The effects of pH value and precursors concentrations in the
growth solution as well as of ultrasound irradiation on the
properties of CdS:Mn nanoparticles in PVA matrix are studied.
Analysis of the photoluminescence spectra indicates that
the increase of the pH value in the growth solution leads to
the increase of the density of surface electron states that participate
in radiative transitions and to the enlargement of
CdS:Mn nanoparticles.We discuss a stabilization of the sizes of CdS:Mn nanoparticles
at pH=6 and change of nanoparticle size with decreasing
pH value at elevated concentrations of ions HS- . It is shown
that in the ultrasound-treated samples the size of the particles
does not depend on pH value. The effects of ultrasound result
primarily from acoustic cavitation. It is demonstrated that the
action of two external factors (the value of the medium pH
and ultrasound treatment) on the characteristics of nanoparticles
is correlated

Fediv, V.I.

Gule, E.G.

Ivanchak, S.A.

Makoviy, V.V.

Rudko, G.Yu.

Savchuk, A.I.

Voloshchuk, A.G.

Цифровий Репозиторій - Інтелектуальні Фонди Буковинського державного медичного університету

p s sreprintsstatussolidiwww.interscience.wiley.comphysicaREPRINTwww.pss-rrl.comwww.pss-c.comwww.pss-b.comwww.pss-a.com  © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim p s scurrent topics in solid state physicscstatussolidiwww.pss-c.comphysicaPhys. Status Solidi C 7, No. 6, 1510–1512 (2010) / DOI 10.1002/pssc.200983196  Evolution of CdS:Mn nanoparticle  properties caused by pH of colloid  solution and ultrasound irradiation A. I. Savchuk*, 1, G. Yu. Rudko2, V. I. Fediv1, A. G. Voloshchuk1, E. G. Gule2,  S. A. Ivanchak1, and V. V. Makoviy1 1 Chernivtsi National University, 2, Kotsyubynsky Street, Chernivtsi 58012, Ukraine 2 V.E. Lashkaryov Institute of Semiconductor Physics NASU, 41, pr. Nauki, Kyiv 03028, Ukraine Received 17 September 2009, revised 18 November 2009, accepted 21 November 2009 Published online 24 March 2010 Keywords CdS:Mn, nanoparticles, organic–inorganic hybrids, photoluminscence, electrochemistry                                * Corresponding author: e-mail a.savchuk@chnu.edu.ua, Phone: +38 0372 584 755, Fax: +38 0372 552914    © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim   1 Introduction Nanocomposites such as poly-mer/semiconductor nanoparticles have been intensely stud-ied during last decades due to their quantum-dimensional properties and prospective applications. Combined control over the sizes, surface properties and aggregation of nano-particles permits to obtain high-quality low-dimensional materials, opening up new possibilities of their use in op-tics, electronics, biology [1, 2]. The production of II-VI semiconductor nanoparti-cles with predicted physicochemical properties remains a challenge. In order to minimize the effect of uncontrolled factors the nanoparticles can be covered with the layers of organic and inorganic substances [3, 4]. Organic passiva-tion is often reversible and incomplete. In addition, the largely mismatched core/shell structures may result in the formation of defects that cause a degradation of optical properties of nanoparticles [5, 6].   One of the effective trends of nanotechnology devel-opment is to use external factors that permit to control the growth process and modification of nanostructures. Exam-ples of the influence of such factors as pressure, pH of the medium, where the synthesis occurs, X-ray irradiation, high-energy electron irradiation have been investigated [7-11].   It has been ascertained [12] that the specific feature of the action of ultrasonic vibrations on the qualitative and quantitative characteristics of nanoparticles is a continuous formation and collapse of bubbles within a static fluid. It has been established that in the process of bubble collaps-ing in water the temperature reaches the values 2000-4000 oC and pressure 100-400 atm [13].  Here we report the influence of the precursors concen-tration, pH values, and ultrasound (US) treatment on the properties of the nanocomposite polyvinyl alcohol (PVA) / CdS:Mn nanoparticles. 2 Experimental CdS:Mn nanoparticles were ob-tained at room temperature by using chemically pure so-dium sulfide, cadmium and manganese chlorides as the precursors. The synthesis took place in 5% aqueous solu-tion of PVA which played the role of a stabilizing agent. The effects of pH value and precursors concentrations in thegrowth solution as well as of ultrasound irradiation on theproperties of CdS:Mn nanoparticles in PVA matrix are stud-ied. Analysis of the photoluminescence spectra indicates thatthe increase of the pH value in the growth solution leads tothe increase of the density of surface electron states that par-ticipate in radiative transitions and to the enlargement ofCdS:Mn nanoparticles.   