We apply the Thomas theory of ferroelectricity to bulk and thin film perovskite ferroelectrics in the paraelectric regime above the transition temperature. From available data on bulk SrTiO3 we are able to fully determine the parameters in the Thomas theory for this material, with overall reasonable results, supporting its validity. In a new application of the Thomas theory to the surface of a thin ferroelectric film in the linear response regime, it is found that there is anticipated to be an intrinsic “dead layer effect” on the surface of a dielectric film which significantly reduces the effective dielectric constant observed in capacitor applications. Two predictions of the theory are verified from recent experimental data. An experiment is suggested to distinguish between linear and nonlinear surface effects.

Photocatalytic reduction of CO 2 toward eight-electron CH 4 product with simultaneously high conversion efficiency and selectivity remains great challenging owing to the sluggish charge separation and transfer kinetics and lack of active sites for the adsorption and activation of reactants. Herein, a defective TiO 2 nanosheet photocatalyst simultaneously equipped with AuCu alloy co-catalyst and oxygen vacancies (AuCu-TiO 2−x NSs) was rationally designed and fabricated for the selective conversion of CO 2 into CH 4 . The experimental results demonstrated that the AuCu alloy co-catalyst not only effectively promotes the separation of photogenerated electron−hole pairs but also acts as synergistic active sites for the reduction of CO 2 . The oxygen vacancies in TiO 2 contribute to the separation of charge carriers and, more importantly, promote the oxidation of H 2 O, thus providing rich protons to promote the deep reduction of CO 2 to CH 4 . Consequently, the optimal AuCu-TiO 2−x nanosheets (NSs) photocatalyst achieves a CO 2 reduction selectivity toward CH 4 up to 90.55%, significantly higher than those of TiO 2−x NSs (31.82%), Au-TiO 2−x NSs (38.74%), and Cu-TiO 2−x NSs (66.11%). Furthermore, the CH 4 evolution rate over the AuCu-TiO 2−x NSs reaches 22.47 μmol•g −1 •h −1 , which is nearly twice that of AuCu-TiO 2 NSs (12.10 μmol•g −1 •h −1 ). This research presents a unique insight into the design and synthesis of photocatalyst with oxygen vacancies and alloy metals as the co-catalyst for the highly selective deep reduction of CO 2 .

Utilizing photonic crystals to fabricate information encryption materials has attracted widespread interest due to their tunable optical properties and responsiveness to external stimuli. In most of the previously reported systems, the information is hidden at a specific angle and the angle-dependent invisibility is a limitation. Meanwhile, poor structural stability is still a key issue that needs to be solved for potential applications. In this paper, a bilayer heterostructure photonic crystal containing ordered hollow silica inverse opal arrays, amorphous silica opal arrays, and poly­(vinyl alcohol) (adhesive) is successfully constructed. It makes the information highly invisible at any angle and also achieves information encryption. With this strategy, the information can be hidden by the noniridescent structural color derived from the strong scattering effect of light from the top layer of amorphous silica sphere arrays. After wiping with ethanol or a refractive-index-matching solvent, the scattering effect vanishes and the amorphous silica sphere arrays become transparent. The reflected light of the bottom layer caused by the increasing refractive index contrast between the inside and outside of the hollow silica spheres could rapidly reveal the hidden information. The bilayer photonic crystal exhibits robust structural stability, and the hiding/revealing process is completely reversible, which shows great potential applications in steganography and information encryption.

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