Abstract
The highly dispersed nanostructured NiTiO3 pigments and NiTiO3/TiO2 composite pigments can be synthesized at relative low temperature. The activation energy of crystal growth of NiTiO3 during calcinations via salt-assistant combustion method is 9.35 kJ/mol. The UV-vis spectra results revealed that the absorbance decreased with the increasing of calcinations temperature due to small size effect of nanometer particles. The optical data of NiTiO3 nanocrystals were analyzed at the near-absorption edge. SEM showed that the obtained NiTiO3 nanocrystals and NiTiO3/TiO2 nanocomposite were composed of highly dispersed spherical-like and spherical particles with uniform size distribution, respectively. The chromatic properties and diffuse reflectance of samples were investigated. The obtained NiTiO3/TiO2 composite samples have higher NIR reflectance than NiTiO3 pigments.
1. Introduction
The temperature inside buildings can be potentially raised due to intense solar radiation to exterior surfaces. Rising energy cost drives people to explore new technologies designed to improve energy efficiency across the globe [1]. Recently, more and more interest has attended roofing materials with high NIR reflectance [2, 3]. The high NIR pigments as “cool” coating can help in maintaining lower exterior surface temperature of building; as a result, the indoor thermal comfort levels in hot season are improved. In recent years, many inorganic pigments have been extensively used as cool materials for building roofs and facades [4]. TiO2, a white pigment with a high solar reflectance of about 87%, is regarded as the best pigment for coating materials, but its application is restricted because it easily causes “white pollution” [5]. There is an interest to develop new environment-friendly and sustainable “colored” pigment without toxic elements such as Pb, Hg, Cd, Sb, As, and Cr [6]. To date, special attentions have been paid to perovskite-type NiTiO3, a traditional yellow-colored pigment, as chemical and electrical materials [7, 8]. In the structure of NiTiO3, both Ni and Ti atoms prefer octahedral coordination with alternating cation layers occupied by Ni and Ti atoms alone, which gives it good stability [9, 10]. In particular, incorporation of NiTiO3 into TiO2 can be expected to obtain better “colored” pigment with high NIR reflectance.
A study indicated that nanocrystalline pigments had better solar reflectance properties compared with macrocrystalline pigments. It is well known that structure and properties of materials may be tailored by processing control. Recent efforts have focused on tailoring nanopigments by energy-saving method. Traditionally, in commercial processes, nanocrystalline NiTiO3 can be prepared by solid state reaction, coprecipitation, high-energy ball milling process, sol-gel, sol-gel assisted electrospinning, and pyrolysis of polymeric precursor [7, 9, 11, 12]. Although these routes can produce nanosized NiTiO3 particles, the dispersibility of nanoscale particles that has been reported is not so ideal. The salt-assistant self-propagation combustion method has been developed by our group for preparation of pyrochlore-type and spinel-type nanoparticles [13, 14]. In this paper, we presented the fabrication and properties of highly dispersed NiTiO3 and NiTiO3/TiO2 nanoyellow pigments.
2. Experimental
2.1. Preparation of Materials
All reagents were of analytical grade and were used without further purification. In this work, NiTiO3 was synthesized by a salt-assisted self-propagating combustion method (SSCM). Tetra-n-butyl titanate (Ti(C4H9O)4) and nickel nitrate (Ni(NO3)2·6H2O) were used as the precursors of Ti and Ni, respectively. Critic acid was used as fuel. KCl was used as inert salt. Critic acid can not only act as fuel, but also act as a complexing agent with a variety of metal ions. According to the formula NiTiO3, stoichiometric amount of (Ni(NO3)2·6H2O), Ti(C4H9O)4, and KCl were added to critic acid solution in turn. After a series of steps of magnetic force stirring, evaporating, and self-propagating combustion, the loose combustion product was obtained. The fabrication procedure was similar to the literature [13]. The combustion product was ground into powders and then submitted to calcine at 700°C~900°C for 4 h.
NiTiO3/TiO2 nanocomposite was prepared by a sol-gel procedure. A solution of Ti(C4H9O)4 (as TiO2 is 0.5 g) in 15 mL ethanol was dropped into a 100 mL NiTiO3 (2 g, above obtained samples calcined at 700°C) water solution under stirring. The mixture was kept stirring for 5 h, followed by standing at room temperature for 24 h to get opaque gel. The obtained gels were kept at 80°C for 4 h and then dried at 90°C for 2 h. Finally, the obtained samples were calcined at 400°C for 3 h.
