53 Journal homepage: www.fia.usv.ro/fiajournal Journal of Faculty of Food Engineering, Ştefan cel Mare University of Suceava, Romania Volume XIV, Issue 1 – 2015, pag. 53 - 57 NAN OS IZ ED IM PLANT S O F Ag + , Cu+ AND Cu 2 + IONS INTO T HE S UR- FA CE LAYE R OF POLYC RYS TAL L INE CdS PHOT OCAT AL YST FO R TH E OX ID ATIO N R EACT IO N OF I OD IDE ION S *Igor KOBASA1, Lyubomyra ODOSIY2 1Yu. Fedkovych National University of Chernivtsy, Ukraine, 2Petro Sagaydachnyi Academy of Ground Forces, Lviv, Ukraine i.kobasa@chnu.edu.ua * Corresponding author Received 5th March 2015, accepted March 31st 2015 Abstract: The process of construction of the nanosized photoactive systems based on cadmium sul- fide has been developed. Additional ions Ag+, Cu+, Cu2+ can be implanted into the surface layer of CdS microcrystals leading to the formation of nanosized particles of corresponding sulfides. High re- dox photocatalytic efficiency was determined in the model reaction of potassium iodide oxidation for the low-doped samples of CdS containing silver and copper ions and the composite nanostructured CdS-based sulfides containing nanoparticles of Ag2S, Cu2S і CuS. This effect is caused by the depres- sion of electron-hole recombination because of reduction of the dope agent ions by free electrons, which results in the formation of reactive neutral atoms. Alternatively, this effect can also be caused by the transfer of photogenerated charges between likely charged clusters in nano- and microcompo- nents of the composite material. Keywords: cadmium sulfide, potassium iodide, photocatalytic activity, nanostructured composite ma- terial, implantation. 1. Introduction Photocatalytic decontamination of some environment pollution agents through the reaction of oxidative destruction is quite a topical issue discussed widely in some ar- ticles [1, 2] and reviews [3, 4]. As seen from analysis of the recent publications, the processes of photocatalytic decontami- nation (by oxidation or reduction) of vari- ous inorganic and organic pollution agents by highly effective semiconducting photo- catalysts is interesting for many scientific groups worldwide and the problem of de- velopment of such photoactive materials seems very topical [5-8]. The process of electron-hole recombina- tion is naturally inherent for any photoex- cited semiconductor and this is the key li- miting factor that put obstacles on the cata- lyst’s efficiency. Therefore, the decrease in the photocatalytic efficiency of the compo- site materials containing two different semiconductors can be caused by sponta- neous electrons transfer between the con- ductivity bands or the analogous process of the holes transfer between the valence bands [9, 10]. The above mentioned trans- fer processes can take place only if they are allowed thermodynamically. Moreover, since the back transfer of the charges is prohibited thermodynamically, the excited semiconducting particles with divided http://www.fia.usv.ro/fiajournal mailto:i.kobasa@chnu.edu.ua Food and Environment Safety - Journal of Faculty of Food Engineering, Ştefan cel Mare University - Suceava Volume XIV, Issue 1 – 2015 I g o r K OB AS A, L yu bo m yr a O D O SI Y, N a no s i z ed i m pla nt s of t h e i on s A g+ , C u + a n d C u 2 + i nt o t he s u r - f ac e l a y er o f p o l y cr y s t al l i n e cd s ph o t oc a t a l y s t f o r t h e r ea c t i o n of ox id at i o n o f i od id e i o ns , F o od a nd E nvir o nme nt Sa f e t y, V o l ume X IV , Is s ue 1 – 2 0 15 , p ag . 5 3 - 5 7 54 charges can exist during the period of time that is required for interaction between the reaction components causing rise in the quantum yield. Various model systems were used in inves- tigations of the photogenerated charges separation [11] but the entire photocatalyt- ic process has been thoroughly investi- gated for the two model systems only – photodecomposition of water and some alcohols, which produces molecular hy- drogen [12] and reduction of methylene blue to its leucoform [13, 14]. It should be emphasized that the photoefficiency of both process is quite high. That is why it seems important to understand if the simi- lar approach can be widened on some other redox systems, for instance, photocatalytic oxidation of iodides. 