Nanoparticles by Thermal Decomposition of Bis

DOI: 10.1002/slct.201600298

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z Materials Science inc. Nanomaterials & Polymers

Synthesis and Characterization of Cd1-xZnxS (x = 0-1) Nanoparticles by Thermal Decomposition of Bis(thiourea) cadmium–zinc acetate Complexes Rama Gaur and Pethaiyan Jeevanandam*[a] The present study reports synthesis of a complete series of Cd1xZnxS solid solution nanoparticles (x = 0-1) via thermal decomposition of bis(thiourea)cadmium-zinc acetate complexes. First, bis(thiourea)cadmium-zinc acetate complexes were synthesized which were then decomposed at 200 8C for 1 h in diphenyl ether. The bis(thiourea)cadmium-zinc acetate complexes and Cd1-xZnxS nanoparticles were characterized using different analytical techniques. The crystallite size of Cd1-xZnxS nanoparticles varies from 2.1 nm to 4.8 nm and the phase changes from hexagonal to cubic with an increase in zinc content. The optical

Introduction CdS is one of the most important II–VI semiconductors with a direct band gap (2.42 eV for bulk) and has been extensively investigated due to its applications in diverse areas such as field effect transistors, light emitting diodes, photocatalysis, photovoltaics and sensors.[1] CdS nanoparticles face a few limitations such as, short lived charge carriers, photocorrosion under visible light, limited optical window and toxicity issues which sometimes restrict the application of CdS.[2, 3] Crystal structure of CdS affects reactivity and it has been reported that wurtzite CdS exhibits higher photocatalytic activity compared to zinc blende CdS.[4] Band gap of a nanostructured semiconductor is an important parameter for applications, and it can be tuned by changing the size. This is due to quantum confinement that normally occurs within a few nanometers for most of the semiconductors.[5] The quest for semiconductors with controllable properties led to search for alternative ways to synthesize semiconductors with composition controlled and tunable structural and optical properties. Incorporation of foreign elements (Zn, Mn, Ni, etc.) into the lattice of CdS to form solid solutions such as, Cd1-xZnxS, Cd1-xMnxS, Cd1-xNixS, etc. is a useful strategy to achieve tunable optical properties and good material stability.[6, 7] Composites based on Cd1-xZnxS solid solutions have been reported as good photocatalysts. For example, titania nanotube-Cd0.65Zn0.35S has

[a] R. Gaur, Dr. P. Jeevanandam Department of Chemistry Indian Institute of Technology Roorkee Roorkee, Uttarakhand-247667, India E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/slct.201600298

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properties of Cd1-xZnxS solid solution nanoparticles depend on composition and the band gap varies from 2.37 eV to 3.69 eV with an increase in the concentration of zinc. Photoluminescence spectral studies indicate shift of excitonic emission peaks towards lower wavelength (465 nm – 503 nm) compared to pure CdS nanoparticles (505 nm) with increasing zinc content in the Cd1-xZnxS nanoparticles. The Cd1-xZnxS nanoparticles were explored as catalysts for the photodegradation of methylene blue in an aqueous solution under sunlight.

been reported to be a better photocatalyst for H2 evolution compared to the constituents.[8] Cd1-xZnxS/reduced graphene oxide nanocomposite shows superior photocatalytic activity with good photocorrosion resistance for the photodegradation of methyl orange.[9] In addition, CdS quantum dots sensitized Zn1-xCdxS solid solution exhibits enhanced H2 production under visible light.[10] Cd1-xZnxS nanoparticles have several advantages over the parent semiconductor (CdS) and introduction of zinc into CdS has been widely used to enhance the characteristics of optoelectronic devices. The optical properties of Cd1-xZnxS solid solutions can be tuned by varying the ratio between Zn and Cd, thereby tuning their band gap from UV to visible region.[11] Introduction of Zn2 + into CdS decreases absorption losses and leads to utilization of complete solar spectrum. The charge carriers in the solid solution can move continuously in conduction and valence bands rather than in the discrete donor levels of the solid solution. Moreover, the replacement of Cd2 + by Zn2 + makes the conduction band of CdS more negative[3, 12] and because of their sufficiently negative flat band potentials, Cd1-xZnxS solid solutions are good candidates for visible-light-driven photocatalysis.[13] Therefore, the synthesis of Cd1-xZnxS solid solution nanoparticles is of immense importance for various applications, e. g. H2 production, environmental remediation, etc. Various chemical and physical methods have been developed for the synthesis of Cd1-xZnxS solid solution nanoparticles. The conventional approaches include chemical bath deposition,[14] chemical reduction,[15] cation-exchange,[16] mechanical alloying,[17] precipitation,[18] successive ionic layer adsorption,[19] spray pyrolysis,[19] solvothermal,[20] sonochemical,[21] sol-gel method,[22] sulfuration,[23] and thermal decomposition method.[6, 24] In recent times, synthesis of semiconductor nanoparticles from single molecular precursors has attracted great inter2687

