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Diffuse reflectance and reflective flexible coatings of capped ZnS nanoparticles
Materials Chemistry and Physics 142 (2013) 734e739
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Diffuse reflectance and reflective flexible coatings of capped ZnS nanoparticles Sanjeev Kumar a, b, c, *, N.K. Verma b, M.L. Singla c a
Sri Guru Granth Sahib World University, Fatehgarh Sahib 140 406, India Nano Research Lab, School of Physics and Materials Science, Thapar University, Patiala 147 004, India c Central Scientific Instruments Organisation, Chandigarh 160 030, India b
h i g h l i g h t s Diffuse reflectance of synthesised uncapped and thioglycerol capped ZnS nanoparticles has been studied. The capped nanoparticles showed maximum diffuse reflectance. Using this property and adjusting pH, binder ratio, etc., the reflective flexible coatings were made.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 May 2011 Received in revised form 20 August 2013 Accepted 21 August 2013
Thioglycerol capped ZnS nanoparticles were synthesized by chemical precipitation method and the effect of capping concentration on diffuse reflectance was studied. The comparative study of diffuse reflectance of commercial bulk ZnS particles with synthesised uncapped and capped nanoparticles has been made. ZnS nanoparticles capped with 0.50% of thioglycerol were found to be of uniform shape and size, with maximum diffuse reflectance of 93e94% in the visible region. The reflectors developed using these nanoparticles showed reflectance of 96.5e97.5% in the visible region with optimum coating thickness of 0.25 mm. These reflectors have also been found to be flexible. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Chemical synthesis Optical properties Coating Visible and ultraviolet spectrometers
1. Introduction ZnS is a II-VI semiconductor with direct band gap, 3.68 eV, and refractive index, 2.37 [1,2]. It exists in two phases, zinc blende and wurtzite; zinc blende phase is thermodynamically stable at room temperature. ZnS finds wide applications in areas such as optical devices, photo-catalyst, bio-labelling [2,3]. Apart from these applications, ZnS can be used as a reflective pigment, due to wide band gap and high refractive index. In industry, bulk ZnS is prepared by reacting its respective mineral ores. The so obtained ZnS particles are non-uniform in shape and size, and are absorptive in the visible region [4]. ZnS can be used as reflective pigment provided it scatters incident light rather than absorbing it, in the visible region. The size, shape and
* Corresponding author. Nano Research Lab, School of Physics and Materials Science, Thapar University, Patiala 147 004, India. Tel.: þ91 9653994114; fax: þ91 0175 2364498. E-mail addresses: [email protected], [email protected] (S. Kumar). 0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2013.08.032
uniformity of particles play a crucial role in determining their reflective property; these parameters can be optimized by reducing their size to nanoscale (particle size <100 nm) [5]. Nanoparticles (NPs) have attracted widespread attention because of their unique optical and electronic properties, arising due to quantum confinement and large surface area, which is not observed in their bulk counterpart [6,7]. Controlling the reaction parameters viz. molar concentration of precursors, nature of solvent, reaction temperature and capping/stabilizing agent, NPs of different sizes and shapes can be synthesised [5]. ZnS NPs capped with cetyltrimethylammonium bromide, thioglycerol, ethylene glycol, methacrylic acid, polyvinyl pyrrolidone have been prepared, respectively, using microemulsion [8], hydrothermal [9], solvothermal/hydrothermal [10], solegel [11], chemical vapour deposition [12] methods. These methods have their own drawbacks such as low yield, time consuming, secondary phase formation and expensive. So, these methods cannot be applied in industry for bulk production of ZnS pigments. However, the chemical precipitation method is cost effective, less time consuming and has high yield.
S. Kumar et al. / Materials Chemistry and Physics 142 (2013) 734e739
735
In this paper, authors have synthesised uncapped and capped ZnS NPs using chemical precipitation method, and compared their diffuse reflectance with that of commercially available ZnS bulk particles. The capped NPs have also been used to prepare reflective flexible coatings. To the best of our knowledge, no work, till date, in the area of diffuse reflectance of capped ZnS NPs, has been reported. 2. Experimental All the chemicals and reagents used were of analytical reagent grade, and were used without any further purification. The synthesis has been carried out by chemical precipitation technique [1,13]. Fig. 1. Schematic illustration of synthesised ZnS NPs capped with TG.
2.1. Synthesis of ZnS nanoparticles The aqueous solution of sodium sulphide (0.5 M) was added drop-wise into zinc acetate (0.5 M) solution with constant stirring. Different concentrations by volume (0e1.00%) of thioglycerol (TG), as a surfactant, were added to cap ZnS particles. The precipitates of ZnS, capped with TG, were washed and dried. The NPs, so obtained, were crushed to powder form. 2.2. Preparation of coating material and reflectors The coating material was prepared by dispersing ZnS NPs and acrylic binder in de-ionized water, using a planetary ball mill, for 5 min at 1000 rpm. For the preparation of coating material, acrylic has been chosen as a binding material. Acrylic has gained a strong foothold in the coatings due to its higher flexibility, easy adhesion and resistance to ultraviolet degradation as compared to the other emulsions and latex. Acrylic binder used in the preparation of coating is of non-convertible type. These are polymerized binders which are dispersed in a medium. The medium evaporates after the coating has been applied to leave a coherent film on the substrate surface. To maintain stable pigment dispersion, pH of the coating material was adjusted at 6.5. Plastic sheet substrate with area 2 cm2 and thickness 1 mm was cleaned [14]. These sheets were then spray coated with the coating material to different thicknesses. For each spraying application, the surface was sprayed until it had a glossy appearance, without running the coating material. The coating was air dried and again sprayed before the glossiness disappeared. By adjusting pigment to solvent ratio, i.e., viscosity, air pressure of spraying gun and nozzle hole size, the thickness of the film was controlled.
