrinsic and extrinsic semiconductor nanostructures are generally used as heterogeneous photo-catalysts due to their high oxidizing power and non-requirement of oxidizing reagents.Heterogeneous photo-catalytic processes are widely utilized to oxidize or mineralize organic/inorganic substances that could not be degraded with biological techniques.Semiconductor photo-catalysts offer the potential for completing elimination of toxic chemicals through their efficiency and potentially broad applicability.Doped calcium sulfide (CaS) nanophosphor has a great potential for its applications as a phosphor in CRTs, fluorescent screen, displays panels and even in thermoluminescence dosimetry [11]- [13].Numerous researchers have studied the irradiation effects in doped CaS during last several decades [14]- [16].Potential of doped CaS nanocrystals as UV dosimeters has been also reported [17] [18].But to the best of our knowledge, nobody has tried to explore the potential of intrinsic and extrinsic CaS nanocrystals for photo-catalytic applications.
In the present investigations emphasis has been given to explore the photo-catalytic activity potential of Ca 1−x Ni x S (0 ≤ x ≤ 0.1) nanocrystals synthesized via facile solid state reaction method.The photo-catalytic behavior of synthesized nano photo-catalysts has been studied by recording MB dye degradation in aqueous media under UV radiation exposure.Photo-catalytic activity dependence on the nanocrystals morphology and dopant concentration has been thoroughly studied.

Experimental
Ca 1−x Ni x S (0 ≤ x ≤ 0.1) nanocrystals have been synthesized by the solid state diffusion method [19] [20].Analytical reagent (AR) grade chemicals; calcium sulphate, nickel nitrate, sodium thoisulphate, carbon powder and ethanol were used as the starting materials without further purification.All these chemicals were purchased from Sigma Aldrich and purity content of all these chemicals was 99.9%.Carbon acts as a reducing agent for the reduction of sulphate to sulphide at high temperature, and nickel acts as an activator.Sodium thoisulphate acts as flux for the reaction.The calculated quantities of calcium sulphate, carbon powder, nickel nitrate and flux were mixed thoroughly with the help of agate pestle and mortar.For the uniform distribution of dopant, initially dopant precursor was dissolved in small volume of ethanol, and then this solution was mixed with the entire charge.Then, the resulting mixture was transferred to a clean graphite crucible (which was already baked at the firing temperature) and a thin layer of carbon powder spread over it.The charge in the crucible was covered with another similar crucible.The thin layer of the carbon over the charge created a reducing environment at high temperature.This whole arrangement was placed in the muffle furnace, and the charge was fired at 950˚C for two hour.Firing at high temperature causes the incorporation of nickel in the host lattice.After two hour, the red hot charge was taken out and rapidly crushed.Finally, the prepared powder sample was collected in dry sample tubes for further studies.For the further reduction of the particle size, the synthesized samples were etched with dilute hydrochloric acid.The particles were dispersed in etching solution (HCl aqueous solution) using mechanical stirring.The solution was allowed to settle for a couple of minutes.Then, the etched nanoparticles were collected by centrifugation and redispersed in ethanol.Then these particles were dried in vacuum oven at ambient temperature for further studies.
Panalytical's (Netherlands) X 'Pert Pro Powder X-ray diffractometer with Cu K α radiation (λ = 1.541Å) was used to record diffraction patterns of the synthesized samples in the 2θ range 20˚ -80˚.Average crystallite size has been calculated from the line broadening of the X-ray diffraction (XRD) diffractograms using Scherrer formula [21].Hitachi, [(H-7500), Japan] transmission electron microscope (TEM) was used to record the electron micro-graphs for average particle size determination.For TEM studies, a drop of well ultrasonicated ethanol dispersed nanocrystals was placed on the carbon coated copper grid.
The photo-catalytic activity of synthesized Ca 1−x Ni x S (0 ≤ x ≤ 0.1) nanocrystals was studied by monitoring the degradation of methylene blue (MB) (C 16 H 18 ClN 3 S•2H 2 O) dye in an aqueous suspension containing 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 Ca 1−x Ni 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 -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.Suspen-sion sample was centrifuged to filter out the nanophoto-catalyst particles, then nanophoto-catalyst free aqueous dye solution was examined using UV-vis absorption spectrophotometer (Hitachi U-2900) to study the photodegradation of the MB dye.