2. Experimental The following source materials were used to build the nanostructured composites Ag2S/CdS, Cu2S/CdS and CuS/CdS: cad- mium sulfide with specific surface area SBET = 4.0 m2/g, AgNO3, Cu2Cl2, Cu- SO4·5H2O. The solubility product of Ag2S, Cu2S and CuS is much lower than that of CdS (these values are 1,0·10-51, 2,5·10-50, 4,0·10-38 and 4,0·10-29 simultaneously [15]). Therefore, the former ions can subs- titute the latter one in the materials. The reaction of substitution has been carried out by stirring of CdS suspension in the solution containing the ions of the above mentioned less soluble sulfides. Reaction dynamics has been controlled by periodic sampling of the solution followed by the atom-absorption determination of concen- trations of the free ions Ag+, Cu+, Cu2+ and Cd2+ in the samples. The substituted sus- pension was centrifuged and the sediment was washed by warm water in order to separate the unreacted components and then dried at the room temperature. Redox photocatalytic activity of the composite sulfides was evaluated through amount of the free iodine formed in the reaction of potassium iodide oxidation according to the method described in [16, 17]. Efficien- cy of the free iodine formation (r, %) was calculated by the methods reported in [18]. Oxidation of potassium is a suitable model reaction because it is comparatively simple and can be widely applied in the solar cells production technologies [19, 20]. It was considered that the specific surface area of the source cadmium sulfide remained un- changed throughout all chemical modifica- tions. 3. Results and discussion Similarly to syntheses of Bi2S3/CdS com- posites [13], the above mentioned method ensured obtaining of the CdS-based mate- rials with exact preplanned values of the surface ions substitution ratio. It was con- sidered that the distance between the sur- face ions of cadmium and sulfur was equal to the sum of their ionic radiuses while the total surface area of all nanocrystals was equal to the sum of all their ionic radiuses. The former parameter of the 1 g sample makes its specific surface area. This rela- tion was used to calculate the total number of ions Cd2+ in the surface monomolecular layer. Fig. 1. Dependence of the surface occupational ratio for CdS and the following substitute ions: Ag+ (a), Cu+ (b) and Cu2+ (c) on their concentra- tion in the reacting mixture. On the other hand, results of quantitative analysis of the surface contents of silver Food and Environment Safety - Journal of Faculty of Food Engineering, Ştefan cel Mare University - Suceava Volume XIV, Issue 1 – 2015 I g o r K OB AS A, L yu bo m yr a O D O SI Y, N a no s i z ed i m pla nt s of t h e i on s A g+ , C u + a n d C u 2 + i nt o t he s u r - f ac e l a y er o f p o l y cr y s t al l i n e cd s ph o t oc a t a l y s t f o r t h e r ea c t i o n of ox id at i o n o f i od id e i o ns , F o od a nd E nvir o nme nt Sa f e t y, V o l ume X IV , Is s ue 1 – 2 0 15 , p ag . 5 3 - 5 7 55 and copper ions provided the data required for evaluation of the surface occupational ratios for each substitute ion throughout a series of the CdS samples [13]. A depen- dence of the surface occupational ratio on concentration of the substitute ions in the reaction mixture is shown in Fig. 1 (rest of the synthesis conditions was kept con- stant). At the initial stages of embedding of the substitute ions into the surface layer of CdS, only few separated defects could ap- pear. Due to the chemical bonds formed between the defects and the substrate lat- tice, they can be considered either as mole- cular ions AgS-, CuS- or as molecules Ag2S, Cu2S and CuS. The number of the defects is rising in course of the substrate processing, and interaction between them appears and grows resulting formation of the nanosized semiconducting islands of the substitutes on the surface of CdS. Thus, two separate groups can be identified among the substituted CdS materials. The low concentrated substitute products fall into the first group. As the substitute com- ponents concentrations are low, they can be considered as two-component systems: molecule (or molecular ion)/semiconductor. The second group in- cludes the highly concentrated substituted products. As content of silver and copper sulfides are high in the products, they ex- hibit no molecular properties and play role of classic semiconductors only. According to [11, 12], the latter materials are nano- structured semiconducting composites con- taining a mixture of nanoparticles of CdS and the other