The phase purity and crystal structure of the sample was determined by PANanalyticals X’Perto Pro X-ray diffractometer with CuKa (l ¼ 1.542 A) as a radiation source. The size and morphology of NPs was studied on Hitachi (H-7500) transmission electron microscope (TEM). The optical absorption spectra were recorded using Perkin Elmer 350 UVevis spectrophotometer to calculate the band gap. The samples (nanoparticles) were dispersed in a solvent using ultrasonicator. The solvent used was chemically inert and did not lead to any agglomeration of the particles. The final measurement was made on the sample contained in a cuvette. The variation of absorption with the wavelength was obtained from the absorbance vs. wavelength plot. The optical band gap has been determined from this absorbance against wavelength plot using the equation (1) [15].
hc ƛint
3. Results and discussion 3.1. Reaction mechanism
2.3. Characterization
Eg ¼
to the intersection of extension of linear parts of the spectrum of yaxis and x-axis. Surface passivation by surfactant molecules has been carried out using FTIR spectra with Perkin Elmer Spectrum BX II spectrometer. Measurement of diffuse reflectance: Reflectance spectra provide similar and complementary information to the absorption measurements. Diffusion reflectance is generally used for unpolished or powdered samples. For diffuse reflectance measurements, an integrating sphere (a sphere with a fully reflective inner surface) is used. The sphere has a pinhole through which the light enters and is transmitted towards the sample. The diffuse reflected light reaches the detector after suffering multiple reflections in the inner surface of the sphere. There is no standard method of sample preparation in diffuse reflectance spectroscopy. Finely grounded powder is carefully packed into a sample holder with a circular hole having surface of a few tens square millimeters. No special care is exercised when the holder is placed horizontally at the bottom of the integrating sphere. In diffuse reflectance measurements, the sample is placed at a port opening opposite to the entrance port. The incident flux is reflected by the sample. The total hemispherical diffuse reflectance is collected by integrating sphere and graph of diffuse reflectance vs. wavelength is plotted. The same set up was used to measure diffuse reflectance of the developed reflectors. Calibration of the reflectance scale was done by standard reference material (WS-1-SL, Spectralon). Measurement of film thickness has been done using ElekrtoPhysik MiniTest 720.
Zinc acetate and sodium sulphide dissociate into their respective ions when dissolved in de-ionized water. Nucleation growth of
(1)
where, h is Plank’s constant (4.135 106 eV nm); c, the velocity of light (3 108 m s1), and ƛint , the wavelength (nm) corresponding
Fig. 2. X-ray diffraction patterns of ZnS NPs (a) uncapped, and TG capped, (b) 0.25%, (c) 0.50%, (d) 0.75%, (e) 1.00%.
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S. Kumar et al. / Materials Chemistry and Physics 142 (2013) 734e739
Table 1 NP-crystallite size and band gap variation with capping concentrations. Sample name
Sample/capping concentration by volume
Crystallite size (nm)
Band gap energy (eV)
(a) (b) (c) (d) (e)
ZnS uncapped ZnS: 0.25% ZnS: 0.50% ZnS: 0.75% ZnS: 1.00%
17.20 16.95 15.85 16.66 17.05
3.64 3.81 4.16 3.82 4.05
ZnS NPs starts due to reaction between Zn2þ and S2 ions. The growth mechanism can be explained on the basis of Ostwald ripening [16]. The chemical potential of the smaller NPs is higher as that of the larger ones. Due to this, the large NPs grow at the expense of the smaller ones. To avoid further growth, NPs are capped with TG which acts as a barrier by inducing steric hindrance [17], and consequently uniform shape, size, and stable NPs were obtained. The schematic illustration for the synthesis of ZnS NPs capped with TG has been shown in Fig. 1. Following is the reaction mechanism for the synthesis process:
nðC4 H6 O4 Zn$2H2 OÞ þ nðNa2 SÞ/ðZnSÞn TG þ ðZnSÞn /TGðZnSÞn
Fig. 4. FTIR spectra of ZnS NPs (a) uncapped, and TG capped, (b) 0.25%, (c) 0.50%, (d) 0.75%, (e) 1.00%.
3.2. Phase and structural analysis Fig. 2 shows the XRD patterns of synthesised uncapped and capped ZnS NPs. These NPs have zinc blende crystal structure, with three broad diffraction peaks, corresponding to (111), (220), and (311) planes of the cubic ZnS (JCPDS card No. 050566). The broadening in the diffraction peaks is due to size and strain effect. Ignoring the lattice strain in the nanocrystallites, owing to
Fig. 3. TEM micrographs of ZnS NPs (a) uncapped, and TG capped, (b) 0.50%, (c) 0.50% with higher resolution, (d) 1.00%.
