Journal of Nanomaterials
Volume 2016 (2016), Article ID 5464978, 6 pages
http://dx.doi.org/10.1155/2016/5464978

Research Article

Synthesis, Characterization, and NIR Reflectance of Highly Dispersed NiTiO₃ and NiTiO₃/TiO₂ Composite Pigments

Yuping Tong, Jing Fu, and Zheng Chen

School of Civil Engineering and Communication, North China University of Water Resources and Electric Power, Zhengzhou 450011, China

Received 6 January 2016; Revised 22 March 2016; Accepted 27 March 2016

Academic Editor: Jean M. Greneche

Copyright © 2016 Yuping Tong et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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].

Table 1: The lattice constant and crystal size of NiTiO₃ nanocrystals.

700°C
750°C
800°C
850°C
900°C

2θ of crystal plane (104)
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

Figure 1: 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 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.

Figure 2: XRD patterns of samples: (a) NiTiO₃, (b) NiTiO₃/TiO₂; “▼” denotes the diffraction peaks of TiO₂.

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)

Figure 3: (a) UV-vis absorption spectra of NiTiO₃ precursor calcined at different temperatures; (b) versus , showing the fit to linear portion corresponding to the direct band gap transition.

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)

Figure 4: SEM images of NiTiO₃ precursor calcined at 700°C; (b) NiTiO₃/TiO₂ composites.

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)

Figure 5: EDS analysis of NiTiO₃ nanocrystals (a) and NiTiO₃/TiO₂ composites (b); insets indicate the amount of elements.

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).

Figure 6: 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 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.

Table 2: 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 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.

Figure 7: NIR reflectance spectra of NiTiO₃ nanopigments and NiTiO₃/TiO₂ composites.

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).

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