Applied Surface Science 270 (2013) 33– 38 Contents lists available at SciVerse ScienceDirect Applied Surface Science j our nal ho me p age: www.elsevier.com/loc ate/apsusc Evaluation of shape and size effects on optical properties of ZnO pigment Narges Kiomarsipour a,∗ , Reza Shoja Razavi a , Kamal Ghani b , Marjan Kioumarsipour c a Department of Materials Engineering, Malek Ashtar University of Technology, Shahin Shahr P.O. Box 83145/115, Isfahan, Iran b Department of Chemistry, Malek Ashtar University of Technology, Shahin Shahr P.O. Box 83145/115, Isfahan, Iran c Department of Physics, University of Kashan, Kashan, Iran a r t i c l e i n f o Article history: Received 16 July 2012 Received in revised form 30 November 2012 Accepted 30 November 2012 Available online 10 December 2012 Keywords: Zinc oxide pigment Light scattering efficiency Optical property Hydrothermal method a b s t r a c t The pigment with optimized morphology would attain maximum diffuse solar reflectance at a lower film thickness and reduce the pigment volume concentration required. This factor would contribute to a reduction in overall weight and possibly extend the durability of the system to longer time scales, specially in space assets. In the present work, five different morphologies of ZnO pigment were synthesized by hydrothermal method. The ZnO pigments were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM) and N 2 adsorption (BET). The optical property of the ZnO pigments was investigated by UV/VIS/NIR spectrophotometer. The results indicated that the optical properties of ZnO powders were strongly affected by the particle size and morphology. The nanorods and microrods ZnO structures showed the minimum spectral reflectance in visible and near infrared regions, whereas the novel nanoparticle-decorated ZnO pigment revealed the maximum spectral reflectance in the same regions. The reflectance spectra of scale-like and submicro- rods ZnO were in the middle of the others. The higher surface roughness led to higher light scattering in nanoparticle-decorated ZnO pigment and multiple-scattering in them. These results proved that a signif- icant improvement in the scattering efficiency of ZnO pigment can be realized by utilizing an optimized nanoparticle-decorated pigment. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Zinc oxide is used in the manufacture of paints, rubber prod- ucts, cosmetics, pharmaceuticals, floor coverings, plastics, printing inks, soap, storage batteries, textiles and electrical equipment. Addition of pigments to coatings is a common industrial practice. Pigments not only provide esthetics to the coatings but also help in improving many properties of the coatings such as UV resistance, corrosion resistance and mechanical properties like scratch and abrasion [1]. Direct fabrication of special structures with controlled crystalline morphology represents significant challenge in various fields, because it can provide a better model for investigating the dependence of electronic and optical properties on the size confine- ment and dimensionality [2–5]. Various ZnO structures including nanobrushs [6], nanowires [7], nanobowls [8] and nanopellets [9] have been produced. They are widely used in many important areas, such as solar cells [10], gas sensors [11], electronics [12] and photo- catalysts [13]. One of the most important applications of ZnO is in ∗ Corresponding author. Tel.: +98 312 5225041; fax: +98 312 5228530. E-mail address: na.kiomarsipour@yahoo.com (N. Kiomarsipour). the paint industry as pigment. The ZnO is a white powder that usu- ally used as pigment and by volume is the second most significant white pigment [14]. There are several potential benefits to optimize the scatter- ing efficiency of the ZnO pigment through control of particle size. An ideal coating design would obtain the theoretical maximum reflectance (i.e. opacity) with the lowest pigment volume concen- tration (PVC) and dry film thickness (DFT). Any additional pigment does not contribute to scattering and is detrimental to the physi- cal properties of the film [15]. The surface texture of a scattering particle, in addition to the overall particle geometric shape, is also an important morphological factor in determining the opti- cal properties of the scatter. In the past two decades, the effect of asphericity of a particle on its single-scattering parameters (e.g., phase function and cross section) has been extensively investi- gated. However, only a handful of studies have investigated the effect of surface texture or roughness on particle optical properties [16,17]. The present paper is focused on the development of a novel morphology of ZnO pigment that can potentially raise the scatter- ing efficiency of ZnO pigment. In the present work, five different morphologies of ZnO were synthesized by hydrothermal method and then the morphology effects on the spectral reflectance were studied. 