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Full Papers est.[25–28] In the present study, a facile route for the synthesis of Cd1-xZnxS solid solution nanoparticles by thermal decomposition of bis(thiourea)cadmium-zinc acetate complexes in diphenyl ether has been reported. Use of single molecular precursors for the synthesis of Cd1-xZnxS solid solution nanoparticles is advantageous over the use of multiple precursors since the single molecular precursors contain all elements of the desired product within the precursor.[25] The use of single molecular precursors aids in effective mixing at the molecular level which leads to products with uniform composition.[29, 30] They help to maintain stoichiometry in the final products and ligands present in the precursors often act as in-situ capping agents for the semiconductor nanoparticles; the ligands induce unusual crystal growth/metastable phase formation.[30, 31] Cd1-xZnxS nanoparticles have been synthesized by the thermal decomposition of multiple precursors (cadmium acetate, zinc acetate and thiourea) in a solvent in a previous report.[6] But solid solutions were obtained only at lower zinc concentration (x 0.6) and phase separation occurs at higher zinc concentration (x > 0.6). Only a few authors have reported the synthesis of Cd1-xZnx S nanoparticles using single molecular precursors (cadmiumzinc bis(N,N-diethyldithiocarbamate) and zinc cadmium thiourea complex).[25–27, 32] The reported methods employ harsh synthetic conditions like high temperature (180 8C – 500 8C), long reaction time (3 h - 12 h) and presence of an inert atmosphere is often required for the preparation of Cd1-xZnxS nanoparticles. In the present report, first, bis(thiourea)cadmium-zinc acetate complexes were prepared using a reported method.[33] Varying amount of zinc was introduced to obtain a series of bis (thiourea)cadmium-zinc acetate complexes. Then, thermal decomposition of bis(thiourea)cadmium-zinc acetate complexes in diphenyl ether at 200 8C results in the formation of a continuous series of Cd1-xZnxS solid solution nanoparticles. Unlike the previous report, no phase separation is observed.[6] After characterization, the Cd1-xZnxS nanoparticles with composition controlled optical properties were explored for the photocatalytic degradation of methylene blue in an aqueous solution under solar irradiation.

Results and Discussion Bis(thiourea)cadmium–zinc acetate complexes The characterization of bis(thiourea)cadmium-zinc acetate complexes was carried out using powder X-ray diffraction, FT-IR spectroscopy, elemental analyses (CHNS analysis and atomic absorption spectroscopy) and TGA measurements. The observed results are well in agreement with the reported results.[33–35] Figure S1 shows the XRD patterns of bis(thiourea) cadmium-zinc acetate complexes and the observed results are in agreement with the reported ones.[33, 36] Figure S2 shows the FT-IR spectra of bis(thiourea)cadmium-zinc acetate complexes. The list of IR bands and their assignments are given in Table S1. In bis(thiourea)cadmium/zinc acetate, the Cd2 + /Zn2 + ions coordinate with thiourea through sulfur and with acetate via oxygen in a tetrahedral arrangement.[33] The coordination of Cd2 + ChemistrySelect 2016, 1, 2687 – 2697

with thiourea through CS bond is confirmed by the shift of C S stretching from 630 cm1 in pure thiourea to 616 cm1 in the cadmium complex and also an increase in the CN stretching frequency from 1082 cm1 in the free ligand to 1043 cm1 in the complex.[34] When zinc is introduced into bis(thiourea)cadmium acetate, it causes isomorphic replacement of Cd2 + by Zn2 + without affecting the coordination sphere, and hence no appreciable shift in IR band positions is observed.[35] Minor difference in the Lewis acid character between Zn2 + and Cd2 + accounts for slight shift in the IR bands. The elemental analyses for the complexes were carried out using atomic absorption spectroscopy and CHNS analysis and the observed results and the proposed formulae are given in Table S2. The observed results indicate that the elemental composition of bis(thiourea) cadmium-zinc acetate complexes, in general, match with the expected values. Figure S3 shows TGA/DTG and DTA plots of bis(thiourea) cadmium acetate and bis(thiourea)zinc acetate. The TGA pattern of bis(thiourea)cadmium acetate (Figure S3a) exhibits a sharp weight loss (40.2 %) at about 186 8C, followed by a gradual weight loss (10.2 %) upto about 580 8C. These weight loss steps are ascribed to decomposition of bis(thiourea)cadmium acetate and partial oxidation of CdS to CdO.CdS, respectively. A small weight gain (~ 1.9 %) is observed at around 643 8C due to partial oxidation of CdS to CdS.CdSO4, and a weight loss (~ 8.9 %) at about 1000 8C is attributed to complete oxidation of CdS to CdO. The TGA pattern of bis(thiourea)zinc acetate (Figure S3c) exhibits three weight loss steps at about 76 8C (3.6 %), 200 8C (41.4 %) and 646 8C (23.5 %) These weight loss steps are attributed to loss of moisture, decomposition of bis(thiourea) zinc acetate to ZnS and oxidation of ZnS to ZnO, respectively. The DTA plot of bis(thiourea)cadmium acetate (Figure S3b) exhibits an endothermic peak at about 186 8C and an exothermic peak around 646 8C, attributed to decomposition of bis(thiourea)cadmium acetate to CdS and oxidation of CdS to CdS.CdSO4, respectively. The DTA plot of bis(thiourea)zinc acetate (Figure S3d) exhibits endothermic peaks at about 76 8C, 180 8C and 200 8C, attributed to loss of moisture, loss of acetate molecules and decomposition of bis(thiourea)zinc acetate, respectively. The exothermic peak at around 646 8C corresponds to oxidation of ZnS to ZnO. The thermal decomposition of bis(thiourea) cadmium-zinc acetate complexes is observed at temperatures ranging from 175 8C to 196 8C depending on the composition (Figure S4a). The increase in decomposition temperature of cadmium-zinc acetate complexes compared to pure bis(thiourea)cadmium acetate is attributed to the incorporation of Zn2 + ions in bis(thiourea)cadmium acetate. The observed overall weight loss values for the bis(thiourea)cadmium-zinc acetate complexes linearly increases with zinc content from 58 % (Cd10Zn0) to 68.5 % (Cd0Zn10) (Figure S4b, Table S3). At temperature close to 600 8C, partial oxidation of ZnS and CdS to ZnO and CdS.CdSO4, respectively, takes place.[37] On further heating beyond 900 8C, a detectable weight loss is observed due to complete oxidation of ZnS and CdS to ZnO and CdO, respectively.

