EXPERIMENTAL STUDY OF CHARGE SPLITTING HETEROSTRUCTURES OF CdS/ZnO NANOPARTICLES. Doc. Anton Fojtík and Bc. Štěpán Svoboda 1

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1 EXPERIMENTAL STUDY OF CHARGE SPLITTING HETEROSTRUCTURES OF CdS/ZnO NANOPARTICLES Doc. Anton Fojtík and Bc. Štěpán Svoboda 1 1 Faculty of Nuclear Sciences and Physical Engennering CTU in Prague Abstract Both experimental and theoretical studies on semiconductor nanoparticles have emerged in large numbers in the recent decades. Semiconductor nanomaterials based on CdS, ZnO, TiO, CdSe and many others offer new possibilities in material sciences, optics, electronics and chemistry. Therefore detailed informations concerning those systems are needed to fully understand the behavior and nature of those nanoparticles. In this work we have mainly focused on experimental studies of CdS nanoparticles. Surface states and possibilities to handle theme were studied using chemical and spectroscopic methods. That involved surface passivation, modification to heterostructure and combination with ZnO nanoparticles. This work main aim was to understand and be able to prepare nanomaterials with desired properties like charge separating nanoparticles. 1. INTRODUCTION It is useless once again emphasize the importance of nanoparticles and enumerate research teams interested in this area. But what is still worth of mentioning is the fact that there exist several ways to explore the immense world of nanoparticles. Wet and physical synthesis. This work is fully focused on wet chemical synthesis and doesn't even try to compare with MOCVD, MBE, lithography and other physical methods used to create analogue nanoparticles. What more is worth of mentioning is the fact, that there are two kinds of extremely interesting nanoparticles which behave in totally different ways. Better understood and mastered is the world of metal nanoparticles where synthesis methods, shape and size control, properties tuning and theoretical models are quite advanced. Contrary to that fact the world of semiconductor nanoparticles is still a little cloudy. There is still a lot to study, explore and explain. Each system seams to have its own rules and specifications. Sure, there is still a lot of common properties like quantum size effect which is considered to be the unique property of quantum states nanoparticles. But on the other hand, there is only a little of methods applicable on different systems. In these work we present the results of investigations on CdS nanoparticles created in a simple wet synthesis. Our main tools in this study were absorption and photoluminiscence spectroscopy for the basic characterization. Mesurements on SEM was performed too, but results were not as striking. CdS nanoparticles are difficult to find because there is a lot of surface stabilizer that prevents good resolution of the nanoparticles.

2 CdS nanoparticles were surface passivated by oxidation of surface, modified to composed nanoparticles (nanoparticles formed of two species) and combined with ZnO nanoparticles. All those studies were performed in order not only to better understand the CdS nanoparticles, but also to be able to manipulate the surface states, hence charges created by absorption.. EQUIPEMENT AND CHEMICALS All chemicals were used as received without further purification. Water used during experiments was a 1MΩ mili-q water. For the synthesis of CdS nanoparticles were used: 99,9% Cd(ClO ) from Alfa products, 9%+ Na S.9H O from Aldrich, hexamethaphospahte (HMP) and water. Other used chemicals: Propan--ol, methanol, LiOH.OH p.a. from Penta, zinc acetate dihydrate (ZAH) p.a. from Penta, NaOH, AgClO >99% from Fluka. Absorption mesurements were performed on Perkin-Elemer Lambda UV spectrophotometre, Ocean Optics QE5000 spectrophotometr using a DT-minilamp. For excitation at 00nm UV LED was used. Quenching of luminescence was measured by groupe of Doc. Michl. 3. SYNTHESIS AND SAMPLE PREPARATIONS CdS nanoparticles were prepared by a simple chemical route just by mixing two solutions containing reactants. Synthesis conditions were a little varied to obtain ideal results. Cd + ions were obtained by dissolution of Cd(ClO ) in mili-q water in order to obtain. - M solution and S - ions were obtained by dissolution of Na S in mili-q water in order to obtain. - M solution. It should be noted, that S solution was prepared fresh every few days because of H S escaping from the solution. Those were stock solutions used for the synthesis as working with those species in solution is much more comfortable and safer. For most of the experiments Cd:S ratio in finial solution was 3: unless mentioned. Analytical concentration of Cd ions was kept at unless mentioned. For the stabilization of nanoparticles sodium hexamethaphosphate (HMP) was used in concentrations ranging form (the same as Cd + ions) to -3. 0,01g of HMP (to obtain in the final mixture -3 M HMP) was dissolved in 13,5ml H O and 1,5ml Cd stock solution was added. And 1,5ml of S stock solution was added to 13,5ml H O. Both those solutions were cooled to slow down the growth of nanoparticles and to enable more homogeneous mixing before reaction take place. After cooling the S solution was dropwise added to Cd solution under stirring. A few modifications were investigated. Amongst them: the influence of HMP amount on reactivity and particle size.

