Solar energy as a sustainable energy source has attracted much attention, and the development of highly efficient photothermal conversion materials becomes a key to coping with the energy crisis due to an increase in global energy consumption and demand. Conventional photothermal materials generally have some bottlenecks such as complication preparation process, high cost, and lack of durability, and there is an urgent need to develop novel and efficient materials. As a crystallization of Chinese ceramic craftsmanship inherited for thousands of years, black porcelain glaze with its rabbit hair, oil drop and other classic glazes show a unique artistic value due to the low cost of raw materials, mature and stable technology, especially for the solar spectra of the wide-area absorption characteristics in photothermal conversion to show a significant advantage. However, there is still controversy in the academic community regarding the coloring mechanism of black glaze, and there is a lack of systematic research on its photothermal properties. In this work, we investigated a relationship between the iron oxide content and the glaze layer by using a basic glaze of SiO₂–Al₂O₃–CaO–MgO–Na₂O–ZnO–Fe₂O₃ in the microstructure and glaze color, revealing the mechanism of its black color formation and photothermal conversion properties.

The raw materials were mixed in a ratio of 1.0:1.3 and ground in a ball mill, and the ground slurry was sieved through a 200–mesh sieve. Afterwards, the glaze paste was dipped and coated on a 40 mm diameter billet for several times, keeping its thickness of approximately 1 mm. The samples were dried in an oven at 75 ℃ for 12 h, and then fired in an electric furnace in air according to the following firing procedure, i.e., the temperature increased to 400 ℃ at a rate of 2.5 ℃/min and kept warm for 30 min, then increased to 1250 ℃ at a rate of 4 ℃/min and kept warm for 90 min, and then decreased to 1100 ℃ at a rate of 2.5 ℃/min and kept warm for 30 min. Finally, the glaze sample was naturally cooled to room temperature in the furnace.

The parameters such as L*, a* and b* values in the CIE space of the samples were determined by a model WSD-3C whiteness meter. The light reflection spectra of the glaze were analyzed by a model Lambda 850 UV/Vis diffuse reflection spectrometer. The Raman spectra was characterized by a model InVia type Raman spectroscope (Renishaw Co., UK) with an excitation wavelength of 532 nm. The physical phase of the glaze was detected by a model D8-Advance X-ray diffractometer (XRD, Bruker Co., Germany), with a 2θ of 10°–80°. The glaze samples were firstly etched with 10% hydrofluoric acid solution for 30 min, and then the microstructure was determined by a model SU8010 scanning electron microscope (SEM, Hitachi Co., Japan). The elemental composition was determined by a model 550I energy dispersive X-ray spectroscope (EDS). The morphology and structure were analyzed by a model JEM2100F transmission electron microscopy (TEM, JEOL Co., Japan) at a voltage of 200 kV, and the surface temperature of the glaze samples was tested by a model UT320D high-precision K/J dual-purpose thermocouple thermometer (UNI-T Co., China). The thermograms of the samples were obtained by a Ti400+ thermal infrared image analyser (FLUKE Co., USA).

The experiment results show that the color of the glaze changes with the increase of iron oxide content. The glaze is transparent when the iron oxide content is 0.0%. The glaze surface turns brownish yellow when the iron oxide content increases to 2.5%. The glaze surface appears dark black when the content further increases 6.5%. A micro-/nano- array structure of resembling zinc ferrite crystals forms when the excess of iron oxide reaches 9.5%, which affects the color of the glaze surface. Clearly, the iron oxide goes through the process of participating in the network structure of the glaze layer and precipitating from the glaze layer. The in-depth analysis of the ink-black colored glaze samples reveals that a large number of iron-rich agglomerates or even crystal nuclei of approximately 3 nm are precipitated from the glaze layer, and the glaze samples are characterized by an increase in the refractive index gradient from the glaze surface to the billet. This special micronaire structure can strengthen the glaze layer's absorption of the incident light, resulting in the glaze with a black color effect and also greatly improving the performance of its photothermal conversion.

This study investigated the effect of iron oxide variation on the microstructure and color of the glaze layer in the SiO₂–Al₂O₃–CaO–MgO–Na₂O–ZnO–Fe₂O₃ multicomponent glaze system. The results indicated that when the content of iron oxide was low, the trivalent iron ions could participate in the network structure of the glaze layer in the form of [FeO4] tetrahedra, and the glaze surface dominated due to the chemical coloring of Fe ions, presenting a brownish-yellow color. A large number of iron-rich aggregates and even crystal nuclei of approximately 3 nm could precipitate from the glaze layer when the content of iron oxide increased to 6.5%, and the glaze layer exhibited a structural characteristic of increasing refractive index gradient from top to bottom. The special micro-/nano-structure of this glaze layer could greatly increase the absorption and consumption of sunlight, and the glaze surface had a black effect and structural coloring characteristics. The results of photothermal performance test showed that the glaze could have the maximum temperature difference of 12.2 ℃ with the blank color, indicating its good photothermal conversion characteristics and a great potential in the green utilization of solar energy.