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Full Papers Characterization of Cd1-xZnxS solid solution nanoparticles The synthesized Cd1-xZnxS solid solution nanoparticles were thoroughly characterized for their elemental, structural, morphological and optical properties using different techniques and the results are discussed in detail as follows. Structural and elemental analysis Figure 1 shows the XRD patterns of Cd1-xZnxS nanoparticles synthesized by the thermal decomposition of bis(thiourea)cad-

Further evidence for the presence of hexagonal phase comes from SAED results discussed later. In the case of ZnS, the peaks at 28.558, 48.048 and 56.678 are due to (111), (220) and (311) reflections of cubic ZnS (JCPDS file no. 80–0020). The XRD patterns of Cd1-xZnxS nanoparticles exhibit shift of peaks towards higher 2q values with an increase in zinc content (Figure 1). The ionic radius of Zn2 + (0.74 ) is smaller than that of Cd2 + (0.97 ); substitution of Zn2 + in CdS causes decrease in the dspacing and hence the observed shift of XRD peaks towards higher 2q values. The lattice parameters for CdS and ZnS were calculated from the XRD patterns (CdS: a = 4.14 and c = 6.72 ; ZnS: a = 5.39 ). The lattice parameters ‘a’ and ‘c’ were calculated using the following relations [Eqs. (1) and (2)]. The calculated values are in agreement with the reported values. The lattice constant (a) for the Cd1-xZnxS samples was calculated using Vegard’s law [Eq. (3)]. 1 h2 þ k 2 þ l 2 ¼ d2 a2 2 1 4 h þ hk þ k 2 l2 þ 2 ¼ 2 2 d 3 a c Cx ¼ CCdS x ðCZnS CCdS Þ

Figure 1. (a) XRD patterns of Cd1-xZnxS nanoparticles synthesized by thermal decomposition of bis(thiourea)cadmium-zinc acetate complexes and (b) variation of lattice parameter ‘a’ with zinc mole fraction in the Cd1-xZnxS nanopaticles. The lattice parameter was calculated using Vegard’s law.

mium-zinc acetate complexes. The peaks at 26.528, 43.918 and 52.228 in the XRD pattern of CdS are attributed to reflections of hexagonal CdS (JCPDS file no. 01–0783). As the crystallite size of CdS nanoparticles is very small (~ 2.7 nm), separate peaks for (100), (002) and (101) reflections for hexagonal CdS are not observed and instead, a single broad peak at 26.528 is observed. ChemistrySelect 2016, 1, 2687 – 2697

ð1Þ

ð2Þ ð3Þ

Where, Cx, CCdS, CZnS are lattice constants (a) for Cd1-xZnxS, CdS, and ZnS, respectively and x is the molar concentration of zinc in the solid solutions. The lattice constant (a), estimated for Cd1-xZnxS samples using Vegard’s law, varies from 5.42 to 5.81 and it lies in between that of CdS and ZnS (aZnS(cubic) = 5.39 ; aCdS(cubic) = 5.85 ). In general, the composition of Cd1-x ZnxS solid solutions derived from Vegard’s law is close to the nominal composition (Table 1). Figure 1b shows the variation of lattice parameter (a) with an increase in zinc mole fraction in the Cd1-xZnxS nanoparticles. The linear variation of lattice parameter and the successive shift of XRD peaks with increasing concentration of zinc suggest the substitution of Cd2 + ions by Zn2 + ions in the CdS lattice. The crystallite size of Cd1-xZnxS nanoparticles was calculated using Debye-Scherrer equation using the prominent reflection ((111)/ (002)). The crystallite size in the solid solution varies from 2.1 nm to 4.8 nm and it decreases with an increase in the zinc concentration. For the Cd1-xZnxS samples with very broad XRD peaks (x = 0.3, 0.6), the crystallite size could not be calculated. Elemental analysis of the Cd1-xZnxS nanoparticles was carried out using atomic absorption spectroscopy and EDX analysis and formulae for the Cd1-xZnxS solid solution nanoparticles were derived (Table 1). The weight percent and atomic percent of zinc, cadmium and sulfur present in the Cd1-xZnxS nanoparticles, as estimated by the EDS analysis, are given in Table S4. Figure 2 shows the EDX spectra of some representative Cd1-xZnxS solid solution nanoparticles (x = 1, 0.8, 0.5, 0.2, 0). Figure 3 shows the EDS mapping images for the Cd1-xZnxS solid solution nanoparticles. The elemental mapping shows uniform distribution of zinc, cadmium and sulfur in the Cd1-xZnxS solid solution nanoparticles which confirms homogeneity of the solid solutions.