Results and Discussion
Nominally broadened XRD patterns have been recorded for all the synthesized Ca 1−x Ni x S samples.Figures 1-3 show X-ray diffractograms recorded for pristine CaS, etched CaS and Ca 0.95 Ni 0.05 S nanocrystals, respectively.Recorded diffractograms confirm the formation of nanocrystallites having average crystallite size extremely larger than Bohr exciton radius of the calcium sulphide.Crystallographic characteristics are strongly dependent on the synthesis conditions and hence the morphology.In case of pristine CaS nanocrystals, major diffraction peak has been observed at 25.3˚, on the other hand, major diffraction peak at 26.5˚ has been observed for the etched CaS and Ni doped CaS samples.It can be clearly observed from the recorded diffractograms that the addition of nickel ions in the host lattice augments the crystalline texture of synthesized nanocrystals.
Figure 4 shows the TEM micrograph recorded for synthesized CaS nanocrystals.Although, synthesized   nanoparticles appear in the form of agglomerates due the absence of capping agents, but average size of the fundamental units of agglomerates are less than 20 nm.
Figure 5 shows the UV-Vis.absorption spectra of synthesized nanocrystals dissolved in aqueous media.Recorded spectra show the blue shift in the absorption edge of the etched samples compared to the pristine samples.The blue shift in the etched samples reveals the reduction in the particle size, which is also confirmed by the XRD results.All the synthesized samples have sturdy absorption in UV region.Decrease of absorbance with increasing irradiation time shows the efficient degradation of MB dye, hence the synthesized nanocrystals have excellent photo-catalytic activity potential to purify the dye contaminated water.But, comparative study of the recorded results shows that the pristine CaS nanocrystals have high photo-catalytic activity than the etched and doped CaS nanocrystals.Etching may enhance the surface defect states, which may act as charge carrier recombination centers and thus diminishes the probability of charge carrier's participation in oxidation and reduction reactions.Initially, addition of nickel in CaS causes decline in the photo-catalytic activity, as these dopant ions may introduce ladder like levels in the forbidden gap of the host material for the non-radiative relaxation of charge carriers, but increasing dopant concentration slightly boost the photo-catalytic activity.

Conclusion
Solid state reaction method is an efficient method, which gives high yield of good quality Ca 1−x Ni x S (0 ≤ x ≤ 0.05) nanocrystals.Crystallographic and morphological characteristics of the synthesized nanomaterials are tunable with synthesis reaction parameters.Dilute hydrochloric acid acts as an efficient nanomaterial etchant, although the nanomaterial morphology can easily altered with etching, but etching enhances the surface defect states, which may act as charge carrier recombination centers to reduce the photo-catalytic activity.Pristine CaS nanocrystals have high photo-catalytic activity than etched and doped CaS nanocrystals.These eco-friendly nanocrystals can be used for purification of polluted water.

Figure 6
Figure6shows the absorption spectra of the MB dye solution in the presence of CaS nanocrystals for different durations of UV radiation exposure.Figure7& Figure8show the absorption spectra of dye solution for different durations of UV-radiation exposure in the presence of etched CaS and Ca 0.95 Ni 0.05 S nano-photocatalysts.

Figure 7 &
Figure6shows the absorption spectra of the MB dye solution in the presence of CaS nanocrystals for different durations of UV radiation exposure.Figure7& Figure8show the absorption spectra of dye solution for different durations of UV-radiation exposure in the presence of etched CaS and Ca 0.95 Ni 0.05 S nano-photocatalysts.

Figure 6 .
Figure 6.Absorption spectra of dye solution for different durations of UVradiation exposure in the presence of pristine CaS nanocrystals.

Figure 7 .
Figure 7. Absorption spectra of dye solution for different durations of UVradiation exposure in the presence of etched CaS nanocrystals.

Figure 8 .
Figure 8. Absorption spectra of dye solution for different durations of UVradiation exposure in the presence of Ca 0.95 Ni 0.05 S nanocrystals.