Trans. Nonferrous Met. Soc. China 25(2015) 2646-2652

Preparation of Cr₂O₃-based pigments with high NIR reflectance via thermal decomposition of CrOOH

Shu-ting LIANG^1,2,3, Hong-ling ZHANG ^1,2, Min-ting LUO^1,2, Hong-xia LIU⁴, Yu-lan BAI⁵, Hong-bin XU^1,2, Yi ZHANG^1,2

1. National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China;

2. Key Laboratory of Green Process and Engineering, Chinese Academy of Sciences, Beijing 100190, China;

3. University of Chinese Academy of Sciences, Beijing 100049, China;

4. Shandong Provincial Academy of Building Research, Jinan 250031, China;

5. College of Chemistry and Pharmaceutical Sciences, Qingdao Agricultural University, Qingdao 266109, China

Received 22 October 2014; accepted 7 May 2015

Abstract: In order to reduce greenhouse gas emission and urban heat island mitigation, pure and titanium(Ti)-doped Cr₂O₃ cool pigments were prepared via the thermal decomposition of CrOOH. The result reveals that the pure Cr₂O₃ pigment presents both a high near-infrared reflectance and excellent yellowish-green color. Meanwhile, titanium was doped to improve the NIR reflectance and strengthen the color. The color of the designed pigments was brighter, and most importantly, the NIR reflectance increased from 84.04% to 91.25% with increasing Ti content from 0 to 0.006% (mole fraction). However, excessive doping of Ti⁴⁺ for Cr³⁺ in Cr₂O₃ (x(Ti)≥0.008%) decreased the NIR reflectance. One possible reason is that the conductivity type of the Cr_2-xTi_xO_3+δ changed from p-type conduction to n-type conduction with increasing Ti content, accompanied by the change of the electrical resistivity and the NIR reflectance. The prepared yellowish-green Cr₂O₃ pigments have a great potential for extensive applications in construction and military.

Key words: CrOOH; cool pigments; NIR reflectance; Ti-doped Cr₂O₃

1 Introduction

The world is facing disruptive global climate change from greenhouse gas emission and increasingly expensive energy supplies. Cool pigments that absorb less near-infrared reflecting (NIR) radiation provide a number of benefits, including reduced energy use and greenhouse gas emission, urban heat island mitigation [1-3]. Therefore, cool pigments have been used in the construction, military and plastics [4]. As one of the green pigments, Cr₂O₃ with a high NIR reflectance has been progressively applied to cool roofing pigments [5,6] and green military camouflage paint and netting [7]. It is required to exhibit green color in the visible spectrum, and also simulate the high reflectivity of chlorophyll in the near infrared portions of the spectrum. Therefore, much interest has attended in preparing Cr₂O₃ with a high NIR reflectance and green color performance.

The industrial production procedures of the high NIR reflectance Cr₂O₃ pigment employ conventional techniques: the thermal decomposition of CrO₃ [8,9] and the reduction of alkali metal chromate with ammonium sulfate [10]. There are serious problems with the traditional production procedures including the environmental pollution resulted from Cr(VI)-containing dusts, and the expensive cost [11-14]. Thus, there is a strong incentive to develop colored, high NIR reflectance Cr₂O₃ pigments through a green production that is less hazardous to environment.

The NIR reflectance of pure Cr₂O₃ was in the range of 50% to 57% [15,16]. After small amounts of metal ions, such as Al, Ti, V, Co and Bi, are normally introduced into the Cr₂O₃, the modified Cr₂O₃ produced an excellent NIR reflectance (70%-82%) [17-20]. The doping with rare earths, such as La and Pr, which have been reported to be environmentally benign, resulted in nano-oxides with high reflective properties (85%) [21-24]. However, the chromatic properties of these resulting high NIR reflectance products are single dark-green and unsatisfactory. Moreover, the reason why doping of other metal elements in a host component Cr₂O₃ significantly changes the NIR reflectance has been seldom reported previously.