We discuss a stabilization of the sizes of CdS:Mn nanoparti-cles at pH=6 and change of nanoparticle size with decreasingpH value at elevated concentrations of ions HS- . It is shownthat in the ultrasound-treated samples the size of the particlesdoes not depend on pH value. The effects of ultrasound resultprimarily from acoustic cavitation. It is demonstrated that theaction of two external factors (the value of the medium pHand ultrasound treatment) on the characteristics of nanoparti-cles is correlated. Phys. Status Solidi C 7, No. 6 (2010) 1511 www.pss-c.com © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ContributedArticle400 500 600 700 8000,00,20,40,60,81,0400 500 600 700 8000,00,20,40,60,81,0  pH 5pH 4pH 5  PL intensity (normalized)Wavelength (nm)pH 6 Figure 1 Photoluminescence spectra of compositePVA/CdS:Mn nanoparticles synthesized at different pHvalues (Na2S concentration is 0.5·10-2 mol·l-1). The physicochemical properties of nanoparticles were con-trolled by variation of pH of colloid solution within the range of 4-6 as well as by US treatment of the colloid dur-ing 30 min. Frequency of ultrasonic irradiation was kept 22 kHz. The synthesis occurred under the conditions of an ex-cess of Cd2+ ions. The films of the PVA/CdS:Mn nanopar-ticles composite were formed by adsorptive dessication method. For this purpose the colloid solution was trans-ferred to glass Petri dishes which were placed in a pres-sure-tight vessel that contained an absorbent. The tempera-ture of drying was 20 oC.  The photoluminescence (PL) spectra were measured using the grating spectrometer with a spectral resolution of 0.5nm and LED source (λ=375 nm) for excitation at room temperature.   3 Results and discussion The PL spectra of nano-composites PVA/CdS:Mn nanoparticles obtained at vari-ous synthesis conditions are shown in Figs. 1-3. All spectra can be pretty well fitted by four Gaussian curves peaking at ~ 430 nm, ~470 nm, ~565 nm, and ~ 625 nm (see the in-sert in Fig. 1). Relative intensities of the bands depend on the conditions of synthesis (concentration of precursors, values of pH) and post-synthetic US processing. The band at ~470 nm is the near-band-edge luminescence. The bands at ~565 nm and at ~625 nm correspond to the transitions involving d-electrons of Mn2+ ions and recombination via surface localized states, respectively. The weak high-energy band at ~430 nm arises due to radiative transitions in PVA [14,15].   Figure 1 shows the influence of the pH value of the colloid solution on the PL spectra. It is seen that the in-crease in the pH leads to a decrease in the ratio of the 470 nm PL band intensity over the 630 nm PL band inten-sity (І470/І630). This indicates that the density of surface electron localized states that participate in the radiative transitions increases. It can be caused by the adsorption of OH- ions on the surface of CdS:Mn nanoparticles. More-over, with the increase of pH, the spectral position of the near-band-edge PL band is shifted to longer wavelengths thus pointing to the enlargement of the CdS:Mn nanoparti-cles and surface terminations, such as OH- [16]. This shift can be related to:  i) An elevated concentration of OH- ions. It increases from 1·10-10 mol·l-1 at pH=4 to 1·10-8 mol·l-1 at pH=6.0.  ii) A change of a sulphurizing agent concentration. It is known that in aqueous solutions Na2S is hydrolyzed, the products of hydrolysis being either HS- ions or H2S mole-cules, depending on the pH value. Calculations show that the ratios of equilibrium concentrations of sulphurizing agents in an aqueous solution of Na2S at pH=4 are equal to [H2S]:[HS-]:[S2-]=1000:1:(2,5·10-7), whereas at рН=6  [H2S]:[HS-]:[S2-]=10:1:(2,5·10-5). Since the concentration of sulfide-ions is the lowest, real sulphurizing agent are the ions HS-, whose concentration increases with pH. At low HS- concentrations the rate of CdS seeds grown is low while at highest HS- concentrations the growth rate in-creases and, as a result, the average diameter of nanoparti-cles increases as well.   iii) An effect of pH on the solubility of nanoparticles. An equation that describes the dependence of molar solubility SCdS on pH, derived on the basis of the theory of heteroge-400 500 600 700 8000,00,20,40,60,81,0  PL intensity (normalized)Wavelength (nm)1 2a400 500 600 700 8000,00,20,40,60,81,0  21 PL intensity (normalized)Wavelength (nm)b Figure 2 Photoluminescence spectra of compositePVA/CdS:Mn nanoparticles synthesized at pH = 5 (a)  andpH = 6 (b). Curves 1, 2 correspond to  Na2S concentrationsof 0.5·10-3 mol·l-1 and 0.5·10-2 mol·l-1, respectively. 