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their negligible value (0.07), the crystallite size of ZnS NPs has been calculated using Debye Scherrer’s equation [18]. The crystallite size has been determined from the full width at half maximum (FWHM) of the most intense peak (Fig. 1), shown in Table 1. FWHM has been calculated using Lorentz best fit curve (R2 >0.98) method. No peak related to other material is observed in the diffraction pattern, which confirms the purity of the ZnS particles. 3.3. Morphology Fig. 3(a)e(d) shows TEM micrographs of uncapped, 0.50%, and 1.00% of TG capped ZnS NPs. It reveals that the uncapped particles (Fig. 3(a)) get aggregated and coalesce non-uniformly due to their high surface energy in the absence of surfactant. But surface passivation of the ZnS NPs with 0.50% TG (Fig. 3(b) and (c)) leads to spherical symmetry. The NPs, so obtained, were nearly spherical with size 20e 28 nm (core size, 15e20 nm and shell (TG layer) thickness, 5e8 nm). Such core/shell type structure changes the optical properties of the materials [19]. Further increase in surfactant concentration resulted in agglomeration of the ZnS NPs (Fig. 3(d)). Above a certain concentration, self-interaction in-between molecules of capping agent occurred. Due to this, partial passivation of zinc surface took place which led to agglomeration of ZnS particles [20].
Fig. 5. UVevis absorption spectra of ZnS NPs (a) uncapped, and TG capped, (b) 0.25%, (c) 0.50%, (d) 0.75%, (e) 1.00%.
3.4. Fourier transform infrared (FTIR) analysis ZnS NPs were mixed with KBr to prepare pellets. Fig. 4 shows the FTIR spectra of uncapped and capped ZnS NPs. A broad intermolecular hydrogen bond OeH stretch, found around 3398 cm1 (curves ‘a’ to ‘e’) is due to the hygroscopic nature of KBr. The absorption band, 3520e3200 cm1, is due to the free hydroxyl group. A weak band appearing around 2350 cm1 is because of the presence of the SeH group [21]. The peak around 1615 cm1 in uncapped ZnS is due to pure water, HeOeH vibration, showing partial evaporation of water in the sample [21]. A high absorption peak near 1590 cm1 might be due to the presence of acetate group of the zinc precursor. The group of peaks around 1000 cm1 is indicative of the sulphide group [21]. The band, 400e600 cm1, is because of the SeS bond stretching in capped NPs. The SeH stretching peak appeared around 2350 cm1; this being very weak, so the corresponding bonds break. This leads to formation of SeS bonds in ZnS capped with TG, giving absorption band in the 400e 600 cm1 region.
3.5.2. Diffuse reflectance analysis The diffuse reflectance spectra, both for commercially available bulk ZnS particles and synthesized uncapped as well as capped ZnS NPs are shown in Fig. 6. The diffuse reflectance of capped ZnS NPs (93e94%) in comparison to the uncapped ZnS NPs (91e92%) and bulk ZnS particles (82e70%), has been found to be higher in the visible region. Table 2 shows diffuse reflectance of the uncapped and capped ZnS NPs for different TG concentrations as well as that of the bulk ZnS particles. On increasing the capping concentration from 0 to 0.50%, the diffuse reflectance increases, whereas, it decreases on further increase in concentration from 0.50 to 1.00%. These results show attainment of maximum diffuse reflectance for 0.50% TG capped ZnS NPs. The uncapped ZnS NPs form a powdered layer, which acts as a thin film. The light incident on it undergoes destructive interference, consequently decreasing the reflected light intensity [23]. On
3.5. Optical analysis 3.5.1. UVevis absorption analysis The optical absorption spectra of ZnS NPs (Fig. 5) show blueshift in the absorption edges for ZnS NPs vis-a-vis the bulk ZnS particles (Eg ¼ 3.68 eV). The band gap energy values of uncapped and capped NPs (Table 1) were also found to be higher than those of the bulk ZnS particles, as calculated from the UVevisible absorption spectra (Fig. 5) [13]. With increase in the capping concentration, 0e 0.50%, the energy band gap increased. But, for further increase in capping concentration, 0.75e1.00%, the energy band gap decreased. This blue-shift is due to the quantum confinement effects [13]. Band gap plays an important role in the reflective properties of the pigments. According to the band theory [22], a substance (pigment) with a large band gap (>3 eV) does not absorb visible light [23]. Such pigments transmit the entire incident light and are, therefore, colourless in their pure form. However, in powder form, light is reflected back and so they appear white. Thus, ‘large band gap substance’ is an excellent reflecting pigment for visible light.
Fig. 6. Diffuse reflectance spectra of ZnS particles.
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S. Kumar et al. / Materials Chemistry and Physics 142 (2013) 734e739
Table 2 Diffuse reflectance of ZnS particles. Sr. No.