In this work, we try to do four works: 1) preparing pure Cr₂O₃ through a non-toxic and convenient process which does not produce Cr(VI)-containing dusts; 2) improving the NIR reflectance of pure Cr₂O₃ from 50%-57% to 84.04%; 3) improving the color performance of Cr₂O₃ from single dark-green to bright yellowish-green while maintaining its high NIR reflectance; 4) discussing the mechanism that doping of other metal element in Cr₂O₃ significantly changed the NIR reflectance. The Cr₂O₃ was prepared from the thermal decomposition of CrOOH. This process was expected to prepare a pure Cr₂O₃ with both high NIR reflectance and good color performance. Finally, Ti has been doped into CrOOH to replace the Cr³⁺ ion and further improve the NIR reflectance. The mechanism that doping of Ti element in Cr₂O₃ changed the NIR reflectance was also discussed.

2 Experimental

CrOOH used in this work was manufactured via hydrogen reduction of K₂CrO₄ [25]. It was calcined in an electric muffle furnace at 1150 °C for a soaking time of 1.5 h and then rapidly cooled. The resulting Cr₂O₃ samples were lixiviated with distilled water. The final products were recovered, generally by filtration and drying.

The Cr_2-xTi_xO_3+δ (x ranges from 0 to 0.02) were synthesized by a solid state reaction method, using precursor CrOOH (99.5%) and TiO₂ (99.9%) as starting materials. Stoichiometric proportions of the precursors were transferred into an agate mortar and homogenized by milling. The resultant powders were calcined in a high temperature electric furnace at an optimized temperature of 1150 °C for 1.5 h. Finally, the Cr₂O₃ products were recovered, generally by filtration and drying.

The color performance data were reported using the CIE L*a*b* (1976) colorimetric system. A Datacolor 110 colorimeter, manufactured by Datacolor CO., Ltd., USA, equipped with an illuminant D65 and 10° complementary observer as required, was employed. The value of CIE-L* denotes the degree of lightness and darkness of the color in relation to the scale extending from white (L*=100) to black (L*=0). The value of CIE-a* denotes the scale extending from the green (-a*) to red (+a*) axes. The value of CIE-b* denotes the scale extending from the blue (-b*) to yellow (+b*) axes. For each colorimetric parameter of the analyzed sample, three values were measured, and the average value was chosen as the measurement result.

The optical properties of the samples were then analyzed by diffuse reflectance spectroscopy (UV-Vis-NIR), which was performed using a Perkin-Elmer (lambda 750) spectrophotometer in the wavelength range of 380-2500 nm employing barium sulphate as a reference. The NIR reflectance was calculated in accordance with the ASTM standard EN410 [26]. The solar reflectance (R) in the wavelength range from 780 to 2500 nm can be figured out.

                      (1)

where r(λ) is the spectral reflectance obtained from the experiment and i(λ) is the standard solar spectrum.

Microstructural characterization of the powders was conducted using a JSM-6700F NT scanning electron microscope (SEM), supplied by JEOL. X-ray diffraction (XRD) patterns were recorded using a Rigaku diffractometer employing Cu K_α radiation (2θ from 5° to 90°, with steps of 0.02°, and a counting time of 2 s per step).

3 Results and discussion

3.1 Pure Cr₂O₃ pigment sample

The Cr₂O₃ which was prepared by calcining a pure CrOOH at 1150 °C for 1.5 h was named as Sample S₁. Thermal decomposition of CrO₃ was a representative industrial method for the production of Cr₂O₃. Commercial Cr₂O₃ pigment produced by this method was chosen as the standard NIR reflection samples. The reflectance spectra of the Sample S₁ and standard Cr₂O₃ are given in Fig. 1(a). Multiplying the normalized spectral irradiance of the sun i(λ) by the spectral reflectivity yields the NIR solar reflection spectrum presented in Table 1. As can be seen in Table 1, the standard Cr₂O₃ exhibited an NIR reflectance of 82.51%. Sample S₁ has a NIR reflectance of 84.04%. The result demonstrates that the Cr₂O₃ prepared from the thermal decomposition of CrOOH has an excellent NIR reflectance.

Fig. 1 NIR reflectance (a) and absorption spectra (b) of standard Cr₂O₃ and Sample S₁

The absorption spectra of the two samples are also determined and compared in the visible region of 380-780 nm from Fig. 1(b). Both samples show two distinct absorption bands at about 460 nm and 600 nm. According to the Tanabe-Sugano diagram, these bands can be attributed to ⁴A_2g→⁴T_2g and ⁴A_2g→⁴T_1g (F) spin-allowed transitions, respectively [27]. As can be seen from the absorption spectrum of Cr₂O₃ shown in Fig. 1(b), in Sample S₁, a progressive two-fold splitting of the high absorption band can be seen. Moreover, the absorption edge of sample S₁ is somewhat steep, indicating a very pure and brilliant color. The positions of absorption bands and their intensities and shapes, are important factors in the determination of color. Compared with standard Cr₂O₃, the positions of the absorption edges of Sample S₁ undergo a red-shift, thus the pigment product S₁ exhibits a more yellowish-green color than that of standard Cr₂O₃.