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Full Papers Table 1. Theoretical and proposed formulae of Cd1-xZnxS nanoparticles derived from AAS, EDXA results and Vegard’s law Sample ID 2+

2+

[Cd :Zn ] = 0:1 [Cd2 + :Zn2 + ] = 0.1:0.9 [Cd2 + :Zn2 + ] = 0.2:0.8 [Cd2 + :Zn2 + ] = 0.3:0.7 [Cd2 + :Zn2 + ] = 0.4:0.6 [Cd2 + :Zn2 + ] = 0.5:0.5 [Cd2 + :Zn2 + ] = 0.6:0.4 [Cd2 + :Zn2 + ] = 0.7:0.3 [Cd2 + :Zn2 + ] = 0.8:0.2 [Cd2 + :Zn2 + ] = 0.9:0.1 [Cd2 + :Zn2 + ] = 1:0

Theoretical formula

Proposed formulae From AAS

From EDXA

From Vegard’s law

ZnS Cd0.1Zn0.9S Cd0.2Zn0.8S Cd0.3Zn0.7S Cd0.4Zn0.6S Cd0.5Zn0.5S Cd0.6Zn0.4S Cd0.7Zn0.3S Cd0.8Zn0.2S Cd0.9Zn0.1S CdS

Zn0.95S Cd0.1Zn0.91S Cd0.2Zn0.83S Cd0.32Zn0.68S Cd0.37Zn0.54S Cd0.48Zn0.52S Cd0.61Zn0.48S Cd0.7Zn0.32S Cd0.84Zn0.17S Cd0.93Zn0.09S CdS

Zn0.92S1.07 Cd0.13Zn0.85S Cd0.22Zn0.82S0.95 Cd0.35Zn0.8S1.04 Cd0.4Zn0.54S1.05 Cd0.61Zn0.31S1.07 Cd0.68Zn0.30S0.93 Cd0.72Zn0.32S0.95 Cd0.81Zn0.24S0.94 Cd0.99Zn0.07S0.93 Cd1.09S0.90

ZnS Cd0.09Zn0.91S Cd0.15Zn0.85S Cd0.24Zn0.76S Cd0.33Zn0.67S Cd0.41Zn0.59S Cd0.48Zn0.52S Cd0.63Zn0.37S Cd0.78Zn0.22S Cd0.91Zn0.09S CdS

Figure 2. Typical EDX spectra of Cd1-xZnxS nanoparticles with different [Cd2 + :Zn2 + ] ratios: (a) 1:0, (b) 0.8:0.2, (c) 0.5:0.5, (d) 0.2:0.8 and (e) 0:1.

The BET surface area of representative Cd1-xZnxS nanoparticles was measured and the values are given in Table S5. The solid solution nanoparticles possess higher surface area than that of CdS but lower than that of ZnS. The zinc rich samples possess higher surface area (119.9 m2/g (x = 0.6), 152.3 m2/ ChemistrySelect 2016, 1, 2687 – 2697

g (x = 0.8)) compared to those which are cadmium rich (69.3 m2/g (x = 0.4), 71.7 m2/g (x = 0.2)). This is attributed to smaller crystallite size of zinc rich Cd1-xZnxS nanoparticles (see XRD results).

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Figure 3. EDS mapping images of the Cd1-xZnxS solid solution nanoparticles.

Figure 4. FE-SEM images of Cd1-xZnxS solid solution nanoparticles with [Cd2 + :Zn2 + ] ratio as (a) 1:0, (b) 0.9:0.1, (c) 0.8:0.2, (d) 0.7:0.3, (e) 0.6:0.4, (f) 0.5:0.5, (g) 0.4:0.6, (h) 0.3:0.7, (i) 0.2:0.8, (j) 0.1:0.9 and (k) 0:1. Scale bar, 500 nm.

Morphological Studies The morphological studies of Cd1-xZnxS nanoparticles were carried out using field emission scanning electron microscopy and transmission electron microscopy. Figure 4 shows the FE-SEM ChemistrySelect 2016, 1, 2687 – 2697

images of CdS, ZnS and Cd1-xZnxS nanoparticles. The FE-SEM image of CdS shows aggregate of nanoparticles (Figure 4a). The FE-SEM image of ZnS shows presence of small nanoparticles (Figure 4k). The Cd1-xZnxS samples (x = 0.1-0.5) exhibit aggregates of particles (Figures 4b – 4 f). The Cd1-xZnxS samples 2691