1 2 3 4 5 6
Capping concentration
0.0 (uncapped) 0.25% ml 0.50% ml 0.75% ml 1.00% ml Bulk
% Reflectance at wavelength 400 nm
450 nm
500 nm
550 nm
600 nm
650 nm
700 nm
91.42 91.57 93.43 91.94 92.52 81.68
92.17 92.29 94.18 92.65 93.24 88.94
92.04 92.20 94.06 92.58 93.15 90.29
91.98 92.13 94 92.51 93.09 90.76
91.97 92.11 93.98 92.49 93.07 90.23
91.92 92.08 93.93 92.45 93.03 83.55
91.92 92.07 94.94 92.44 93.02 69.12
capping ZnS NPs (refractive index (n) ¼ 2.37) with TG (n ¼ 1.52), the coating behaves like a homogeneous and isotropic film because of uniform shape and size of the particles (Fig. 3(b) and (c)) [22]. Consequently, this results in specular reflections, and the reflected light interferes constructively, resulting in increase in the intensity of the reflected light with capping concentration (0e0.50%). With further increase in the capping concentration from 0.50 to 1.00%, the shape and size of the particles no longer remain uniform because of agglomeration (Fig. 3(d)). Therefore, the capping (surface passivation) results in uneven surface, thereby decreasing reflectance. The reflectance of bulk ZnS particles decreases sharply after 625 nm (Fig. 6), it has been reported earlier as well [4]. Therefore, the bulk ZnS particles cannot be used as reflective pigment for the visible region. From the current diffuse reflectance study, it is concluded that the ZnS NPs possess stable and high diffuse reflectance as compared to that of the bulk ZnS particles. 3.5.3. Diffuse reflectance analysis of coating and their flexibility ZnS NPs capped with 0.50% TG show maximum diffuse reflectance (Fig. 6) and therefore, have been employed as reflective pigment for the preparation of coating material. Such a coating material has been applied to plastic sheets to develop reflectors. Fig. 7 shows the diffuse reflectance spectra of reflectors having different coating thicknesses. It reveals that with increase in coating thickness, the reflectance increases. For coating thickness, 0.15, 0.20, and 0.25 mm, the diffuse reflectance has been found to be, respectively, 93.5e94.5%, 95.5e96.5%, and 96.5e97.5%. Further increase in coating thickness from 0.25 to 0.30 mm, resulted in undesired change in reflectance. The coating thickness, 0.25 mm, is
the optimum coating thickness. At the optimum coating thickness, the critical volume pigment concentration (above it no substantial difference in the behaviour and appearance of the coating is observed) plays an important role in determining reflectance. Flexibility of the coating has been determined to ensure the durability of the reflector. Coating must be flexible enough so as not to develop crack or split with shrinkage of substrate due change in temperature [24]. A simple method to check the flexibility of the coating has been applied by changing the working temperature from 30 to 45 C, which showed, respectively, 5.0% and 10.5% decrease in the sheet surface area without any crack or split being developed in the coating. Thus, the developed reflectors have been found to be flexible. 4. Conclusions The comparative study on diffuse reflectance of bulk ZnS particles vis-a-vis synthesised uncapped and capped ZnS NPs, with different capping concentrations of TG, show that capped ZnS NPs are more stable and are highly reflective in visible region. This study establishes that ZnS NPs capped with 0.50% TG are uniform in shape, small in size and large in band gap; they form an excellent reflective pigment in visible light. These NPs show maximum diffuse reflectance (93e94%) in visible region. The reflectors of different coating thicknesses (0.15e0.30 mm) have been developed using these NPs on a plastic substrate. It has been found that the reflector with coating thickness, 0.25 mm, possesses maximum reflectance, 96.5e97.5%. The reflectors with 0.25 mm thick coating have been found to be flexible. These reflective flexible coatings may be potential candidates for applications in areas demanding reflector flexibility. Acknowledgement Sanjeev Kumar is extremely grateful to the Department of Science and Technology, Government of India, for awarding him the INSPIRE fellowship to carry out the current research work. The constructive discussions with Mr. Harvinder Kumar, Silky Paint Industries are also acknowledged. References
Fig. 7. Diffuse reflectance spectra of reflectors.
[1] S. Kumar, N.K. Verma, M.L. Singla, J. Coat. Technol. Res. 8 (2011) 223. [2] C. Li, Z. Cui, Z. Li, B. Yang, J. Shen, J. Mater. Chem. 13 (2003) 526. [3] E.E. Lee, M.J. Gunzburg, T.L. Nguyen, G.J. Howlett, J. Rothacker, E.C. Nice, H.A. Ckayton, P. Mulvaney, Nano Lett. 8 (2008) 2883. [4] G. Buxbaum, G. Pfaff, Industrial Inorganic Pigment, third ed., WILEY-VCH, Weinheim, 2005. [5] C. Burda, X. Chen, R. Narayanan, M.A. El-Sayed, Chem. Rev. 105 (2010) 1025. [6] T. Takagahara, K. Takeda, Phys. Rev. B 46 (1992) 15578. [7] Y.S. Jung, Y.J. Hong, Y. Li, J. Cho, Y.J. Kim, G.C. Yi, Nano 2 (2008) 637. [8] S.K. Mehta, S. Kumar, S. Chaudhary, K.K. Bhasin, M. Gradzielski, Nanoscale Res. Lett. 4 (2009) 17. [9] M.S. Niasari, F. Davar, M. Mazaheri, J. Alloys Compd. 470 (2009) 502. [10] M.P. Anil, S.G. Shivram, J. Crystal Growth (2011) http://dx.doi.org/10.1016/ j.jcrysgro.2011.01.087.