1512 A. I. Savchuk et al.: Evolution of CdS:Mn nanoparticle properties © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim  www.pss-c.com physicap s sstatus solidi cneous equilibria using the product of solubility of cadmium sulfide, assumes the form: lgSCdS=-4,105-pH,  (1) i.e., the higher the pH the lower the solubility. The effect of Na2S precursor concentration on the PL properties of CdS:Mn nanoparticles is shown in Fig. 2(a,b). It is seen that the increase of Na2S concentration in-fluences the size and surface states of the particles differ-ently, depending on pH of the colloidal solution. Indeed, at pH=5  the increase of Na2S concentration leads to the in-crease of both average particles size and surface states den-sity; at pH=6 the spectral position of the band at 470 nm in the PL spectrum does not change with the growth of the Na2S concentration, although all the conditions for their enlargement (elevated concentrations of ions HS- and ОН-) are fulfilled. The observed stabilization of the sizes of CdS:Mn nanoparticles at pH=6 мay be due to the decrease of the concentration of Cd2+ ions owing to a formation of hydroxocomplexes [Cd(OH)]+ and [Cd(OH)2]0 which are less reactively capable and are characterized by considera-bly lower coefficients of diffusion.  The influence of US irradiation is shown in Fig. 3. It is seen that in the US-treated samples the particles size and surface states density do not depend on the pH value (see Fig.3) contrary to the behavior of the untreated samples (Fig.1). The effect of US derives primarily from acoustic cavitation. It is known that cavitation in the fluid-solid sys-tem is a source of highly energetic phenomena [13]. Cavi-tation in fluids may exert a considerable effect on the sur-face of nanoparticles. The effect of microcurrents, shock waves and interparticle collisions on the chemical compo-sition and physical morphology of nanoparticles may con-siderably enhance the chemical reactive activity of the in-organic phase and the absorptive capacity of a polymer.  For pH = 5 and pH = 6 US-treated samples, the PL band edge emission position (Fig. 3), located at lower wavelength compared to the untreated sample ones (Fig. 1) and then corresponding to nanoparticles of smaller size, can be explained by the following: i) Destruction of the ab-sorptive layer around the nanoparticles caused by the de-sorption of ОН- ions from the surface of nanoparticles by ultrasonic treatment; ii) Change of nanoparticles dissolu-tion depending on pH (see Eq. (1)) and stimulation by the action of US vibrations.  Note, that the action of two external factors (decrease of the pH value and US treatment) on the characteristics of nanoparticles is correlated.  An analysis of the findings obtained and their compari-son with the available data indicate that the effect of US treatment on the properties of PVA/CdS:Mn nanoparticles composites is a result of mechanical action of ultrasonic waves that changes the structure of the adsorptive layer around the nanoparticles, whereas the effect of pH is im-plemented via the change of ОН- and HS- ions concentra-tions in the reactive medium.   References  [1] A. P. Alivisatos, Science 271, 933 (1996).  [2] C. Brechignac, P. Houdy, and M. Lahmani (eds.), Nanoma-terials and Nanochemistry (Springer-Verlag, Berlin, 2007), p. 747.  [3] X. Zhong, Y. Feng, W. Knoll, and M. Han, J. Am. Chem. Soc. 125, 13559 (2003).  [4] C. N. R. Rao, A. Muller, and A. K. Cheetham (eds.), The Chemistry of Nanomaterials: Synthesis, Properties and Ap-plications, Vol. 1 (Wiley-VCH Verlag, Berlin, 2004), p. 741.  [5] X. Zhong, M. Han, Z. Dong, T.J. White, and W. Knoll, J, Am. Chem. Soc. 125, 8589 (2003).  [6] L. Manna, E. C. 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Evolution of CdS:Mn nanoparticle properties caused by pH of colloid solution and ultrasound irradiation

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Цифровий Репозиторій - Інтелектуальні Фонди Буковинського державного медичного університету