The CIE 1976 color coordinates of the standard Cr₂O₃ and Sample S₁ are shown in Table 1. Compared with the standard Cr₂O₃, Sample S₁ has an increase in the b* value from 8.88 to 20.38, which shows that the yellowness of the pigment sample is enhanced. At the same time, the increase of the green hue of the Sample S₁ that is evident from the higher values of the color coordinate –a* (a* changes from -12.04 to -19.23). The L* values of the samples are from 38.27 to 47.60. As a result, the Sample S₁ shows an excellent yellowish-green color when compared with the dark-green standard Cr₂O₃. The significant differences in the color performance of the samples can be observed in Fig. 2. Physicochemical properties of Sample S₁ including the stability and capacity to absorb oil and sieve residue were also measured and discussed. The Cr₂O₃ content of Sample S₁ as main chemical composition was more than 99% (mass fraction), and water-soluble matter content was lower than 0.1%, the contents of volatile matter and moisture were lower than 0.15%, the sieve residue content was lower than 0.01%, capacity to absorb oil (g/100g) was 20.98, which were conformed to commercial pigment standard. It was found that Cr₂O₃ which was obtained by calcining the CrOOH has both an excellent yellowish- green color and a commensurate NIR reflectance compared with the standard Cr₂O₃.

Table 1 Solar reflectance and chromatic coordinates of standard Cr₂O₃ and Sample S₁

Fig. 2 Photographs of Cr₂O₃ pigments

The industrial production of the standard Cr₂O₃ pigments employs the thermal decomposition of CrO₃. The different decomposition mechanisms between CrOOH and CrO₃ may result in different morphologies and grain sizes of Cr₂O₃ pigments; as a result, the optical reflections or absorptions of the Sample S₁ and the standard Cr₂O₃ were also different. Earlier investigations showed that the decomposition of CrO₃ into Cr₂O₃ involved the formation of three detectable intermediate phases: CrO_2.66, CrO_2.5 and CrO_2.25 [9], which were previously characterized by XRD and infrared spectroscopy. However, CrOOH showed a sharp exothermic peak centered at 430 °C corresponding to the dehydroxylation of the compound and the crystallisation of Cr₂O₃ [25].

The different decomposition courses of CrO₃ and CrOOH may result in different particle sizes and shapes of Cr₂O₃. The morphology of Sample S₁ is compared with that of standard Cr₂O₃ in Fig. 3. According to Fig. 3(b), the particles of the Sample S₁ had a flat and round shape and well dispersed. These particles are fully crystallized with a uniform size of 300-700 nm. However, when compared with the Sample S₁, the standard Cr₂O₃ turns to be much more irregular. The morphology of standard Cr₂O₃ appears to be cobblestone, with non-uniform particles having diameters ranging from 0.3 to 2.2 μm.

Fig. 3 SEM images of samples

It is known that when a beam of light falls on a sample, reflection, transmission, and absorption can occur. If the Cr₂O₃ is thick enough, the transmitted light can be neglected. There are many factors that affect the reflectance of pigments, in which the particle size plays a crucial role. When the particle size of Cr₂O₃ decreases from 3 μm to 300 nm, the number of grains in the same volume tends to increase, thus the reflection path of light inside the pigment tends to increase [17]. As a result, the total reflection increases. Thus, the Sample S₁ is more reflective compared with the standard Cr₂O₃. On the other hand, with the particle size of Cr₂O₃ increases, the scattering powder decreases and the color is darker, bluer, and less saturated [28]. From the experimental results, CrO₃ produced a Cr₂O₃ pigment with a large size and dark-green color performance, while Cr₂O₃ obtained by calcining the CrOOH had a yellowish-green color.

CrOOH plays a desirable role in regulating crystal growth and produces a Cr₂O₃ pigment with a uniform size. Cr₂O₃ pigment with a high NIR reflectance and a yellowish-green hue was obtained. The high reflectance exhibited by the designed yellowish-green pigment indicated that this pigment can serve as an excellent cool pigment.

