Synthesis of green cool pigments (Co_xZn_1-xO) for application in NIR radiation reflectance

Highlights

• Synthesis clean and without generating waste.

• Cool green pigments inherent in the Co_xZn_1-xO composition.

• Shades influenced by cobalt precursor salt.

• Reflectivity NIR greater than 30%.

Abstract

Reconciling green synthesis, sustainability and production of pigments for protection of solar radiation is a continuous topic of search for unique, versatile and environmental compatible methods. In this perspective, we present the green synthesis of inorganic pigments with reflective properties of infrared radiation in the NIR region. Green powder pigments described by Co_xZn_1-xO, were obtained via the gelatinization method of starch colloidal suspension with inorganic salts (zinc and cobalt), calcined at different temperatures. Structural, morphological and spectroscopic characterization showed the pigments present Wurtzite crystalline phase with compact hexagonal structure, and monophasic when obtained at a calcination temperature above 800 °C. Colorimetric measurements obtained by the CIEL*a*b* method presented values related to green pigments attributed to cobalt ions, which replace the zinc ions in the Wurtzite matrix and corroborate the composition estimated by X-ray fluorescence analysis for Co_0.1Zn_0.9O. The pigments' NIR reflectivity increased with the calcination temperature increasing, with R% > 35%; and when the pigment is dispersed in a white paint the increase is R% > 50%. Therefore, our green pigments present great potential to be used as an active material in cold coatings and in solar devices.

Introduction

The solar radiation covers a wide wavelength range, ca. 295–2500 nm, comprising approximately 5% ultraviolet light (∼295–400 nm), 50% of visible light (∼400–700 nm) and 45% of near infrared radiation (NIR ∼ 700–2500 nm). The highest incidence is between 700 and 1200 nm and it corresponds to the region of the greatest heat generation [1,2]. This heating is of special importance for surfaces, such as walls in buildings or paved highways that causes an increase in the environment temperature [3,4]. With the growing of urban centers and less afforestation, the absorption on this energy range increases and it results in the intensification of the so-called “urban heat island” phenomenon. This entails a higher consumption of electric energy, due to the expenses with refrigeration systems, thus demanding a great concern regarding the preparation of materials that reflect the radiation in the near infrared (NIR) region, with the property of lowering the temperature inside the buildings. Konopacki and Akbari [5] presented in their studies a daily energy savings of 11% in a retail store in Austin, through the application of a reflective membrane in the building roofs, showing high impact in energy saving.

Thus, NIR reflective pigments are currently being developed to be used in cold coatings [3,6,7]. They are used on building surfaces, such as roofs, walls and facades [8,9], cars [10] and windows [11]. They can have any color, because the reflectivity and the pigment absorption are independent of each other [1]. Therefore, the strategy is the formulation of pigments with high reflectivity from mixed metal oxides [12], being this group known for its high absorption in the region of visible and high NIR reflectivity [13]. The inorganic pigments are mostly synthesized from different routes, highlighting the sol-gel method [6,14]; solid state reaction [15,16]; co-precipitation [17]; combustion method [18], hydrothermal [19]; Pechini [20] and others.

Pigments obtained from the doping of different matrices are widely known [[21], [22], [23]], due to the great variety of colors obtained from the insertion of metallic ions in the matrix studied. Among these, the zinc oxide matrix is used, due to its versatility and applicability as a catalyst, luminescent materials, gas sensors, sunscreen and pigments [24,25]. The ZnO presents the thermodynamically stable Wurtzite phase, with tetrahedral coordination in a compact hexagonal arrangement [26] and it presents a relatively open structure which facilitates the incorporation of dopant metals, which allows changes such as the position of the conduction and valence bands, band gap variation, sintering temperatures among others [27].

Preferably used as the white inorganic pigment, when doped with metal ions have different color, for example with the Co(II/III) ion their color is green [19,21,28]. Cobalt-based pigments have been studied and proposed as a viable alternative, due to their low toxicity [29], chemical stability and optical characteristics [3], in addition to meeting the requirements of color stability of building materials [30]. However, Co-doped pigments must be prepared with low cobalt content, due to their scarcity and possible environmental problems that may arise from the production of new pigments [31].

In this work, doped zinc oxide pigments with low cobalt content were synthesized. Although it is a known green pigment and reported in the literature [19,21,28], the Co-doped zinc oxide applied as reflectors of NIR radiation were poorly explored. Recently, there has been reported [32] the reflective properties of Zn_1-xCo_xO pigments, but their application in ink has not been explored, a property that was discussed in this study. So, the green pigment (Co_0.1Zn_0.9O) was synthesized by the gelatinization method based on starch that is simple, low cost and requires lower calcination temperatures when compared with traditional synthesis, for example the solid state reaction. Powders with high color homogeneity and with good reproducibility were obtained and characterized.