Full Papers with an average size of 4.0 0.7 nm. Figures 5(b-d) show the TEM images of Cd1-xZnxS (x = 0.2, 0.5, 0.8). The TEM results confirm the presence of nanoparticles with mean size of 6.3 0.7 nm (Cd0.8Zn0.2S), 3.4 0.5 nm (Cd0.5Zn0.5S) and 4.3 0.7 nm (Cd0.2Zn0.8S). The TEM images of Cd1-xZnxS (x = 0.1, 0.3, 0.4, 0.6, 0.7, 0.9) nanoparticles are shown in Figure S5. The TEM image of Cd0.9Zn0.1S nanoparticles (Figure S5a) shows aggregation of elongated nanoparticles with average length and width of 23.1 6.2 nm and 14.1 5.0 nm, respectively. The TEM results (Figures S5b-S5 f) again indicate the presence of nanoparticles with mean size of 8.6 1.3 nm (Cd0.7Zn0.3S), 7.9 1.3 nm (Cd0.6Zn0.4 S), 6.9 1.2 nm (Cd0.4Zn0.6S), 7.3 1.2 nm (Cd0.3Zn0.7S) and 4.5 1.1 nm (Cd0.1Zn0.9S). The insets in Figure 5 show HRTEM images of pure CdS, ZnS and Cd1-xZnxS nanoparticles (Cd0.8Zn0.2S, Cd0.5Zn0.5S and Cd0.2 Zn0.8S). The HRTEM images display well-resolved lattice fringes. The estimated interplanar distance values are 3.07 (111), 3.21 (111), 3.39 (100), 3.48 (100) and 3.57 (100) for ZnS, Cd0.2Zn0.8S, Cd0.5Zn0.5S, Cd0.8Zn0.2S and CdS, respectively (Table 2). The interplanar distance of CdS nanoparticles (Figure 5a) and that of ZnS nanoparticles (Figure 5e) match well with the d-spacing of (100) plane of hexagonal CdS and (111) plane of cubic ZnS. However, the interplanar distance values for Cd1-xZnx S (x = 0.2, 0.5, 0.8) nanoparticles match neither with that of hexagonal CdS nor cubic ZnS and they lie in between that of CdS and ZnS. The reduction in d-spacing from 3.57 (CdS) to 3.27 in Cd1-xZnxS (x = 0.8) is attributed to the substitution of larger Cd2 + ions (r = 97 pm) with smaller Zn2 + ions (r = 74 pm).[38] This suggests that Cd1-xZnxS nanoparticles have been formed.[24] Figure 6 shows selected area electron diffraction patterns of CdS, ZnS and Cd1-xZnxS nanoparticles (x = 0.2, 0.5, 0.8). The presence of rings and spots in the SAED pattern of CdS (Figure 6a) suggests polycrystalline nature and the rings are indexed to wurtzite phase. On the other hand, the SAED pattern Figure 5. TEM images of pure CdS (a), ZnS (e) and Cd1-xZnxS solid solution of ZnS (Figure 6e) exhibits bright rings, indicating polycrystalnanoparticles with different [Cd2 + :Zn2 + ] ratios; (b) 0.8:0.2, (c) 0.5:0.5 and (d) line nature and the rings are indexed to the cubic phase. The 0.2:0.8. The HRTEM images are shown in the insets. SAED patterns of Cd0.8Zn0.2S (Figure 6b) and Cd0.5Zn0.5S (Figure 6c) match well with the Table 2. Interplanar distance values from HRTEM images of Cd1-xZnxS nanoparticles hexagonal phase, while the (hkl) Interplanar distance () SAED pattern of Cd0.2Zn0.8S (FigZnS CdS Cd1-xZnxS (Observed) ure 6d) matches with the cubic # Observed Reported phase. Figure S6 shows the seHexagonal Cubic x = 0.2 x = 0.5 x = 0.8 Reported* Observed lected area electron diffraction 3.56 – 3.57 3.48 3.39 3.27 (100)h patterns of the Cd1-xZnxS solid 3.15 3.54 – 3.20 – 3.21 3.08 3.07 (111)c/(101)h solution nanoparticles (x = 0.1, 2.06 2.05 – – 2.07 – 1.12 – (220)c/(110)h 1.75 1.55 – – – – 1.61 – (311)c/(112)h 0.3, 0.4, 0.6, 0.7, 0.9). The pres# * ence of rings in the SAED patJCPDS File No. 01–0783 (Hexagonal CdS) and 80–0019 (Cubic CdS) ; JCPDS File No. 80–0020 (cubic ZnS); h – hexagonal; c – cubic terns suggest polycrystalline nature of the samples. The SAED patterns of Cd0.9Zn0.1S, Cd0.7Zn0.3S, Cd0.6Zn0.4S, Cd0.4Zn0.6 0.8). The TEM image of CdS nanoparticles (Figure 5a) shows agS and Cd0.3Zn0.7S (Figs. S6a-S6e) match well with hexagonal gregation of elongated nanoparticles with average length and phase while the SAED pattern of Cd0.1Zn0.9S (Figure S6 f) matchwidth of 25.3 6 nm and 17.0 3.5 nm, respectively. TEM imes with cubic phase. The transition of phase from hexagonal to age of ZnS shows presence of small nanoparticles (Figure 5e) cubic with increase in zinc content in the Cd1-xZnxS nano(x = 0.6-0.9) show the presence of small nanoparticles (Figures 4g–4j). Figure 5 shows the TEM images and high resolution TEM images of CdS, ZnS and Cd1-xZnxS nanoparticles (x = 0.2, 0.5,


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Full Papers utions, the band gap varies from 2.45 eV (x = 0.1) to 2.77 eV (x = 0.9) (Table 3) The band gap for the Cd1-xZnxS nanoparticles

Table 3. PL excitonic emission band position and band gap values for Cd1-x ZnxS solid solution nanoparticles

Figure 6. SAED patterns of pure CdS (a), ZnS (e) and Cd1-xZnxS solid solution nanoparticles with different [Cd2 + :Zn2 + ] ratios; (b) 0.8:0.2, (c) 0.5:0.5 and (d) 0.2:0.8.

particles is clear from the SAED patterns. The SAED patterns show the presence of hexagonal phase for Cd1-xZnxS nanoparticles with x < 0.8 and cubic phase with x 0.8. It is known that statistical substitution in a lattice site to leads to lattice contraction (for smaller ions) and lattice expansion (for larger ions).[24] With increasing zinc content, the crystal phase of Cd1-x ZnxS solid solution changes from hexagonal to cubic phase.[26] Similar results have been reported by Mariappan et al. for Cd1-x ZnxS thin films synthesized by chemical bath deposition where the structure is hexagonal for the solid solutions with x 0.6 and cubic for x 0.8.[39] Optical Studies The optical properties of Cd1-xZnxS solid solution nanoparticles were investigated using diffuse reflectance spectroscopy and photoluminescence spectroscopy. Figure 7a shows the diffuse reflectance spectra of Cd1-xZnxS nanoparticles. The estimated band gap values, from the Tauc plots (Figure 7b), for pure CdS and ZnS are 2.37 eV and 3.69 eV, respectively. These values agree well with the reported values. For the Cd1-xZnxS solid solChemistrySelect 2016, 1, 2687 – 2697

