Compositional, structural, and surface characterization of heat-treated halloysite samples: Influence of surface treatment on photochemical activity

Quasi-homogeneous nanoparticulate halloysite clay with layered scroll morphology is an ideal material for photochemical activity due to the lack of interference from Fe contaminants and mixed mineral phases. In this work, halloysite was used as a model system for studying the influence of compositional defect sites on the structure and photochemical activity as a result of thermal modifications in the range of 400–480 °C and with 30–180 min exposure. The change in the Al-coordination environment was studied by 27Al, 1H magic-angle spinning (MAS) and cross-polarization (CP)/MAS nuclear magnetic resonance (NMR), and infrared (IR) spectroscopic techniques. The structural and morphological changes were followed by powder X-ray diffraction (XRD) and transmission electron microscope (TEM) techniques. Pore structure description was obtained from nitrogen gas adsorption measurements. The amount of residual hydroxide groups was determined by thermogravimetric analysis (TGA). Temperature-controlled desorption (TPD) and 1H MAS NMR techniques were used for the characterization of acid-base properties of thermally created defect sites. These defect sites showed a decrease in Al-coordination number and an increase in the number of acidic sites. However, evidence for the formation of basic sites during long exposure experiments can be seen, which emphasises the impact of the duration of the heat treatment. The emergence of acidic sites was correlated with the increase in photochemical activity. A significantly different photochemical activity was found for thermally treated halloysite than expected from the semiconductor-type photocatalysis model on the basis of the number of hydroxyl radicals generated as a function of the irradiation time.