S. Kumar et al. / Materials Chemistry and Physics 142 (2013) 734e739 [11] L.Q. Yue, F.J. Zhang, J.Z. Huang, L.W. Wang, J. Nanosci. Nanotechnol. 8 (2008) 1199. [12] R. Maitya, U.N. Maitia, M.K. Mitrab, K.K. Chattopadhyaya, Physica E 33 (2006) 104. [13] Z. Jindal, N.K. Verma, J. Mater. Sci. 43 (2008) 6539. [14] R. Talbert, Paint Technology Handbook, Taylor & Francies Group, USA, 2008. [15] A. Zachariah, K.V. Baiju, S. Shukla, K.S. Deepa, J. James, K.G.K. Warrier, J. Phys. Chem. C 112 (2008) 11345. [16] J.H. Yao, K.R. Eleder, H. Gao, M. Grant, Phys. Rev. B 45 (1992) 8173. [17] G.N. Sastry, S. Shaik, J. Am. Chem. Soc. 120 (1998) 2131.
[18] [19] [20] [21]
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G. Cao, Nanostructures and Nanomaterials, Imperial College Press, London, 2004. P. Reiss, M. Protiere, L. Li, Small 5 (2009) 154. M. Sharma, T. Jain, S. Singh, O.P. Pandey, AIP Adv. 2 (2013) 012183. R.M. Silverstein, R.C. Basslwe, T.C. Morrill, Infrared Spectroscopy, fifth ed., John Wiley & Sons, Singapore, 1991. [22] S.M. Karvinen, Ind. Eng. Chem. Res. 42 (2003) 1035. [23] F.L. Pedrotti, L.M. Pedrotti, S.L. Pedrotti, Introduction to Optics, third ed., Addison-Wesley, America, 2006. [24] G. Sward, Paint and Coating Testing Manual, fourteenth ed., ASTM Manual Series, Philadelphia, 1995.
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Diffuse reflectance and reflective flexible coatings of capped ZnS nanoparticles Sanjeev Kumar a, b, c, *, N.K. Verma b, M.L. Singla c a
Sri Guru Granth Sahib World University, Fatehgarh Sahib 140 406, India Nano Research Lab, School of Physics and Materials Science, Thapar University, Patiala 147 004, India c Central Scientific Instruments Organisation, Chandigarh 160 030, India b
h i g h l i g h t s Diffuse reflectance of synthesised uncapped and thioglycerol capped ZnS nanoparticles has been studied. The capped nanoparticles showed maximum diffuse reflectance. Using this property and adjusting pH, binder ratio, etc., the reflective flexible coatings were made.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 May 2011 Received in revised form 20 August 2013 Accepted 21 August 2013
Thioglycerol capped ZnS nanoparticles were synthesized by chemical precipitation method and the effect of capping concentration on diffuse reflectance was studied. The comparative study of diffuse reflectance of commercial bulk ZnS particles with synthesised uncapped and capped nanoparticles has been made. ZnS nanoparticles capped with 0.50% of thioglycerol were found to be of uniform shape and size, with maximum diffuse reflectance of 93e94% in the visible region. The reflectors developed using these nanoparticles showed reflectance of 96.5e97.5% in the visible region with optimum coating thickness of 0.25 mm. These reflectors have also been found to be flexible. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Chemical synthesis Optical properties Coating Visible and ultraviolet spectrometers
1. Introduction ZnS is a II-VI semiconductor with direct band gap, 3.68 eV, and refractive index, 2.37 [1,2]. It exists in two phases, zinc blende and wurtzite; zinc blende phase is thermodynamically stable at room temperature. ZnS finds wide applications in areas such as optical devices, photo-catalyst, bio-labelling [2,3]. Apart from these applications, ZnS can be used as a reflective pigment, due to wide band gap and high refractive index. In industry, bulk ZnS is prepared by reacting its respective mineral ores. The so obtained ZnS particles are non-uniform in shape and size, and are absorptive in the visible region [4]. ZnS can be used as reflective pigment provided it scatters incident light rather than absorbing it, in the visible region. The size, shape and
* Corresponding author. Nano Research Lab, School of Physics and Materials Science, Thapar University, Patiala 147 004, India. Tel.: þ91 9653994114; fax: þ91 0175 2364498. E-mail addresses: [email protected], [email protected] (S. Kumar). 0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2013.08.032
uniformity of particles play a crucial role in determining their reflective property; these parameters can be optimized by reducing their size to nanoscale (particle size <100 nm) [5]. Nanoparticles (NPs) have attracted widespread attention because of their unique optical and electronic properties, arising due to quantum confinement and large surface area, which is not observed in their bulk counterpart [6,7]. Controlling the reaction parameters viz. molar concentration of precursors, nature of solvent, reaction temperature and capping/stabilizing agent, NPs of different sizes and shapes can be synthesised [5]. ZnS NPs capped with cetyltrimethylammonium bromide, thioglycerol, ethylene glycol, methacrylic acid, polyvinyl pyrrolidone have been prepared, respectively, using microemulsion [8], hydrothermal [9], solvothermal/hydrothermal [10], solegel [11], chemical vapour deposition [12] methods. These methods have their own drawbacks such as low yield, time consuming, secondary phase formation and expensive. So, these methods cannot be applied in industry for bulk production of ZnS pigments. However, the chemical precipitation method is cost effective, less time consuming and has high yield.
