Synthesis and characterization of Fe^3 + doped Co_0.5Mg_0.5Al₂O₄ inorganic pigments with high near-infrared reflectance

Highlights

• High NIR reflecting pigments Co_0.5Mg_0.5Al₂O₄ were synthesized by the Pechini-type sol–gel method.

• The doping of Fe^3 + changed the pigment color from blue to black.

• Pigment properties make it a potential candidate for use as cool pigments.

Abstract

In this work, a series of near-infrared reflective inorganic pigments with a general formula Co_0.5Mg_0.5Al_2 − xFe_xO₄ (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) were successfully prepared by Pechini-type sol–gel method. Comprehensive analyses were carried out to characterize the developed pigment powders including thermogravimetry and differential scanning calorimetry, X-ray diffraction, field emission scanning electron microscopy, ultraviolet–visible near infrared diffuse reflectance spectroscopy, and CIE-L^⁎a^⁎b^⁎ 1976 color scales. The results demonstrated that the single-phase Co_0.5Mg_0.5Al_2 − xFe_xO₄ was synthesized at an optimum temperature of 900 °C. The resulting calcined powders have a well-developed cubic spinel structure. The substitution of Fe^3 + for Al^3 + in Co_0.5Mg_0.5Al_2 − xFe_xO₄ changes the color from blue to black and the band gap shifts from 4.40 eV to 3.50 eV. And Fe doped pigments possess high near-infrared solar reflectance (> 43%) in the range of 780–2500 nm. Therefore, these Co_0.5Mg_0.5Al_2 − xFe_xO₄ powders have great potential in serving as cool pigments for building coatings.

Introduction

Solar radiation consists of ~ 5% UV radiation, 43% visible radiation and 52% near-infrared radiation (NIR; ~ 780–2500 nm). Solar energy is absorbed by buildings and paved surfaces, which leads to temperature rise of urban structures. These surfaces thereby warm up the surrounding, causing measurable ambient temperature rise [1], [2]. This phenomenon, termed “urban heat island” (UHI), has been urging a drastic increase in the demand for energy, especially-energy consumed by central air conditioning systems in large buildings for cooling. One of the most effective methods to alleviate Heat Island Effect involves developing near-infrared reflective (NIR) pigments [3], [4]. Induced by energy conservation, these pigments and the materials related have been gaining increased attention from researchers over the years. Particularly, one of these materials, the spinel-type oxide have been widely studied as attributed to the potential in electrochemical, catalytic and pigment applications [5], [6], [7]. The spinel-type oxides are generally formulated as A^2 + B₂^3 + O₄, in which the anions arranged in a cubic close-packed lattice that is classified as cubic. In addition, the A and B cations can occupy up to all the octahedral and tetrahedral sites in this lattice. Notably, there are two ideal structures of significance: normal spinel structure and inverse spinel structure [8]. For example, CoAl₂O₄ is one of the best known members of cobalt spinel-type oxides possessing various uses in ceramic, glass, paint industries; e.g., contrast-enhancing luminescent CoAl₂O₄ pigments for television tubes to produce Thenard's blue color [9].

One of the common methods for manufacturing pigment powder involves a solid-state reaction in which the oxides are mechanically ground at high calcination temperatures above 1200 °C for a considerable amount of time. Though the mentioned process is relatively uncostly, undesired byproducts can be produced due to lack of homogeneity indicated by larger and uneven grains in the final product as a result of the weak control [10], [11], [12]. Recently, scientists have proposed various novel wet-chemical synthetic routes that would alleviate this problem, which include sol–gel method (polymer precursor method) [13], [14], low temperature combustion technique [15], [16], polymer pyrolysis method [17], [18], co-precipitation reaction [19], [20], emulsion precipitation [21], and hydrothermal crystallization [22].

In this work, Co_0.5Mg_0.5Al_2 − xFe_xO₄ pigment was synthesized using Pechini-type sol–gel method. Over the years, other kinds of compound powders, such as LaFe_1 − xAl_xO₃ yellow pigment powder [2], Zn_1 − xMg_xFe₂O₄ [23], and Co_xZn_1 − xAl₂O₄ [24] , have also been synthesized using sol–gel method. This method has many great advantages as compared to others; e.g., lower energy-consumption, relatively low temperatures of synthesis, good stoichiometric and morphological control. Most importantly, these pigment powders exhibit higher homogeneity and stronger chemical activity.

CoAl₂O₄ is one of the commercially important materials which has super properties and extensively used as a pigment. But cobalt is scarce and expensive, thus increasing the production costs of cobalt-based pigments. Therefore, in the present work, Mg^2 + for Co^2 + ion substitution in the CoAl₂O₄ was done. On the other hand, spinel-type materials have attracted more attention from researchers. Due to their capabilities of accommodating different cations, they display various colors and tonalities [25]. Doping of Fe^3 + for Al^3 + in Co_0.5Mg_0.5Al₂O₄ changes the color properties of pigments. Therefore, in this paper, a series of NIR reflectance inorganic pigments with the formula Co_0.5Mg_0.5Al_2 − xFe_xO₄ was used as a cool pigment. Co_0.5Mg_0.5Al_2 − xFe_xO₄ samples were synthesized through a sol–gel route within a defined calcination temperature range. The crystal structure, NIR reflective chromatic properties, thermal and chemical stability were investigated.