2.2. Instrumentation
The crystalline phase structure was determined by Bruker D8 Advance X-ray diffractometer (XRD) using Cu Kα radiation. UV-vis absorption spectrum was carried out by UV-1700 spectrometer. Scanning electron microscopy (SEM) image was recorded on a JSM-7500F scanning electron microscope; EDS was taken on INCAPentaFET-x3 energy dispersive X-ray detector. The CIE 1976 colorimetric method was used, as recommended by the Commission Internationale de l’Eclairage (CIE). In this method, is the lightness axis [black (0) to white (100)], is the green (−value) to red (+value) axis, and is the blue (−value) to yellow (+value) axis. The parameter (chroma) represents saturation of the color. For each colorimetric parameter of a sample, measurements were made in triplicate and an average value was chosen as the result. Typically, for a given sample, the standard deviation of the measured CIE values is less than 0.10, and the relative standard deviation is not higher than 1%, indicating that the measurement error can be ignored. UV-vis-NIR reflectance of the obtained pigments was carried out by UV-vis-NIR spectrophotometer (PerkinElmer Lambda 950), using polytetrafluoroethylene (PTFE) as a white standard.
3. Results and Discussions
3.1. XRD Analysis
Figure 1 shows the XRD patterns of NiTiO3 combustion product, after being ground and calcined at different temperatures for 4 h. All diffraction peaks are in good agreement with the reflection of ilmenite NiTiO3 phase [11]. The peaks are indexed, corresponding to the lattice planes (012), (104), (110), (113), (024), (116), (124), and (300) of cubic system. It is noted that NiTiO3 nanocrystals can be synthesized at 700°C. The lattice constant of samples is obtained by jade 6 program, the average crystal size is determined from the XRD patterns according to the Scherrer equation, and corresponding data are listed in Table 1. It is clear that, with the increasing of calcining temperature, both of the lattice constant and crystal size become larger to some extent. The activation energy for crystal growth is equal to 9.35 kJ/mol by calculation according to the literature [13].
XRD patterns of NiTiO3/TiO2 nanocomposite are shown in Figure 2. It has been illustrated that there exist the characteristic peaks of TiO2 in the NiTiO3/TiO2 composite as anatase phase (JCPDS 73-1764) in comparison with XRD pattern of pure NiTiO3. No other polymorph of TiO2 is observed. Moreover, the intensity of NiTiO3 peaks is significantly lower. It is indicated that to some extent NiTiO3 are coated by TiO2 particles. The results can be further confirmed by SEM and EDS.
3.2. UV-Vis Spectra Characteristic
Figure 3 shows the UV-vis absorption spectra of NiTiO3 precursor calcined at different temperatures (700°C, 750°C, 800°C, 850°C, and 900°C) in ethanol suspension prepared using 20 W/cm2 sonic intensity for 5 mins. It can be seen that, with the increasing of calcinations temperature, the absorbance decreased, which can be attributed to small size effect of nanometer particles.
(a)
(b)
The optical data were analyzed at the near-absorption edge. According to the literature [12], the relation between the band gap of semiconductors for direct transition materials and absorption coefficient satisfied the following equation:where is Photon Energy, is absorption coefficient, is constant in relation with the materials, and is the band gap of the semiconductors. Figure 4(b) shows the plot of versus . The energy intercept gives for a direct transition when the linear region is extrapolated to zero ordinate. The band gap of NiTiO3 precursor calcined at different temperatures (700°C, 750°C, 800°C, 850°C, and 900°C) is calculated to be 2.63 eV, 2.34 eV, 2.17 eV, 2.03 eV, and 2.09 eV, respectively. Generally, due to the quantization size effects, the band gap value decreased with the increasing of crystal size (Table 1). The value is smaller than that reported in [10], 2.92~3.16. As we all know, high-efficiency visible-light-driven semiconductor photocatalysis should have sufficiently narrow band gap with the value of 1.23 eV < < 3.0 eV [15]. On one hand, the band gap with eV is to harvest visible light; on the other hand, large enough band gap with eV is to provide energetic electrons. So the obtained samples can be considered as interesting candidates for use in photocatalysis.
(a)
(b)
3.3. SEM and EDS Analysis
The sample for SEM and EDS is made by the following steps. First, the sample was dispersed in ethanol with oscillating for 20 min in the ultrasonicator at constant temperature. Then the sample was dropped slowly on the silicon chip.