S. Kumar et al. / Materials Chemistry and Physics 142 (2013) 734e739
735
In this paper, authors have synthesised uncapped and capped ZnS NPs using chemical precipitation method, and compared their diffuse reflectance with that of commercially available ZnS bulk particles. The capped NPs have also been used to prepare reflective flexible coatings. To the best of our knowledge, no work, till date, in the area of diffuse reflectance of capped ZnS NPs, has been reported. 2. Experimental All the chemicals and reagents used were of analytical reagent grade, and were used without any further purification. The synthesis has been carried out by chemical precipitation technique [1,13]. Fig. 1. Schematic illustration of synthesised ZnS NPs capped with TG.
2.1. Synthesis of ZnS nanoparticles The aqueous solution of sodium sulphide (0.5 M) was added drop-wise into zinc acetate (0.5 M) solution with constant stirring. Different concentrations by volume (0e1.00%) of thioglycerol (TG), as a surfactant, were added to cap ZnS particles. The precipitates of ZnS, capped with TG, were washed and dried. The NPs, so obtained, were crushed to powder form. 2.2. Preparation of coating material and reflectors The coating material was prepared by dispersing ZnS NPs and acrylic binder in de-ionized water, using a planetary ball mill, for 5 min at 1000 rpm. For the preparation of coating material, acrylic has been chosen as a binding material. Acrylic has gained a strong foothold in the coatings due to its higher flexibility, easy adhesion and resistance to ultraviolet degradation as compared to the other emulsions and latex. Acrylic binder used in the preparation of coating is of non-convertible type. These are polymerized binders which are dispersed in a medium. The medium evaporates after the coating has been applied to leave a coherent film on the substrate surface. To maintain stable pigment dispersion, pH of the coating material was adjusted at 6.5. Plastic sheet substrate with area 2 cm2 and thickness 1 mm was cleaned [14]. These sheets were then spray coated with the coating material to different thicknesses. For each spraying application, the surface was sprayed until it had a glossy appearance, without running the coating material. The coating was air dried and again sprayed before the glossiness disappeared. By adjusting pigment to solvent ratio, i.e., viscosity, air pressure of spraying gun and nozzle hole size, the thickness of the film was controlled.
The phase purity and crystal structure of the sample was determined by PANanalyticals X’Perto Pro X-ray diffractometer with CuKa (l ¼ 1.542 A) as a radiation source. The size and morphology of NPs was studied on Hitachi (H-7500) transmission electron microscope (TEM). The optical absorption spectra were recorded using Perkin Elmer 350 UVevis spectrophotometer to calculate the band gap. The samples (nanoparticles) were dispersed in a solvent using ultrasonicator. The solvent used was chemically inert and did not lead to any agglomeration of the particles. The final measurement was made on the sample contained in a cuvette. The variation of absorption with the wavelength was obtained from the absorbance vs. wavelength plot. The optical band gap has been determined from this absorbance against wavelength plot using the equation (1) [15].
hc ƛint
3. Results and discussion 3.1. Reaction mechanism
2.3. Characterization
Eg ¼
to the intersection of extension of linear parts of the spectrum of yaxis and x-axis. Surface passivation by surfactant molecules has been carried out using FTIR spectra with Perkin Elmer Spectrum BX II spectrometer. Measurement of diffuse reflectance: Reflectance spectra provide similar and complementary information to the absorption measurements. Diffusion reflectance is generally used for unpolished or powdered samples. For diffuse reflectance measurements, an integrating sphere (a sphere with a fully reflective inner surface) is used. The sphere has a pinhole through which the light enters and is transmitted towards the sample. The diffuse reflected light reaches the detector after suffering multiple reflections in the inner surface of the sphere. There is no standard method of sample preparation in diffuse reflectance spectroscopy. Finely grounded powder is carefully packed into a sample holder with a circular hole having surface of a few tens square millimeters. No special care is exercised when the holder is placed horizontally at the bottom of the integrating sphere. In diffuse reflectance measurements, the sample is placed at a port opening opposite to the entrance port. The incident flux is reflected by the sample. The total hemispherical diffuse reflectance is collected by integrating sphere and graph of diffuse reflectance vs. wavelength is plotted. The same set up was used to measure diffuse reflectance of the developed reflectors. Calibration of the reflectance scale was done by standard reference material (WS-1-SL, Spectralon). Measurement of film thickness has been done using ElekrtoPhysik MiniTest 720.
Zinc acetate and sodium sulphide dissociate into their respective ions when dissolved in de-ionized water. Nucleation growth of
(1)
where, h is Plank’s constant (4.135 106 eV nm); c, the velocity of light (3 108 m s1), and ƛint , the wavelength (nm) corresponding
Fig. 2. X-ray diffraction patterns of ZnS NPs (a) uncapped, and TG capped, (b) 0.25%, (c) 0.50%, (d) 0.75%, (e) 1.00%.