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.11.167 34 N. Kiomarsipour et al. / Applied Surface Science 270 (2013) 33– 38 Table 1 Synthesis conditions of hydrothermally synthesized ZnO pigments. Morphology [Zinc solution] a [Hydroxide solution] a pH of final solution Temprature ( ◦ C) Time (h) Scale-like 1.5 2.0 11 160 18 Submicrorods 0.5 1.5 12 150 20 Microrods 0.5 1.5 12 180 20 nanorods 0.5 1.5 12 120 20 Decorated-nanoparticles 1.0 4.0 12.5 170 14 a The concentration of zinc nitrate and potassium hydroxide solutions (Molar). 2. Experimental 2.1. Preparation of pigments ZnO samples were prepared by hydrothermal method and used for the evaluation of morphology effects of ZnO pigments on their optical properties. The synthesis conditions were according to our previous works [18,19]. Typical synthesis conditions of ZnO pow- ders synthesized in this work, were summarized in Table 1. All of the samples were prepared as follows: The zinc nitrate aqueous solution was prepared by adding appropriate amount of Zn(NO 3 ) 2 ·6H 2 O (Reagent Grade, 98% Sigma–Aldrich) to distilled water. The pH of solution increased by adding dropwise a solution of KOH (appropriate amount of KOH added in distilled water) and stirring vigorously for 10 min at room temperature. Then the resulting slurry mixture was transferred into a 100 mL Teflon-lined stainless steel autoclave up to 80% of the total volume. Hydrothermal reaction was conducted in an oven. After the reaction was completed, the autoclave was cooled very slowly to room temprature and the final product was collected by pressure filtration. Powdered sample was thoroughly washed with distilled water and then dried in air at 120 ◦ C for 12 h. 2.2. Characterization of pigments Crystal structure of as-prepared products was characterized by powder X-ray diffraction (XRD) on a Bruker D8 Advance X-ray diffractometer using Cu-K␣ radiation (40 kV, 40 mA and  = 0.1541 nm). XRD patterns were recorded from 0 ◦ to 90 ◦ with a scanning step of 0.02 ◦ /s. Morphology and size of the samples were analyzed by Hitachi S-4160 Field Emission Scanning Elec- tron Microscopy (FE-SEM) at an accelerating voltage of 15 kV. The diffuse reflectance spectra of prepared powders were mea- sured by JASCO V-670 UV–vis Spectrophotometer (in the range of 220–2200 nm). The specific surface areas were measured by Brunauer–Enmet–Teller (BET) method employing N 2 adsorption at 77 K after treating the sample at 170 ◦ C and 10 −4 Pa for 2 h, using a Tristar-3000 apparatus. Atomic force microscopy (AFM) mea- surements for ascertaining the surface roughness of the scale-like and nanoparticle-decorated pigments were performed by atomic force spectroscope (AFM; DME Dualscope DS-95-200-E; 0.12 nN, 30 mm/s). 3. Results and discussion The typical XRD patterns of the products are shown in Fig. 1. All of the diffraction peaks can be indexed as hexagonal wurtzite ZnO with cell constants a = 3.2490 ˚ AÅ and c = 5.2050 ˚ AÅ for prod- ucts, in good agreement with the reported data for ZnO (JCPDS File, 00-005-0664). The very sharp diffraction peaks indicated the good crystallinity of the prepared crystals and no characteristic peaks were detected from any other impurities such as Zn(NO 3 ) 2 ·6H 2 O and Zn(OH) 2 . Fig. 2(a–k) shows the low-magnification and high-magnification FE-SEM images of the five corresponding obtained pigments: scal-like, submicrorods, microrods, nanorods and nanoparticle- decorated ZnO. In Fig. 2, it can be seen that the morphology of ZnO pigments is greatly affected by the hydrothermal conditions. Fig. 2(a) and (b), shows the scale-like ZnO, which all of the scales have fairly uniform diameters about 300 nm and thicknesses of 50 nm. In Fig. 2(c) and (d), FE-SEM images of the submicrorods ZnO is shown. The diameters of ZnO rods are about 100 nm and their lenghts are up to 2 ␮m. The FE-SEM images of the microrods ZnO pigment are shown in Fig. 2(e) and (f). The lower magnifica- tion Image 2(e) indicates that the highly dispersed microrod ZnO structures have diameters of about 200 nm and lengths of 3 ␮m. The FE-SEM images of nanorod ZnO pigment are shown in Fig. 2(g) and (h). It is found that the product is composed of well-dispersed Fig. 1. XRD patterns of the as-synthesized ZnO pigments. N. Kiomarsipour