3.2 Ti-doped Cr₂O₃ pigments

In order to further improve the NIR reflectivity of Cr₂O₃ pigments, chrome oxide as a host component containing the element titanium as a guest component has been obtained. It is reported that compared with pure Cr₂O₃, Ti-doped Cr₂O₃ has changed many properties including porous structure, grain size [29], surface area [30], conductivity [31], color performance and NIR reflectivity [24]. Ti-doped Cr₂O₃ has a wide application in sensors to detect trace quantities of reducing gases in air as well by changing its resistance [32].

In this work, CrOOH and TiO₂ were mixed into different compositions and then calcined to obtain Cr_2-xTi_xO_3+δ products. The XRD patterns of the Cr_2-xTi_xO_3+δ (x ranges from 0.003 to 0.02) are shown in Fig. 4. All the patterns show the characteristic reflections of the corundum structure of Cr₂O₃ (JCPDS card No. 38-1479) and no impurity phases are detected. It is probable that Ti forms solid solution with corundum- hematite crystalline structure.

Fig. 4 Powder X-ray diffraction patterns of Cr_2-xTi_xO_3+δ pigments

Fig. 5 NIR reflectance spectra of Cr_2-xTi_xO_3+δ pigments

For the Cr_2-xTi_xO_3+δ complex pigment, the electronic transitions of Ti ions result in more absorption and less reflectance in the near infrared region of Cr_2-xTi_xO_3+δ. The NIR reflectance spectra of Cr_2-xTi_xO_3+δ prepared are shown in Fig. 5. The titanium free sample exhibits 84.04% NIR reflectance at 780-2500 nm region. Doping of 0.006% Ti⁴⁺ for Cr³⁺ in Cr₂O₃ increases the NIR reflectance to 91.25%. The NIR reflectance of this sample is superior to that of standard Cr₂O₃. On the other hand, further doping of more Ti⁴⁺ for Cr³⁺ decreases the NIR reflectance. The high NIR reflectance values highlight the potential for the utility of these samples as cool pigments.

The chromatic properties of the synthesized Cr_2-xTi_xO_3+δ (x ranges from 0 to 0.02) pigments can be assessed from their CIE 1976 color coordinate values depicted in Table 2. The standard Cr₂O₃ pigment was produced from the thermal decomposition of CrO₃, while the Cr₂O₃ of Sample S₁ was also prepared by calcining a pure CrOOH at 1150 °C for 1.5 h. The systematic doping of Ti⁴⁺ for Cr³⁺ in Cr₂O₃ (n(Ti):n(Cr) from 0 to 1.01%) results in an increase in the L* value regularly from 47.60 to 52.48, which indicates that the brilliant color of the pigment sample is enhanced. On the other hand, the increase of titanium does not seem to affect the green and yellow hue of the pigments. After being doped with titanium, all samples show a bright yellowish-green color.

Cr_2-xTi_xO_3+δ pigments are found to have a more bright yellowish-green color performance and a high NIR reflectance (91.25%). Through Fresnel formula [33], the NIR reflectance has a relationship between refractive index and extinction index, which are determined by the conductivity and the dielectric constant.

The results of some researchers strongly suggest that the introduction of Ti⁴⁺ to Cr₂O₃ alters its defect structure and electrical conductivity [34]. Cr₂O₃ is usually reported as an insulator with the possibility of low native p-type conductivity [35]. In particular, Ti⁴⁺-doped Cr₂O₃ are famous systems in which doping can influence the electronic property. The mixed valence formation of Cr₂O₃ by Ti⁴⁺ ion doping has been described as follows [36,37]:

                   (2)

                    (3)

Equation (2) represents compensation of Ti⁴⁺ by Cr vacancies, and Eq. (3) represents compensation by electrons and can be regarded as dissolution as Ti³⁺ depending on the degree of ionization of the neutral defect. The variation in measured density of the solid solution indicates that Eq. (2) is the main reaction [37]. NAGAI and OHVAYASHI [38], through measurements of electrical conductivity and thermoelectric power of Cr_2-xTi_xO_3+δ, reported that undoped Cr₂O₃ had a p-type conductivity, and a change in the conduction behavior of Cr_2-xTi_xO_3+δ from p-type conduction to n-type conduction with increasing the TiO₂ content to more than 1.85% [31,38,39].

