Wood is a high-strength lightweight material owing to its orthotropic cellular structure and composite-like constitution. In conventional fabrication of wood-derived functional materials, the removal of the potentially beneficial components, such as lignin and hemicellulose, often leads to the disruption of the native hierarchical wood structure. Herein, we developed a facile method of in situ wood sulfonation followed by hot pressing for pine veneers to prepare high-density transparent thin films with preserved wood components and the natural fiber alignment. An optimum lignin content of the hot-pressed films was found to be 20.6% where both mechanical and optical properties were significantly enhanced with a more dense and compact structure. The hot-pressed transparent wood films also showed UV-blocking capability and could be recycled into discrete wood fibers owing to the sulfonate groups endowed by the in situ sulfonation step. The unique combination of properties achieved for thin wood films marks an important step in engineering functional wood-based materials that utilize both the structure of aligned fibers and the complex components of natural wood.
