Designing efficient catalysts for CO2 reduction is a critical challenge in addressing global carbon emissions and producing valuable chemical feedstocks. Hydrothermal synthesis offers a versatile route to fabricate well-defined nanostructures with controlled morphologies and compositions, particularly for metal oxides like Cu2O and BiVO4. This method leverages aqueous solutions at elevated temperatures and pressures to crystallize materials with tailored properties, enabling precise control over catalytic active sites.

Facet engineering plays a pivotal role in determining the catalytic performance of these materials. Different crystal facets exhibit distinct atomic arrangements and electronic structures, influencing adsorption energies and reaction pathways. For Cu2O, the (100) and (111) facets dominate the reduction process. The (100) facet, rich in coordinatively unsaturated Cu sites, favors CO2 adsorption and activation, while the (111) facet tends to promote C-C coupling, leading to multi-carbon products like ethylene and ethanol. By controlling reaction parameters such as pH, temperature, and precursor concentration, hydrothermal synthesis can selectively expose these facets. For instance, alkaline conditions often favor the formation of (100)-dominant Cu2O cubes, whereas neutral or acidic conditions yield octahedral structures with predominant (111) facets.

Doping further refines the selectivity and activity of these catalysts. Introducing foreign atoms into the lattice modifies electronic properties and creates defect sites that enhance CO2 adsorption and activation. In BiVO4, doping with transition metals like Mo or W improves charge carrier separation and reduces recombination losses, critical for photocatalytic CO2 reduction. Similarly, nitrogen-doped Cu2O exhibits enhanced conductivity and stability due to the introduction of mid-gap states that facilitate electron transfer. The hydrothermal method allows uniform dopant incorporation by co-precipitating metal precursors with dopant sources, ensuring homogeneous distribution. For example, a 2% W-doped BiVO4 synthesized hydrothermally at 180°C demonstrates a 3-fold increase in formate production compared to undoped BiVO4, attributed to improved charge mobility and reduced electron-hole recombination.

Performance metrics for CO2 reduction catalysts include Faradaic efficiency (FE), current density, and overpotential. Cu2O catalysts with optimized (100) facets achieve FE values exceeding 60% for ethylene at moderate overpotentials (-0.6 V vs. RHE), while BiVO4-based photocatalysts show formate selectivity up to 80% under visible light. Stability remains a significant challenge, as many metal oxides suffer from phase transformation or dissolution under reducing conditions. Cu2O is prone to reduction to metallic Cu, which shifts selectivity toward hydrogen evolution. Strategies to mitigate this include protective coatings or alloying with stabilizing elements like Zn. Hydrothermally synthesized core-shell structures, such as Cu2O@TiO2, exhibit prolonged stability, maintaining 90% of initial activity after 20 hours of operation due to the TiO2 shell preventing Cu2O reduction.

The table below summarizes key performance indicators for hydrothermally synthesized CO2 reduction catalysts:

Catalyst Morphology Facet Dopant Product FE (%) Stability (h)
Cu2O Cubes (100) None C2H4 65 10
Cu2O Octahedra (111) N Ethanol 55 15
BiVO4 Nanoplatelets (010) W Formate 80 30
Cu2O@TiO2 Core-shell Mixed None C2H4 70 20

Challenges persist in scaling these materials while maintaining performance. Hydrothermal synthesis, though precise, requires optimization of batch conditions to ensure reproducibility. Additionally, the interplay between facet exposure and dopant effects must be carefully balanced to avoid competing reactions. Future directions include exploring dual-doped systems and hierarchical architectures to further enhance activity and durability.

In summary, hydrothermal synthesis provides a powerful platform for designing CO2 reduction catalysts with controlled facets and dopants. By leveraging these strategies, researchers can tailor materials for selective and stable CO2 conversion, advancing the development of sustainable energy solutions.