Visible Light Photoreduction of CO₂ Using CdSe/Pt/TiO₂ Heterostructured Catalysts

A series of CdSe quantum dot (QD)-sensitized TiO₂ heterostructures have been synthesized, characterized, and tested for the photocatalytic reduction of CO₂ in the presence of H₂O. Our results show that these heterostructured materials are capable of catalyzing the photoreduction of CO₂ using visible light illumination (λ > 420 nm) only. The effect of removing surfactant caps from the CdSe QDs by annealing and using a hydrazine chemical treatment have also been investigated. The photocatalytic reduction process is followed using infrared spectroscopy to probe the gas-phase reactants and gas chromatography to detect the products. Gas chromatographic analysis shows that the primary reaction product is CH₄, with CH₃OH, H₂, and CO observed as secondary products. Typical yields of the gas-phase products after visible light illumination (λ > 420 nm) were 48 ppm g⁻¹ h⁻¹ of CH₄, 3.3 ppm g⁻¹ h⁻¹ of CH₃OH (vapor), and trace amounts of CO and H₂

Inverting Transient Absorption Data to Determine Transfer Rates in Quantum Dot–TiO₂ Heterostructures

Transient absorption spectroscopy is a powerful technique for understanding charge carrier dynamics and recombination pathways. Analyzing the results is not trivial due to nonexponential relaxation dynamics away from equilibrium, leading to a disparity in reported charge-transfer rates. An inversion analysis technique is presented that transforms transient signals back into their original rate equation. The technique is demonstrated on two CdSe/TiO₂ heterostructures with different surface states. Auger recombination is identified at higher carrier densities and radiative recombination at lower carrier densities. The heterostructure with additional surface traps exhibits increased trap-state Auger recombination at high carrier densities and changes to radiative recombination at low carrier densities due to a Shockley–Read–Hall process. Carrier-dependent electron-transfer rates are determined and compared to common methods that only capture the magnitude of the charge transfer at specific carrier densities. The presented transient absorption analysis provides direct understanding of the recombination mechanisms with minimal additional analysis or with presumption of decay mechanisms

Electrocatalytic Oxygen Evolution with an Atomically Precise Nickel Catalyst

The electrochemical oxygen evolution reaction (OER) is an important anodic process in water splitting and CO₂ reduction applications. Precious metals including Ir, Ru. and Pt are traditional OER catalysts, but recent emphasis has been placed on finding less expensive, earth-abundant materials with high OER activity. Ni-based materials are promising next-generation OER catalysts because they show high reaction rates and good long-term stability. Unfortunately, most catalyst samples contain heterogeneous particle sizes and surface structures that produce a range of reaction rates and rate-determining steps. Here we use a combination of experimental and computational techniques to study the OER at a supported organometallic nickel complex with a precisely known crystal structure. The Ni₆(PET)₁₂ (PET = phenylethyl thiol) complex out performed bulk NiO and Pt and showed OER activity comparable to Ir. Density functional theory (DFT) analysis of electrochemical OER at a realistic Ni₆(SCH₃)₁₂ model determined the Gibbs free energy change (Δ<i>G</i>) associated with each mechanistic step. This allowed computational prediction of potential determining steps and OER onset potentials that were in excellent agreement with experimentally determined values. Moreover, DFT found that small changes in adsorbate binding configuration can shift the potential determining step within the OER mechanism and drastically change onset potentials. Our work shows that atomically precise nanocatalysts like Ni₆(PET)₁₂ facilitate joint experimental and computational studies because experimentalists and theorists can study nearly identical systems. These types of efforts can identify atomic-level structure–property relationships that would be difficult to obtain with traditional heterogeneous catalyst samples