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.
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 [
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 [
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 [
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.
The crystalline phase structure was determined by Bruker D8 Advance X-ray diffractometer (XRD) using Cu K
Figure
The lattice constant and crystal size of NiTiO3 nanocrystals.
700°C | 750°C | 800°C | 850°C | 900°C | |
---|---|---|---|---|---|
2 |
33.112 | 33.113 | 33.109 | 33.109 | 33.111 |
Lattice constant | |||||
|
5.02912 | 5.02315 | 5.02985 | 5.02993 | 5.02982 |
|
13.79037 | 13.77461 | 13.79002 | 13.79118 | 13.78966 |
Crystal size | 46.9 | 49.3 | 58.1 | 63.0 | 65.3 |
XRD patterns of NiTiO3 combustion product calcined at (a) 700°C, (b) 750°C, (c) 800°C, (d) 850°C, and (e) 900°C for 4 h, respectively.
XRD patterns of NiTiO3/TiO2 nanocomposite are shown in Figure
XRD patterns of samples: (a) NiTiO3, (b) NiTiO3/TiO2; “▼” denotes the diffraction peaks of TiO2.
Figure
(a) UV-vis absorption spectra of NiTiO3 precursor calcined at different temperatures; (b)
The optical data were analyzed at the near-absorption edge. According to the literature [
SEM images of NiTiO3 precursor calcined at 700°C; (b) NiTiO3/TiO2 composites.
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
The morphologies of NiTiO3/TiO2 nanocomposites are shown in Figure
EDS is used to further confirm the composition of the obtained samples. The EDS analysis of the obtained products (Figure
EDS analysis of NiTiO3 nanocrystals (a) and NiTiO3/TiO2 composites (b); insets indicate the amount of elements.
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
Photographs of NiTiO3 pigments calcined at 700°C, 750°C, 800°C, 850°C, and 900°C for 4 h and NiTiO3/TiO2 composites.
The chromatic properties of the obtained NiTiO3 pigment samples can be assessed from their CIE 1976
Color coordinates of NiTiO3 pigment samples.
Calcining temperature | Color coordinates | |||
---|---|---|---|---|
|
|
|
| |
NiTiO3 | ||||
700°C | 73.0 | −6.2 | 10.2 | 11.9 |
750°C | 67.4 | −3.99 | 42.1 | 42.3 |
800°C | 61.0 | 4.56 | 49.8 | 50.0 |
850°C | 57.2 | 5.14 | 48.7 | 48.9 |
900°C | 54.4 | 6.59 | 48.0 | 48.5 |
NiTiO3/TiO2 | 62.4 | 3.21 | 32.5 | 32.7 |
The NIR reflectance spectra of the NiTiO3 nanocrystals obtained at different temperatures are given in Figure
NIR reflectance spectra of NiTiO3 nanopigments and NiTiO3/TiO2 composites.
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.
The authors declare that there are no competing interests regarding the publication of this paper.
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).