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S. Kumar et al. / Materials Chemistry and Physics 142 (2013) 734e739
Table 1 NP-crystallite size and band gap variation with capping concentrations. Sample name
Sample/capping concentration by volume
Crystallite size (nm)
Band gap energy (eV)
(a) (b) (c) (d) (e)
ZnS uncapped ZnS: 0.25% ZnS: 0.50% ZnS: 0.75% ZnS: 1.00%
17.20 16.95 15.85 16.66 17.05
3.64 3.81 4.16 3.82 4.05
ZnS NPs starts due to reaction between Zn2þ and S2 ions. The growth mechanism can be explained on the basis of Ostwald ripening [16]. The chemical potential of the smaller NPs is higher as that of the larger ones. Due to this, the large NPs grow at the expense of the smaller ones. To avoid further growth, NPs are capped with TG which acts as a barrier by inducing steric hindrance [17], and consequently uniform shape, size, and stable NPs were obtained. The schematic illustration for the synthesis of ZnS NPs capped with TG has been shown in Fig. 1. Following is the reaction mechanism for the synthesis process:
nðC4 H6 O4 Zn$2H2 OÞ þ nðNa2 SÞ/ðZnSÞn TG þ ðZnSÞn /TGðZnSÞn
Fig. 4. FTIR spectra of ZnS NPs (a) uncapped, and TG capped, (b) 0.25%, (c) 0.50%, (d) 0.75%, (e) 1.00%.
3.2. Phase and structural analysis Fig. 2 shows the XRD patterns of synthesised uncapped and capped ZnS NPs. These NPs have zinc blende crystal structure, with three broad diffraction peaks, corresponding to (111), (220), and (311) planes of the cubic ZnS (JCPDS card No. 050566). The broadening in the diffraction peaks is due to size and strain effect. Ignoring the lattice strain in the nanocrystallites, owing to
Fig. 3. TEM micrographs of ZnS NPs (a) uncapped, and TG capped, (b) 0.50%, (c) 0.50% with higher resolution, (d) 1.00%.
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their negligible value (0.07), the crystallite size of ZnS NPs has been calculated using Debye Scherrer’s equation [18]. The crystallite size has been determined from the full width at half maximum (FWHM) of the most intense peak (Fig. 1), shown in Table 1. FWHM has been calculated using Lorentz best fit curve (R2 >0.98) method. No peak related to other material is observed in the diffraction pattern, which confirms the purity of the ZnS particles. 3.3. Morphology Fig. 3(a)e(d) shows TEM micrographs of uncapped, 0.50%, and 1.00% of TG capped ZnS NPs. It reveals that the uncapped particles (Fig. 3(a)) get aggregated and coalesce non-uniformly due to their high surface energy in the absence of surfactant. But surface passivation of the ZnS NPs with 0.50% TG (Fig. 3(b) and (c)) leads to spherical symmetry. The NPs, so obtained, were nearly spherical with size 20e 28 nm (core size, 15e20 nm and shell (TG layer) thickness, 5e8 nm). Such core/shell type structure changes the optical properties of the materials [19]. Further increase in surfactant concentration resulted in agglomeration of the ZnS NPs (Fig. 3(d)). Above a certain concentration, self-interaction in-between molecules of capping agent occurred. Due to this, partial passivation of zinc surface took place which led to agglomeration of ZnS particles [20].
Fig. 5. UVevis absorption spectra of ZnS NPs (a) uncapped, and TG capped, (b) 0.25%, (c) 0.50%, (d) 0.75%, (e) 1.00%.
3.4. Fourier transform infrared (FTIR) analysis ZnS NPs were mixed with KBr to prepare pellets. Fig. 4 shows the FTIR spectra of uncapped and capped ZnS NPs. A broad intermolecular hydrogen bond OeH stretch, found around 3398 cm1 (curves ‘a’ to ‘e’) is due to the hygroscopic nature of KBr. The absorption band, 3520e3200 cm1, is due to the free hydroxyl group. A weak band appearing around 2350 cm1 is because of the presence of the SeH group [21]. The peak around 1615 cm1 in uncapped ZnS is due to pure water, HeOeH vibration, showing partial evaporation of water in the sample [21]. A high absorption peak near 1590 cm1 might be due to the presence of acetate group of the zinc precursor. The group of peaks around 1000 cm1 is indicative of the sulphide group [21]. The band, 400e600 cm1, is because of the SeS bond stretching in capped NPs. The SeH stretching peak appeared around 2350 cm1; this being very weak, so the corresponding bonds break. This leads to formation of SeS bonds in ZnS capped with TG, giving absorption band in the 400e 600 cm1 region.
3.5.2. Diffuse reflectance analysis The diffuse reflectance spectra, both for commercially available bulk ZnS particles and synthesized uncapped as well as capped ZnS NPs are shown in Fig. 6. The diffuse reflectance of capped ZnS NPs (93e94%) in comparison to the uncapped ZnS NPs (91e92%) and bulk ZnS particles (82e70%), has been found to be higher in the visible region. Table 2 shows diffuse reflectance of the uncapped and capped ZnS NPs for different TG concentrations as well as that of the bulk ZnS particles. On increasing the capping concentration from 0 to 0.50%, the diffuse reflectance increases, whereas, it decreases on further increase in concentration from 0.50 to 1.00%. These results show attainment of maximum diffuse reflectance for 0.50% TG capped ZnS NPs. The uncapped ZnS NPs form a powdered layer, which acts as a thin film. The light incident on it undergoes destructive interference, consequently decreasing the reflected light intensity [23]. On
3.5. Optical analysis 3.5.1. UVevis absorption analysis The optical absorption spectra of ZnS NPs (Fig. 5) show blueshift in the absorption edges for ZnS NPs vis-a-vis the bulk ZnS particles (Eg ¼ 3.68 eV). The band gap energy values of uncapped and capped NPs (Table 1) were also found to be higher than those of the bulk ZnS particles, as calculated from the UVevisible absorption spectra (Fig. 5) [13]. With increase in the capping concentration, 0e 0.50%, the energy band gap increased. But, for further increase in capping concentration, 0.75e1.00%, the energy band gap decreased. This blue-shift is due to the quantum confinement effects [13]. Band gap plays an important role in the reflective properties of the pigments. According to the band theory [22], a substance (pigment) with a large band gap (>3 eV) does not absorb visible light [23]. Such pigments transmit the entire incident light and are, therefore, colourless in their pure form. However, in powder form, light is reflected back and so they appear white. Thus, ‘large band gap substance’ is an excellent reflecting pigment for visible light.