Sample ID

Excitonic emission peak position (nm)

Band gap (eV)

[Cd2 + :Zn2 + ] = 0:1 [Cd2 + :Zn2 + ] = 0.1:0.9 [Cd2 + :Zn2 + ] = 0.2:0.8 [Cd2 + :Zn2 + ] = 0.3:0.7 [Cd2 + :Zn2 + ] = 0.4:0.6 [Cd2 + :Zn2 + ] = 0.5:0.5 [Cd2 + :Zn2 + ] = 0.6:0.4 [Cd2 + :Zn2 + ] = 0.7:0.3 [Cd2 + :Zn2 + ] = 0.8:0.2 [Cd2 + :Zn2 + ] = 0.9:0.1 [Cd2 + :Zn2 + ] = 1:0

463 465 474 477 480 490 496 496 501 503 505

3.69 2.77 2.67 2.63 2.61 2.57 2.53 2.53 2.51 2.45 2.37

exhibits blue shift with respect to pure CdS nanoparticles and, in general, the band gap increases with an increase in the zinc concentration (Figure 7c). The blue shift of band gap observed in Cd1-xZnxS nanoparticles provides evidence for the formation of solid solutions. The observed crystallite size values for Cd1-xZnxS solid solution nanoparticles (2.1 nm – 4.8 nm) are close to or marginally larger than the Bohr radius of CdS and ZnS (CdS ~ 2.5 nm, ZnS ~ 3.5 nm) and the situation corresponds to weak quantum confinement regime.[38] It has been reported that the band gap difference between CdS particles with the radii of 4.8 and 2.8 nm is very small (0.08 eV). It is proposed that the significant band gap increase (~ 0.4 eV) in Cd1-xZnxS with an increase in the zinc content is due to the compositional variation and not due to size effect. Figure 8a shows the photoluminescence spectra for CdS, ZnS and Cd1-xZnxS nanoparticles (lexcitation = 220 nm). The PL spectrum for pure CdS exhibits an asymmetric emission band in the region 400 to 600 nm and a weak emission band at about 735 nm. The presence of asymmetric band suggests multiple emissions, and it was deconvoluted into two bands at about 505 nm and 565 nm. The band at 505 nm is attributed to excitonic emission and the bands at 565 nm and 735 nm are attributed to radiative recombination of charge carriers at the surface trap states of cadmium and sulfur vacancies, respectively. The PL spectrum for pure ZnS nanoparticles exhibits a sharp emission band at about 370 nm, a shoulder at 470 nm and a small peak at 735 nm. The emission band at about 370 nm is attributed to excitonic emission and the bands at 470 and 735 nm are attributed zinc and sulfur defects, respectively. Figure 8b shows the PL spectra for Cd1-xZnxS nanoparticles in the range 320 nm to 420 nm and Figure 8c shows the normalized PL spectra in the wavelength range 400 nm 800 nm. The PL spectra of Cd1-xZnxS samples exhibit a sharp emission band at about 370 nm, an asymmetric emission band in the region 400 nm to 600 nm and a small band at 735 nm. The asymmetric emission bands were deconvoluted into two emission bands at 503 nm to 465 nm and 565 nm. The band at 2693


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Figure 7. (a) Diffuse reflectance spectra, (b) Tauc plots for Cd1-xZnxS nanoparticles, and (c) variation of band gap of Cd1-xZnxS nanoparticles with zinc mole fraction.

503 nm – 465 nm is attributed to excitonic emission and the bands at 565 nm and 735 nm are attributed to presence of cadmium and sulfur vacancies of Cd1-xZnxS nanoparticles, respectively. Figure 8d shows the variation of intensity of emission at about 370 nm as a function of concentration of zinc in the solid solution and intensity of the emission increases with an increase in the zinc content. This is attributed to an increase in the internal defects resulting from increased lattice strain due replacement of Cd2 + ions with Zn2 + ions in Cd1-xZnxS.[16] The variation of excitonic emission as a function of zinc concentration in the Cd1-xZnxS solid solution nanoparticles is shown in Figure 8e. The excitonic emission peak exhibits a blue shift (503 nm to 465 nm) with respect to pure CdS (505 nm) on increasing the concentration of zinc (Table 3). The blue shift of excitonic emission observed in Cd1-xZnxS nanoparticles is attributed to the intermixing of ZnS with CdS.[16] Mechanism of formation of Cd1-xZnxS nanoparticles The formation of Cd1-xZnxS nanoparticles by thermal decomposition of bis(thiourea)cadmium-zinc acetate complexes is explained as follows. Complex formation facilitates the bonding ChemistrySelect 2016, 1, 2687 – 2697

of Zn2 + and Cd2 + ions with sulfur of thiourea, which has a strong coordination ability.[40] Pre-conditioned arrangement and availability of all the three ions (Cd2 + , Zn2 + and S2) in a single molecule (i. e. complex) facilitates the formation of Cd1-xZnxS nanoparticles. When the complex is subjected to thermal decomposition, it results in the release of Cd2 + , Zn2 + and S2 ions.[32] These ions react at high temperature (~ 200 8C) to form Cd1-xZnxS nanoparticles. The reaction that takes place during the formation of Cd1-xZnxS nanoparticles is as follows.[41] ½Cd1-x Znx ðCS ðNH2 Þ2 Þ2 ðCH3 COOÞ2 ! Cd1-x Znx S þ other byproducts ðNH3 þ HCNS þ CH3 COCH3 þ CH2 CO þ CO2 þ H2 OÞ The role of thiourea is not only limited to acting as a source of sulfur but it also regulates nucleation rate of Cd1-xZnxS nanoparticles by supplying S2 ions in a controlled manner. Li et al. have reported that thiourea inhibits crystal growth leading to the formation of nanoparticles with small size.[26] For the same reason, Cd1-xZnxS nanoparticles with small crystallite size 2694


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Figure 8. (a) PL spectra of pure CdS, ZnS and Cd1-xZnxS nanoparticles, (b) in the range 320 nm – 420 nm and (c) 400 nm – 800 nm. The variation of peak intensity at 367 nm (d) and the variation of excitonic peak position with concentration of Zn2 + ions (e) are also shown.