3 0, aged Orig Heating nanoparticles to 0 C 0,5 Absorbance 0, 0,3 0, 0,1 0,0 ZnO nanoparticles were obtained by modified synthesis proposed by Spanhel and Anderson. 1 During our previous work it has been proved that ZnO nanoparticles with low dispersion and high quality can be prepared without preparing the precursor solution. This has been further studied to more details by E.Meulenkamp. And we are not about to discuss this formation. Most previous synthesis were carried out in ethanol, but for our purpose the mixture of propan--ol and methanol :1 (in volumes) proved to be more than sufficient. This mixture is denoted IM1 (isopropanolmethanol :1) Wavelenght [nm] Figure 1: Absorption spectra of original and aged CdS 3: nanoparticles. ZnO nanoparticles were synthesised by dissolving 0,07g of ZAH (zinc acetate) in mixture of propan--ol and methanol (IM1) and adding 0,01g of lithium hydroxide monohydrate in powder. Nanoparticles are synthesised during approximatly hours while still stirring the mixture. It should be noted, that ZAH is not fully soluble in IM1 mixture but ZnO nanoparticles are. Therefore as reaction goes on all reactants are dissolved and forms nanoparticles Wavelenght [nm] New Aged Figure : spectra of New and Aged CdS nanoparticles. Peak at 00nm is due to scattering.. EXPERIMENTS AND RESULTS In this paragraph we are about to present the results of this research. In the first part the CdS nanoparticles with Cd:S ration 3: are presented along with modifications and measurements performed on this system. At the end of this paragraph other possibilities of synthesis are going to be discussed as well as partial results are going to be presented. It should be noted at the beginning that all synthesis procedures led to nearly the same absorption and luminescence spectra. The differences between the different procedures were observed after further

4 manipulations. Figures 1 and depicts usual absorption and luminescence spectra for the CdS nanoparticles prepared in 3: ratio. As can be seen absorption spectra doesn't change dramatically and that fact can be used for evaluating the modifications. It is a general feature that absorption spectra doesn't change dramatically and therefore a significant change in absorption spectra could be interpreted as a major change in system. On the other hand luminescence spectra are well known to reflect even a minor changes on the nanoparticles. While ageing the photoluminescence of CdS nanoparticles usually shifts to red wavelengths. This shift to red wavelengths as seen on Fig. is quite a typical for CdS nanoparticles and a lot of reactants can provoke it. Every solution of CdS nanoparticles we have prepared exhibited a very strong scattering. As we used 00nm LED for excitation there was always a scattering peak at this wavelength. Those scattering peaks are not shown in our spectra because of little importance for our experiments. CdS nanoparticles were already studied before with very similar results. 3 During this project similar experiments were performed in order to establish new research line. It was necessary to be sure that we are able to reproduce older results. But our investigations led to new unpublished results. % 1,% 93,3% 0%,% 33,3%,% 0% 13,3%,% 0% Wavelength [nm] Fig 3: Influence of Ag + ions on CdS luminescence. Concentration of Ag + ions is expressed in % to Cd ++ analytical concentration. As mentioned in [3] it has been found that some metal ions increase the luminescence of CdS nanoparticles while some might decrease it. Silver ions have one of the strongest effects in increasing the luminescence. This effect has been measured for our CdS 3: nanoparticles and results are in Figure 3, where