et al. / Applied Surface Science 270 (2013) 33– 38 35 crystals on a large scale, and all of the nanorods have diameters of about 50 nm and lengths of 300 nm. Fig. 2 (i–k) exhibits the FE- SEM images of nanoparticle-decorated ZnO pigment. The surface of ZnO pigment was decorated by nanoparticles with diameters of approximately 20–50 nm. The decorated surface of pigment was led to increase specific surface area and surface roughness of particles and consequently, increase the light scattering effi- ciency [16,20]. N 2 adsorption–desorption results further confirmed Fig. 2. Low- and high-magnification FE-SEM images of ZnO structures: (a, b) Scale-like, (c, d) submicrorods, (e, f) microrods, (g, h) nanorods and (i, j, k) nanoparticle-decorated ZnO. 36 N. Kiomarsipour et al. / Applied Surface Science 270 (2013) 33– 38 Fig. 2. (Continued). the porous surface feature of the nanoparticle-decorated ZnO pig- ment. The representative isotherm of the ZnO pigment is shown in Fig. 3. The N 2 isotherm plot belongs to the type IV isotherm in the Brunauer classification [21]. A hysteresis loop observed at higher relative pressures (P/P 0 = 0.5–0.9) is associated with the filling and emptying of mesoporous (10–30 nm in diameter) by capillary con- densation. According to the IUPAC classifications, the observed loop 0 20 40 60 0 0.5 1 Va/cm3(STP) g-1 p/p 0 ADS DES Fig. 3. Typical N 2 adsorption–desorption isothermal of the nanoparticle-decorated ZnO pigment. indicates the H 3 type, implying the presence of mesoporous [21]. The BET surface area of the nanoparticle-decorated ZnO pigment was calculated to be only 6.3712 m 2 /g. The surface of the particles was decorated by nanoparticles with diameters of approximately 20–50 nm, on which mesoporous with diameter ranging from 10 to 30 nm were formed among nanoparticles simultaneously. The experimentally measured specular-included UV/VIS/NIR reflectance spectra of ZnO pigments are shown in Fig. 4. It can be seen that the optical property was greatly affected by morphology and particle size of pigments. The particle size of the pigment plays an important role in light scattering efficiency [15,22,23]. Wave theories of light account for bending phenomena which give rise to scattering. Light scattering by this mechanism increases with decreasing particle size until an optimum is reached at approximately half the wavelenght of light. Optimal particle size for scattering can be calculated, and also depends on wavelength [24]. The scattering efficiency is extremely dependent upon the par- ticle size distribution of the pigment. It is well known from Mie scattering theory that as the pigment particle diameter approaches approximately one-half that of the incident radiation wavelength, scattering is increased dramatically. The results of the modeling indicated that an optimum particle size of distribution would be between between 0.25 and 0.45 ␮m, and particles greater than 1.5 ␮m provide very little Mie scattering contributions [22,23]. It is quite evident from the contour plot that ZnO particle diameter greater than approximately 1.5 ␮m do not scatter light efficiency at any wavelength [15]. When particle sizes are large (greater than 1.5 ␮m), reflectance is relatively low. As particle size decreases, reflectance increases as the number of first-surface reflections and the amount of multiple scattering increases. Further decreases in particle size result in an improvement of the reflectance. As parti- cle size decreases such that the size is less than the wavelength, the particle as a whole interacts with a wavelength of light [25]. N. Kiomarsipour et al. / Applied Surface Science 270 (2013) 33– 38 37 0 20 40 60 80 100 0 50 0 100 0 150 0 200 0 250 0 Reflectance (%) Wavelenght (nm) Nanop article-decorated Z nO Scale-li ke ZnO Submic rorod s ZnO Microrods ZnO Nanorods Z nO Fig. 4. Experimental specular-included UV/VIS/NIR reflectance spectra of as-synthesized ZnO pigments. From Fig. 4, it can be observed that the spectral reflectances of nanorod and microrod ZnO pigments are lower than the others. Because of the particle sizes of nanorod and microrod ZnO pig- ments are smaller and greater than the optimal particle size range of 0.25–1.5 ␮m, respectively, hence do not scatter light efficienty at any wavelength [15]. The observed high light absorption in UV region (at wavelengths below 366 nm) indicated in Fig. 4 is due to ZnO band gap and this phenomenon is one of the most important ZnO characteristics [14]. The particle sizes of scale-like and submi- crorod ZnO pigments are in the optimum range of 0.25–0.45 ␮m, and their UV/VIS/NIR reflectance spectra have the suitable amount for pigmentation applications. Consequently, the decorated surface of new ZnO pigment led to higher light reflectance. The decorated surface with nanoparticles led to increasing of special surface area and consequently increasing the light scattering. The presence of nanoparticles on the surface of pigment caused increase surface roughness and more chance for light to be refracted. The higher reflectance of new morphology also can be attributed to more dif- ference between the refractive indices ZnO and air-voids in pigment mesoporous structure (between in particles). The nanoparticle- decorated ZnO pigments similar to particles containing air voids led to higher reflectance index due to much difference between the refractive indices of ZnO and air voids and hence increased the light scattering [26]. The AFM images of scale-like and nanoparticle-decorated ZnO pigments are presented in Fig. 5. As can be seen in this figure (Fig. 5), the surface roughness of nanoparticle-decorated ZnO is higher and more uniform than scale-like ZnO. The scattering of light by small particles is determined not only by the composition of the incident light and the optical properties of the particles and the medium but also by the size, shape, con- centration, surface roughness, spatial arrangement of the particles, etc. Many studies had shown that surface roughness can indeed play an important role on the light scattering pattern under certain conditions [27–30]. The initial theoretical studies and subsequent experimental studies of multiple-scattering effects were carried out for randomly rough surfaces characterized by rms heights of the order of 5–10 nm, and transverse correlation lengths of the order of 100 nm, i.e. surfaces with nanoscale roughness. Subse- quent experimental and theoretical work was devoted to the study of surfaces that were significantly rougher than these, e.g. surfaces with microscale roughness. This is because some of the methods Fig. 5. AFM images of: (a) nanoparticle-decorated and (b) scale-like ZnO pigments. 38 N. Kiomarsipour et al. / Applied Surface Science 270 (2013) 33– 38 developed for treating scattering from surfaces with this larger scale of roughness, especially computational methods, can also be used in the study of scattering from surfaces with nanoscale roughness, and some of the results obtained in studies of sur- faces with the larger scale roughness also apply to surfaces with nanoscale roughness [31]. Another study indicated that a good correlation was between the surface roughness from AFM and opti- cal reflection measurements. In addition, angle-resolved reflection measurements gave an account on the decrease in optical scat- tering after polishing the sample surface. In this case, the angular reflection distribution is similar to that of a thin sample with low surface roughness and shows that the measured optical scattering is mainly determined by the surface roughness [32]. On the other hand, it is evident that increasing surface roughness strongly affects the scattering properties of ice particles [33]. The higher surface roughness of nanoparticle-decorated ZnO led to multiple scatter- ing and increasing of the reflectance. Presence of nanoparticles on the pigment surface enhanced the multiple scattering for whole of wavelength range [34,35]. 4. Conclusions Well-dispersed five different morphologies of ZnO pigment was synthesized by a simple hydrothermal method. Evaluation of their optical properties indicated that the spectral reflectance was strongly affected by particle shape and size. Nanorod and microrod ZnO pigments had shown the lower reflectance spectra, because their particle size were not in the optimum range and they do not scatter light efficiently at any wavelength. Scale-like and sub- microrod ZnO pigments had shown mean reflectance. 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Dékány, Hydrothermal synthesis of prism-like and flower-like ZnO and indium-doped ZnO structures, Colloids and Surfaces A: Physicochemical and Engineering Aspects 340 (2009) 1–9. . hydrothermal method and used for the evaluation of morphology effects of ZnO pigments on their optical properties. The synthesis conditions were according . to ZnO band gap and this phenomenon is one of the most important ZnO characteristics [14]. The particle sizes of scale-like and submi- crorod ZnO pigments . the composition of the incident light and the optical properties of the particles and the medium but also by the size, shape, con- centration, surface