Table 2 Solar reflectance and color coordinates of the Cr_2-xTi_xO_3+δ (x ranges from 0 to 0.02)

Thus, it can be deduced that in this work after doping of Ti⁴⁺ for Cr³⁺ in Cr₂O₃, the conductivity decreases with the increase of the Ti content from Sample S₁ to sample S₅, accompanied by the change of the NIR reflectance. When doping of 0.006% Ti⁴⁺ for Cr³⁺ in Cr₂O₃, the conductivity type of Cr_1.987Ti_0.013O_3+δ (Sample S₅) changed from p-type conduction to n-type conduction. However, excessive doping of Ti⁴⁺ for Cr³⁺ in Cr₂O₃, the conductivity type of Cr_2-xTi_xO_3+δ is n-type conduction, and the conductivity increases with the increase of the Ti content from Sample S₅ to Sample S₇. At last, the conductivity goes through a minimum and then increases, accompanied by the change of the dielectric constant and the NIR reflectance. So, the NIR reflectance is influenced by the proportion of Ti doping into Cr₂O₃, and the Cr_1.987Ti_0.013O_3+δ has the highest reflectance (91.25%).

4 Conclusions

1) Cr₂O₃ pigment with a comparable high NIR reflectance and a bright yellowish-green hue was obtained via the simple calcine of CrOOH. The results demonstrated that the produced Cr₂O₃ has a higher NIR reflectance of 84% and good yellowish-green color compared with the dark-green standard Cr₂O₃.

2) Titanium (Ti) was doped to improve the NIR reflectance and a series of Cr_2-xTi_xO_3+δ (x ranges from 0 to 0.02) having a corundum structure displaying color of bright yellowish-green were synthesized by the solid state route. The conductivity type of Cr_2-xTi_xO_3+δ changed from p-type conduction to n-type conduction with Ti doping, and the NIR reflectance increased to the maximum value, and then decreased. As a result, the Cr_1.987Ti_0.013O_3+δ which displaying a bright yellowish- green color has the highest reflectance of 91.25%.

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热分解CrOOH制备具有高近红外反射率的Cr₂O₃颜料

梁书婷^1,2,3, 张红玲^1,2, 雒敏婷^1,2, 刘红霞⁴, 白玉兰⁵, 徐红彬^1,2, 张 懿^1,2

1. 中国科学院 过程工程研究所 湿法冶金清洁生产技术国家工程实验室，北京 100190；

2. 中国科学院 绿色过程与工程重点实验室，北京 100190；

3. 中国科学院大学，北京 100049；

4. 山东省建筑科学研究院，济南 250031；

5. 青岛农业大学 化学与制药学院，青岛 266109

摘  要：为了减少温室气体的排放，缓解城市热岛效应，通过热分解CrOOH制备纯Cr₂O₃冷色颜料和掺Ti的Cr₂O₃冷色颜料。结果表明，所制备的纯Cr₂O₃颜料同时具有较高的近红外反射性能和优良的黄绿色调。同时，在Cr₂O₃中掺杂Ti⁴⁺离子可进一步提高其红外反射率和颜色性能。所制备的Ti掺杂Cr₂O₃颜料的颜色非常明亮光鲜。随着Ti掺杂量由0提高至0.006%(摩尔分数)，样品的近红外反射率由84.04%提高至91.25%。然而，Ti⁴⁺掺杂量过高(x(Ti)≥0.008%)会使近红外反射率降低，这可能是由于随着Ti⁴⁺含量的升高，Cr_2-xTi_xO_3+δ的导电性能由p型导电转变为n型导电，同时伴随电阻率和近红外反射性能的变化。制备的明亮黄绿色调的Cr₂O₃颜料在建筑材料和军事方面具有非常广泛的应用前景。

关键词：CrOOH；冷色颜料；红外反射率；Ti掺杂Cr₂O₃

(Edited by Yun-bin HE)

Foundation item: Project (11204304) supported by the National Natural Science Foundation of China; Project (2013CB632600) supported by the National Basic Research Program of China; Project (2011AA060702) supported by the National High-tech Research and Development Program of China

Corresponding author: Hong-ling ZHANG; Tel: +86-10-82544808; Fax: +86-10-82544810; E-mail: hlzhang@ipe.ac.cn

DOI: 10.1016/S1003-6326(15)63887-0