Figure 4(a) gives the SEM images of the NiTiO3 pigments obtained at 700°C. It is clear that the sample is composed of highly dispersed and spherical-like particles and the size distribution is uniform. However, the size of 50~80 nm is larger than the value (46.9 nm) obtained from XRD patterns by the Scherrer formula. This is due to the fact that the crystal size obtained by the XRD is the grain size and that obtained by the SEM is particles’ size.
The morphologies of NiTiO3/TiO2 nanocomposites are shown in Figure 4(b). It can be seen that the uniform and highly dispersed NiTiO3/TiO2 composite particles with spherical shape are ca. 300 nm in diameter. Furthermore, we can see that the surface of sphere is not smooth and uniform but consists of many particles. The morphologies of NiTiO3/TiO2 nanocomposites reveal that NiTiO3 nanoparticles are coated by TiO2 particles.
EDS is used to further confirm the composition of the obtained samples. The EDS analysis of the obtained products (Figure 5(a)) indicates NiTiO3 nanocrystals are composed of titanium, nickel, silicon, and oxygen with an approximate molar ratio of , which gives stoichiometric formula of as-obtained product NiTiO3 with no chemical segregation phenomenon. The Si peak in the spectrum is from the silicon chip for making sample. It can be seen from Figure 5(b) that NiTiO3/TiO2 nanocomposites are composed of plentiful Ti, which can be explained by TiO2 particles forming and covering at the surface of NiTiO3 nanoparticles.
(a)
(b)
3.4. Chromatic Properties and Diffuse Reflectance of Samples
With the temperature increasing, the band gap decreases from 2.63 to 2.03 eV, which can be attributed to the decrease of O-M distance. The color of the pigment samples changes from yellow-green color to dark yellow and then to light yellow (Figure 6).
The chromatic properties of the obtained NiTiO3 pigment samples can be assessed from their CIE 1976 color coordinate values, which are listed in Table 2. It can be seen for NiTiO3 that, with the temperature increasing, the decreasing of value (from 73.0 to 54.4) indicates the enhancing of the darkness of pigments. value of NiTiO3/TiO2 nanocomposites is lower than that of pure NiTiO3 obtained at 700°C. value regularly changes from −6.20 to 6.59, which indicates weakening of the green hue of the pigments. From value, it can be noted that the yellowness of pigments achieves maximum at calcining temperature of 800°C. The values are reasonable and consistent with the results from Figure 6.
The NIR reflectance spectra of the NiTiO3 nanocrystals obtained at different temperatures are given in Figure 7. The NiTiO3/TiO2 composite sample processes higher NIR reflectance of about 89.7% compared with pure NiTiO3 sample of 87.5%. By composite NiTiO3 and TiO2, the pigments have yellow color and higher NIR reflectance. From this, it can be seen that there exists the synergetic effect between NiTiO3 and TiO2. Therefore, the high NIR reflectance (~89.7%) showed by the yellow composite colored pigments will make them interesting candidates for use as cool colorants.
4. Conclusions
Highly dispersed NiTiO3 nanocrystals and NiTiO3/TiO2 composite pigments have been synthesized. All the peaks in XRD patterns were indexed. SEM micrographs revealed that NiTiO3 sample had good dispersibility and uniform size distribution. The morphologies of NiTiO3/TiO2 nanocomposites are shown as uniform and highly dispersed NiTiO3/TiO2 nanoparticles with spherical shape. The optical data of NiTiO3 sample were analyzed at the near-absorption edge. With the temperature increasing, the band gap decreases from 2.63 to 2.03 eV, which can be attributed to the decrease of O-M distance. The color of the pigment samples changes from yellow-green color to dark yellow and then to light yellow. The NiTiO3/TiO2 composite sample processes higher NIR reflectance of about 89.7%. Moreover, the obtained pigments do not encompass any toxic metal element and can be considered as interesting candidates for use in the surface coating application as cool colorants.
Competing Interests
The authors declare that there are no competing interests regarding the publication of this paper.
Acknowledgments
The authors gratefully acknowledge the financial support of Key Programs for Science and Technology Development of Henan province, China (no. 122102210239), the Fund for Young Teachers in University of Henan Province, China (2012GGJS-103), the key science and technology plan projects of Zhengzhou city (no. 131PPTGG410-12), and the Natural Science Research Projects of Education Department of Henan province, China (13B560115).