5 Wavelength ratio 3:1. As we were interested in long time stability. days 1 day New Fig 5: Increase of luminescence in time.(in this case the Cd:HMP ratio is 1:1 and Ag + concentration is %) luminescence spectra are plotted with different Ag + concentration. As can be clearly seen from Fig. 3 Ag + ions increase the luminescence up to 33% of Ag ions. Similar effect has been observed previously but the amount of maximal increase in luminescence was found to be 0% of AgNO 3. This can be due to nature of anions or due to HMP concentration. It has been found that lowering of HMP concentration leads to increase in sensitivity to Ag + ions, but it lowers the stability of nanoparticles. This particular measurement was performed for nanoparticles prepared with Cd:HMP For this particular measurement we have measured a quantum yield for those nanoparticles. Again in function of Ag + concentration. Quantum yield has been measured (Figure ) by comparing the area under the luminescence peak with the luminescence of rhodamine G. Both species were maintained at the same absorbance at 00nm where they were excited. The dependence of quantum yield on Ag + concentration is plotted on Fig.. It has been known that increasing the amount of ions leads only to quenching of the luminescence. But as far as we know, no one described an increase of luminescence in time. Quantum yield [%] (rhodamin G) % Ag:Cd Fig : Quantum yield as a function fo Ag ions concentration. It has been observed that after several days the luminescence increase dramatically. Moreover even the specimens that were overdosed and had no significant luminescence are again able to convert UV into VIS red light. This effect is illustrated by Fig. 5 where CdS 3: nanoparticles are doped with silver and left several days to measure the effect. It works for all measured silver ratios (~1% up to >0%). But particles with higher content of silver seems to be more reactive and after some time form black precipitate.

6 Another enhancement of luminescence can be produced by addition of NaOH up to ph 11. This alkalization Absorbance 0,07 0,0 0,05 0,0 0,03 0,0 0,01 0,00 τ 1/ = 135ns τ 1/ ~ 1 µs Wavelength [nm] CdS 3: Aged- surface passivated CdS 3: Aged τ 1/ > µs Fig : Absorption and luminescence spectra of aged CdS and surface passivated CdS nanoparticles. Clear emission from band-gap can be observed at 50nm as well as increase of red luminescence.,0 1, can have two absolutely contradictory effects. It can either shift luminescence to red wavelengths either passivate the surface and force the direct recombination from band-gap. Usually both effects are present and it has not been clearly explained which one will be dominant. It seems that the passivation works for aged solutions. Prior to alkalization a little of HClO was added to the solution in order to liberate the surface and to dissolve Cd islands on CdS nanoparticles that could prevent NaOH in passivating the surface. If the passivation doesn't work a clear red shift and a small enhancement photoluminescence is observed. If ph reaches critical point (that is usually around 1) precipitation of CdS occures. A typical surface passivation and its luminescence and absorption spectra are presented on Figure. in Absorbance 1,0 0,5 0, Wavelength [nm] 33nm Fig. : Absorption spectra of ZnO nanoparticles with band-gap at 33nm. The passivation of surface produce luminescence from band gap. Therefore it seems that alkalization partially blocs the non-radiative transitions and increase the amount of absorbed energy. Those effects together leads to enhancement of luminescence and to direct recombination from band-gap.

7 To confirm the fact, that the 50nm emission is from band-gap and not from surface states a time resolved photoluminiscence was measured and the corresponding times of live were obtained. Obtained data are depicted on Figure 7 as well as mentioned on Figure. 000 CdS-passivated@00nm CdS-passivated@0nm CdS@0nm Those decays illustrates very well the dynamics of the electronic states transitions as predicted by theory. What is evident is very fast transition from the band-gap. This luminescent transition at 0nm is exactly at the point of inflexion of the absorption spectra, which indicates the band-gap size. Moreover the time of live at this wavelength is one order of magnitude smaller than for other luminescences. The time of live of 135ns is very fast, even if 135ns is not as fast transition as usual. Counts >us ~1us Time (ns) 135ns Fig 7: Decays of luminescence for passivated nanoparticles (@0 and for non-passivated (@0nm) all excited at 370nm.and corresponding times of live. The transition from surface seems to have a very long time of live as even during µs, used for the measurement, the intensity doesn't fall near zero. And in the case of 00nm emission from surface passivated, the time of live can be effectually lowered by presence of the fast band-gap transition which depopulates the excited electrons fast. After having examined the possibility to handle the surface states either by suppressing them by surface passivation, either by shifting them and increasing the luminescence another experiment was prepared. This time ZnO nanoparticles were added to CdS nanoparticle solution in order to receive charge separation and allow handling with electronic states. ZnO nanoparticles are easy to prepare in alcoholic solutions but CdS nanoparticles are usually prepared in water. Fortunately it has been proved that as prepared ZnO nanoparticles remain stable even in water at least for several hours needed for the experiment. ZnO nanoparticles were added as a. - M solution directly to CdS sols. A major increase in luminescence was observed that is to be explained. Figure presents the absorption spectra of prepared ZnO nanoparticles in IM1. What is to be remembered is the position of band-gap which is obtained from the inflexion point of absorption spectra. Band-gap sets the limit for the absorption which is in that case at 33nm. The excitation wavelength remains at 00nm and therefore it is is clear and evident that ZnO nanoparticles can not be excited at this wavelength. What makes the analysis a little bit uneasy is the fact that luminescence spectra of CdS and ZnO are very close, mainly for nonaged CdS particles.