Fig. 6. Diffuse reflectance spectra of ZnS particles.
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Table 2 Diffuse reflectance of ZnS particles. Sr. No.
1 2 3 4 5 6
Capping concentration
0.0 (uncapped) 0.25% ml 0.50% ml 0.75% ml 1.00% ml Bulk
% Reflectance at wavelength 400 nm
450 nm
500 nm
550 nm
600 nm
650 nm
700 nm
91.42 91.57 93.43 91.94 92.52 81.68
92.17 92.29 94.18 92.65 93.24 88.94
92.04 92.20 94.06 92.58 93.15 90.29
91.98 92.13 94 92.51 93.09 90.76
91.97 92.11 93.98 92.49 93.07 90.23
91.92 92.08 93.93 92.45 93.03 83.55
91.92 92.07 94.94 92.44 93.02 69.12
capping ZnS NPs (refractive index (n) ¼ 2.37) with TG (n ¼ 1.52), the coating behaves like a homogeneous and isotropic film because of uniform shape and size of the particles (Fig. 3(b) and (c)) [22]. Consequently, this results in specular reflections, and the reflected light interferes constructively, resulting in increase in the intensity of the reflected light with capping concentration (0e0.50%). With further increase in the capping concentration from 0.50 to 1.00%, the shape and size of the particles no longer remain uniform because of agglomeration (Fig. 3(d)). Therefore, the capping (surface passivation) results in uneven surface, thereby decreasing reflectance. The reflectance of bulk ZnS particles decreases sharply after 625 nm (Fig. 6), it has been reported earlier as well [4]. Therefore, the bulk ZnS particles cannot be used as reflective pigment for the visible region. From the current diffuse reflectance study, it is concluded that the ZnS NPs possess stable and high diffuse reflectance as compared to that of the bulk ZnS particles. 3.5.3. Diffuse reflectance analysis of coating and their flexibility ZnS NPs capped with 0.50% TG show maximum diffuse reflectance (Fig. 6) and therefore, have been employed as reflective pigment for the preparation of coating material. Such a coating material has been applied to plastic sheets to develop reflectors. Fig. 7 shows the diffuse reflectance spectra of reflectors having different coating thicknesses. It reveals that with increase in coating thickness, the reflectance increases. For coating thickness, 0.15, 0.20, and 0.25 mm, the diffuse reflectance has been found to be, respectively, 93.5e94.5%, 95.5e96.5%, and 96.5e97.5%. Further increase in coating thickness from 0.25 to 0.30 mm, resulted in undesired change in reflectance. The coating thickness, 0.25 mm, is
the optimum coating thickness. At the optimum coating thickness, the critical volume pigment concentration (above it no substantial difference in the behaviour and appearance of the coating is observed) plays an important role in determining reflectance. Flexibility of the coating has been determined to ensure the durability of the reflector. Coating must be flexible enough so as not to develop crack or split with shrinkage of substrate due change in temperature [24]. A simple method to check the flexibility of the coating has been applied by changing the working temperature from 30 to 45 C, which showed, respectively, 5.0% and 10.5% decrease in the sheet surface area without any crack or split being developed in the coating. Thus, the developed reflectors have been found to be flexible. 4. Conclusions The comparative study on diffuse reflectance of bulk ZnS particles vis-a-vis synthesised uncapped and capped ZnS NPs, with different capping concentrations of TG, show that capped ZnS NPs are more stable and are highly reflective in visible region. This study establishes that ZnS NPs capped with 0.50% TG are uniform in shape, small in size and large in band gap; they form an excellent reflective pigment in visible light. These NPs show maximum diffuse reflectance (93e94%) in visible region. The reflectors of different coating thicknesses (0.15e0.30 mm) have been developed using these NPs on a plastic substrate. It has been found that the reflector with coating thickness, 0.25 mm, possesses maximum reflectance, 96.5e97.5%. The reflectors with 0.25 mm thick coating have been found to be flexible. These reflective flexible coatings may be potential candidates for applications in areas demanding reflector flexibility. Acknowledgement Sanjeev Kumar is extremely grateful to the Department of Science and Technology, Government of India, for awarding him the INSPIRE fellowship to carry out the current research work. The constructive discussions with Mr. Harvinder Kumar, Silky Paint Industries are also acknowledged. References
Fig. 7. Diffuse reflectance spectra of reflectors.
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