(2.1 nm – 4.8 nm) are obtained by the present thermal decomposition method. Nanoparticles with very small crystallite size provide high surface area and abundant active sites which aid transport of charge carriers from interior to the surface of nanoparticles. Only a few authors have reported the synthesis of Cd1-xZnxS nanoparticles using single source molecular precursors. Zhang and co-workers have reported the synthesis of hexagonal Cd1-x ZnxS nanoparticles (x = 0.05, 0.195, 0.277, 0.5, 0.8, 0.9 and 1.0) by solid state thermal decomposition (300 8C, 3 h) and nanorods (x = 0, 0.05, 0.1, 0.2 and 0.3) by solvothermal decomposition (180 8C, 12 h) of cadmium-zinc bis(N,N-diethyl dithiocarbamates).[27, 28] Chen et al. have reported solvothermal decomposition (180 8C, 12 h) of cadmium-zinc bis(N,N-diethyl ChemistrySelect 2016, 1, 2687 – 2697

dithiocarbamates) for the synthesis of Cd1-xZnxS nanorods (x = 0.05, 0.195, 0.227, 0.5, 0.8, 0.9 and 1.0).[25] Li et al. have reported the synthesis of Cd1-xZnxS nanoparticles (x = 0, 0.1, 0.3, 0.4, 0.5, 0.6, 0.7, 0.9, 1.0) by solid state thermal decomposition of zinccadmium thiourea complexes at 500 8C for 3 h under N2 atmosphere.[26] Naumov et al. have reported the synthesis of Cd1-xZnxS thin films by aerosol pyrolysis of bis(thiourea)cadmium zinc chloride at 300 8C 550 8C.[32] In the present study, a series of bis(thiourea)cadmium-zinc acetate complexes were prepared and a complete series of Cd1-xZnxS solid solutions (x = 0-1) has been prepared by the thermal decomposition of the complexes. The reaction conditions employed are milder (temperature = 200 8C) with shorter synthesis time (60 mins)) compared to the reported methods. Importantly, no phase separation is 2695


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Figure 9. (a) UV-Vis spectral results indicating the photodegradation of methylene blue using Cd1-xZnxS solid solution nanoparticles and commercial TiO2 under solar irradiation and (b) photodegradation efficiency of Cd1-xZnxS nanoparticles and commercial TiO2 versus the Zn mole fraction in Cd1-xZnxS.

observed unlike the previous report where multiple precursors were employed.[6]

place and the Cd1-xZnxS nanoparticles act as photocatalysts and not as adsorbents.

Photocatalytic degradation of methylene blue in an aqueous solution

Conclusions

The Cd1-xZnxS solid solution nanoparticles were explored as visible light photocatalysts for the photodegradation of methylene blue in an aqueous solution under solar irradiation. Figure 9a shows the UV-Vis spectral results indicating the photodegradation of methylene blue in the presence of different Cd1xZnxS nanoparticles as the catalyst. Among the Cd1-xZnxS nanoparticles, Cd0.5Zn0.5S exhibits good degradation efficiency (87.2 %) compared to that of pure ZnS (32.4 %) and pure CdS (63.3 %) after 60 minutes of irradiation (Figure 9b). The photocatalytic activity of the Cd1-xZnxS solid solution nanoparticles was compared with commercial TiO2 (Sigma Aldrich, 99 %, surface area = 14.2 m2/g). The XRD pattern of commercial TiO2 (TiO2-C) matched with anatase phase (JCPDS No. 86–1157) with an estimated crystallite size of 47.9 nm. The photocatalytic degradation of methylene blue was carried out under identical conditions using commercial TiO2 and it exhibits a photodegradation efficiency of 88.5 % which is comparable to the best photocatalyst among all the solid solution samples (Cd0.5 Zn0.5S (87.2 %)). It is interesting to note that Cd0.5Zn0.5S nanoparticles with low surface area (94.2 m2/g) show good photocatalytic activity, which suggests that suitable band edge alignment and high negative potential play key role in the photocatalytic degradation of methylene blue. Methylene blue is a cationic dye and photocatalysts with more negative conduction band potential are expected to have better efficiency for the photodegradation.[3] Figure S7 shows the UV-Vis spectral results for the adsorption of methylene blue from aqueous solutions under dark in the presence of different Cd1-xZnxS nanoparticles along with commercial TiO2. The results show negligible reduction in the intensity of absorption bands due to methylene blue indicating that negligible adsorption takes ChemistrySelect 2016, 1, 2687 – 2697