sulfides. Investigation of photocatalytic activity of the above mentioned materials was carried out using the model reaction of oxidation of iodides. It proved that the activity de- pends on the nature and concentration of the substitute ions in the material (see Fig. 1). Fig. 2. Efficiency of the free iodine formation by the implanted CdS products with ions of Ag + (a), Cu+ (b), Cu2+ (c) as a function on the surface monolayer occupational ratio. As seen from Fig. 2, the rise in the free iodine production is registered even for the slightly substituted examples. It can hardly be expected that any semiconducting isl- ands of AgS or CuS can be formed on such low-substituted material. Therefore, the first class of substituted materials appears to have some advanced photoactivity simi- larly to the two-semiconductor nanocom- posite materials as it was described for the process of the molecular hydrogen produc- tion [12] or reduction of methylene blue on Bi2S3/CdS [13]. The process of phototransformation in- volving the second class material will run similarly to that for the first class. The only difference in the process scheme is related to the charge separation stage involving transfer of the charges between the likely charged areas. As seen from the energy transformation diagram analysis [10], the conductivity band of the nanosized Ag2S is located above the conductivity band of CdS while the gap between the valence bands of these components is 0.4-0.6 eV wide. That is why the electron transfer processes are thermodynamically hindered while the hole transferring is not. The latter process will result in separation of the pho- togenerated charges. As a result, the elec- tron-holes recombination will be inhibited in such a particle meaning that its redox activity will rise. Food and Environment Safety - Journal of Faculty of Food Engineering, Ştefan cel Mare University - Suceava Volume XIV, Issue 1 – 2015 I g o r K OB AS A, L yu bo m yr a O D O SI Y, N a no s i z ed i m pla nt s of t h e i on s A g+ , C u + a n d C u 2 + i nt o t he s u r - f ac e l a y er o f p o l y cr y s t al l i n e cd s ph o t oc a t a l y s t f o r t h e r ea c t i o n of ox id at i o n o f i od id e i o ns , F o od a nd E nvir o nme nt Sa f e t y, V o l ume X IV , Is s ue 1 – 2 0 15 , p ag . 5 3 - 5 7 56 Analysis of the energy transformations during light absorption by the synthesized sulfides shows that two different mechan- isms of photocatalysis can actually take place. The first mechanism is exhibited by the materials with low content of Ag+, Cu+ and Cu2+ ions sitting in the lattice and bonded chemically with sulfur. This me- chanism implies transfer of the photogene- rated electrons from the conductivity band to the above mentioned ions and formation of highly reactive atoms of silver and cop- per. The second mechanism is more intrin- sic for the copper and silver-enriched ma- terials. In involves separation of the photo- generated charges by their transfer be- tween the likely charged areas of cadmium sulfide and nanoparticles Ag2S, Cu2S, CuS resulting in oppressed charges recombina- tion. 4. Conclusion New CdS-based catalysts were developed on the basis of the low-doped or composite nanostructured sulfides containing nano- particles of Ag2S, Cu2S, CuS. An extremal pattern has been found for the dependen- cies of the materials photoactivity on their composition and synthesis conditions. The first class of photocatalytic materials includes the polycrystalline CdS-based products with Ag+, Cu+ and Cu2+ surface implants similar to those applied to tech- nologies of molecular hydrogen production by photodecomposition of water or photo- reduction of methylene blue. Our results prove that potential applicability of such photoactive materials can also be widened to some other processes. Besides, new reli- able methods of synthesis of various im- planted CdS-based materials with prep- lanned photocatalytic activity have been developed and tested. 5. References [1]. S. FUKAHORI, H. ICHIURA, T. KITAOKA, H. TANAKA. Photocatalytic decomposition of bisphenol A in water using composite TiO2-zeolite sheets prepared by a papermaking technique. Environ. Sci. Technol., 37, 1048-1051 (2003). [2]. S. KANIOU, K. PITARAKIS, I. BARLAGIANNI, I. POULIS. Photo-catalytic oxidation of sulfamethazine. Chemosphere. 60, 372-380 (2005). [3]. D.F. BAHNEMANN. 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