High entropy oxides (HEOs), composed of at least five nearly equimolar principal atoms occupying a similar sublattice, demonstrate promising catalytic potential but limited activity. Low-dimensional HEOs, serving as the metastable phase, possess distinctive electronic structures and fully exposed active sites, which anticipate showcasing appealing performance; however, their synthesis remains challenging. Herein, through the incorporation of clusters for kinetic control, a library of single-phase high entropy oxides (HEOs) with single-unit-cell thickness was synthesized under mild conditions (373 K). By modulating the surface entropies, including vibrational, translational, and rotational entropy, the synthesized high entropy subnano-oxides can exhibit structures, such as subnano-wires, subnano-sheets, and spiral coils. Contributed by the fully exposed active sites and electron delocalization among two-dimensional (2D) layer, HEOs presented as subnano-sheets display enhanced catalytic activity for photocatalytic reduction of CO₂ to CH₄, achieving a yield (5777 ± 230.21 μmol·g⁻¹·h⁻¹), which is 41 times higher than that of bulk HEO obtained from the high-temperature calcination synthetic route.

Design of non-noble metal electrocatalysts for biomass conversion to high-value chemicals and understanding the related catalytic mechanisms are of profound significance but have remained a major challenge. Here, we developed a novel biomass-derived electrocatalyst (denoted as Cu/NC), featuring with electron-deficient copper nanoparticles anchored on N-doped carbon nanosheets, for the electrochemical reduction of 5-hydroxymethylfurfural (HMF) to 2,5-bis(hydroxymethyl)furan (BHMF, a vital precursor of functional polymers). The optimized Cu/NC electrocatalyst exhibited an excellent performance with high Faradaic efficiency (89.5%) and selectivity (90.8%) of BHMF at a low concentration of HMF (18.1 mM). Even at a very high HMF concentration (108.6 mM), the Faraday efficiency and selectivity of BHMF could still reach 74.8% and 81.1%, respectively. This performance approached those of the reported noble metal-based electrocatalysts. Mechanism study revealed that the N doping in the Cu/NC catalyst could regulate the electronic structure of Cu, strengthening the adsorption of the HMF carbonyl group, and thus boosting the selectivity of BHMF. Additionally, strong electronic metal-support interactions of Cu and the N-doped carbon support optimized the charge transfer rate, thus promoting the dissociation of water to the active hydrogen (H*) species and boosting the reaction kinetic rate of H* and HMF.