8 Figure 9 depicts the luminescence spectra of CdS dopped with ZnO. For this experiment CdS 3: nanoparticles with analytical concentration of 3. - of Cd ++. The 0,0M ZnO was added in 0µl drops to 5ml of CdS solution. Therefore 0µl corresponds to,7% of Cd. It can be clearly seen that the luminescence increase. As can be seen form Figure the luminescence is coming from CdS. On Figure the same experiment was measured for CdS nanoparticles that has been heated before further treatment. As reported In Both cases, ph was settled above 7. ZnO dissolves in acidic enviroment. during the heat treatment the same peak as during passivation appears. The presence of that peaks will allow us a better understanding of the phenomena. From Figures 9 and enhancement of emission is very clear, but the mechanism remains unexplained. As can be seen form the Fig. ZnO nanoparticles increase even the luminescence that is supposed to be coming from the band-gap, that is the interior of the particle. That fact confirms the increase of CdS luminiscence Wavelength Fig 11: The luminescence of Wavelength CdS over-doped [nm] with 19% ZnO The absorption of the band-gap is this time situated at 70nm which approximately corresponds to emission peak at 0nm. Only for the specimens with higher content of ZnO the CdS peak at 350nm is observable, even if it is very weak. To be sure that the effect of enhancement of the luminescence comes form interaction between ZnO and CdS nanoparticles, the same experiment was performed but instead of using nanoparticles just 100ul ZnO Several days - smoothed 100ul ZnO 0ul Im1Z 0ul Im1Z 0ul Im1Z 0ul Im1Z Orig (in analytical concentrations) as prepared (red) and after 3 weeks ageing (black) Figure 9: spectra of CdS 3: combined with ZnO. 0µl corresponds to,7% Wavelength Fig : spectra of CdS 3: Heated at ph 9 upon addition of ZnO. Again 0µl corresponds to,7%. ph9 0ul ZnO 150ul ZnO 00ul ZnO 00ul ZnO 100ul ZnO ZAH dissolved in water was used. (0,0M is not dissolvable in IM1) And it was repeated with ZAH dissolved in IM1 but at lower concentration. Both experiments led to negligible increase of luminescence. As usually for CdS system the luminescence measured after addition of ZnO is not stable and has an important time evolution. Just to compare this effect luminescence was remeasured after 3 weeks. An incredible increase in luminescence was observed for the peak around 0nm. This effect was measured only for the over-doped

9 system. We have just remarked the increase in luminescence while checking old samples. This process is to be measured and studied in the future. All this happened only with a little variation in absorption. spectra can be found on the Figure 11.. CONCLUSION We have studied the possibilities to handle electronic states of CdS nanoparticles in order to receive enhancement of luminescence or charge separation. We have measured the enhancement of luminescence for CdS/Ag S systems along with its quantum yield. For this well known system, we have further found, that after a relaxation this luminescence even increase, that has not been reported befor. While combining CdS and ZnO materials in order to receive charge separation we observed a new unpublished result. That was increase of CdS luminescence. We believe, that we were able to block nonradiative transitions and therefore provoke this increase. Further measurements are about to explain thei fenomena. LITERATURE [1] L. Spanhel; Colloidal ZnO nanostructures and functional coatings: A survey; J Sol-Gel Sci Techn; DOI.07/s [] E.A.Meulenkamp; Synthesis and Growth of ZnO Nanoparticles; J. Phys. Chem. B, 199,, [3] L. Spanhel aj.; Photochemistry of Semiconductor Colloids. 17. Strong luminescing CdS and CdS-Ag S Particles; Ber. Bunsenges. Phys. Chem., 197, 91, -9 [] A. Fojtik aj.; Photo-Chemistry of Colloidal Metal Sulfides. Photo-Physics of Extremely Small CdS Particles: Q-State CdS and Magic Agglomeration Numbers; Ber. Bunsenges. Phys. Chem., 19,,