A complete series of Cd1-xZnxS nanoparticles (x = 0 1) has been successfully synthesized by the thermal decomposition of bis(thiourea)cadmium-zinc acetate complexes in diphenyl ether. Cd1-xZnxS nanocrystals with controlled composition were obtained by varying the Cd:Zn ratio in the bis(thiourea)cadmium-zinc acetate complexes. The XRD results were in agreement with Vegard’s law and linear variation of lattice parameter with the composition is observed. TEM results confirm the presence of small nanoparticles in Cd1-xZnxS and transition of phase from hexagonal to cubic phase is observed for the solid solutions with x 0.8. The Cd1-xZnxS nanoparticles exhibit tunable optical properties and the band gap of Cd1-xZnxS nanoparticles can be tuned from 2.37 eV (x = 0) to 3.69 eV (x = 1) by varying the zinc content in the solid solutions. PL results show tunability of band edge emission as a function of zinc concentration in the Cd1-xZnxS nanoparticles. The Cd1-xZnxS solid solution nanoparticles show good photocatalytic activity for methylene blue degradation in aqueous solution under sunlight compared to pure CdS and ZnS nanoparticles. The Cd1-x ZnxS nanoparticles are expected to be useful in different areas including photocatalysis and optoelectronics. Supporting Information The supporting information contains experimental section which consists of synthetic details for bis(thiourea)cadmiumzinc acetate complexes and Cd1-xZnxS nanoparticles, characterization techniques used and details on photocatalysis experiments. In addition, it contains characterization results for the bis(thiourea)cadmium-zinc acetate complexes (XRD, IR, TGA and elemental analysis) and the Cd1-xZnxS nanoparticles (TEM, SAED, EDS and surface area).

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Full Papers Acknowledgements P. J. gratefully acknowledges the financial support from Council of Scientific and Industrial Research (CSIR), New Delhi (Project No. 01/(02726)13/EMR-II). Ms. Rama Gaur acknowledges Council of Scientific and Industrial Research for the award of Junior/Senior Research Fellowship (SRF/JRF). Thanks are due to the Institute Instrumentation Centre, IIT Roorkee for providing some of the facilities used in the present study. Thanks are also due to Dr. Paritosh Mohanty for his help with the BET measurements. Keywords: Nanoparticles · Cd1-xZnxS · Optical properties · Bis (thiourea)cadmium-zinc acetate · Thermal decomposition [1] C. Martinez-Alonso, C. A. Rodriguez-Castaneda, P. Moreno-Romero, C. Selene Coria-Monroy, H. Hu, Int. J. Photoenergy 2014, 453747/1-453747/ 11; F. Qin, B. Bai, D. Jing, L. Chen, R. Song, Y. Suo, RSC Adv. 2014, 4, 34864-34872; W. Zhang, J. Zheng, C. Tan, X. Lin, S. Hu, J. Chen, X. You, S. Li, J. Mater. Chem. B 2015, 3, 217–224. [2] S. A. Macias-Sanchez, R. Nava, V. Hernandez-Morales, Y. J. Acosta-Silva, B. Pawelec, S. M. Al-Zahrani, R. M. Navarro, J. L. G. Fierro, Int. J. Hydrogen Energy 2013, 38, 11799–11810; D. Meissner, R. Memming, B. Kastening, J. Phys. Chem. 1988, 92, 3476–3483. [3] J. A. Villoria, R. M. Navarro Yerga, S. M. Al-Zahrani, J. L. G. Fierro, Ind. Eng. Chem. Res. 2010, 49, 6854–6861. [4] J. Zhang, S. Wageh, A. Al-Ghamdi, J. Yu, Appl. Catal., B 2016, 192, 101–107. [5] M. Li, J. Ouyang, C. I. Ratcliffe, L. Pietri, X. Wu, D. M. Leek, I. Moudrakovski, Q. Lin, B. Yang, K. Yu, ACS Nano 2009, 3, 3832–3838; I. Moreels, K. Lambert, D. Smeets, D. De Muynck, T. Nollet, J. C. Martins, F. Vanhaecke, A. Vantomme, C. Delerue, G. Allan, Z. Hens, ACS Nano 2009, 3, 3023–3030. [6] R. Gaur, P. Jeevanandam, J. Mater. Sci. Mater. Electron. 2015, 26, 7223–7231. [7] S. Kumar, P. Sharma, V. Sharma, J. Nanopart. Res. 2013, 15, 1662/1-1662/ 8; A. Rani, R. Kumar, Appl. Phys. A-Mater. 2015, 120, 775–784. [8] J. Li, L. Wu, L. Long, M. Xi, X. Li, Appl. Surf. Sci. 2014, 322, 265–271. [9] M. Huang, J. Yu, C. Deng, Y. Huang, M. Fan, B. Li, Z. Tong, F. Zhang, L. Dong, Appl. Surf. Sci. 2016, 365, 227–239. [10] J. Yu, J. Zhang, M. Jaroniec, Green Chem. 2010, 12, 1611–1614. [11] M. A. Mahdi, J. J. Hassan, Z. Hassan, S. S. Ng, J. Alloys Compd. 2012, 541, 227–233. [12] G. Yang, Q. Zhang, W. Chang, W. Yan, J. Alloys Compd. 2013, 580, 29–36. [13] W. Wang, W. Zhu, H. Xu, J. Phys. Chem. C 2008, 112, 16754–16758. [14] Y. Dong, L. Zhou, S. Wu, Mater. Sci. Semicond. Process. 2014, 19, 78–82. [15] W. Z. Wang, I. Germanenko, M. S. El-Shall, Chem. Mater. 2002, 14, 3028–3033. [16] D. Choi, J.-Y. Pyo, Y. Kim, D.-J. Jang, J. Mater. Chem. C 2015, 3, 3286–3293.


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Submitted: April 1, 2016 Accepted: June 20, 2016

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