CO₂ Reduction to Methanol on TiO₂‑Passivated GaP Photocatalysts

In the past, the electrochemical instability of III–V semiconductors has severely limited their applicability in photocatlaysis. As a result, a vast majority of the research on photocatalysis has been done on TiO₂, which is chemically robust over a wide range of pH. However, TiO₂ has a wide band gap (3.2 eV) and can only absorb ∼4% of the solar spectrum, and thus, it will never provide efficient solar energy conversion/storage on its own. Here, we report photocatalytic CO₂ reduction with water to produce methanol using TiO₂-passivated GaP photocathodes under 532 nm wavelength illumination. The TiO₂ layer prevents corrosion of the GaP, as evidenced by atomic force microscopy and photoelectrochemical measurements. Here, the GaP surface is passivated using a thin film of TiO₂ deposited by atomic layer deposition (ALD), which provides a viable, stable photocatalyst without sacrificing photocatalytic efficiency. In addition to providing a stable photocatalytic surface, the TiO₂ passivation provides substantial enhancement in the photoconversion efficiency through passivation of surface states, which cause nonradiative carrier recombination. In addition to passivation effects, the TiO₂ deposited by ALD is n-type due to oxygen vacancies and forms a pn-junction with the underlying p-type GaP photocathode. This creates a built-in field that assists in the separation of photogenerated electron–hole pairs, further reducing recombination. This reduction in the surface recombination velocity (SRV) corresponds to a shift in the overpotential of almost 0.5 V. No enhancement is observed for TiO₂ thicknesses above 10 nm, due to the insulating nature of the TiO₂, which eventually outweighs the benefits of passivation

Microscopic Study of Atomic Layer Deposition of TiO₂ on GaAs and Its Photocatalytic Application

We report a microscopic study of <i>p</i>-GaAs/TiO₂ heterojunctions using cross-sectional high resolution transmission electron microscopy (HRTEM). The photocatalytic performance for both H₂ evolution and CO₂ reduction of these heterostructures shows a very strong dependence on the thickness of the TiO₂ over the range of 0–15 nm. Thinner films (1–10 nm) are amorphous and show enhanced catalytic performance with respect to bare GaAs. HRTEM images and electron energy loss spectroscopy (EELS) maps show that the native oxide of GaAs is removed by the TiCl₄ atomic layer deposition (ALD) precursor, which is corrosive. Ti³⁺ defect states (i.e., O vacancies) in the TiO₂ film provide catalytically active sites, which improve the photocatalytic efficiency. Density functional theory (DFT) calculations show that water molecules and CO₂ molecules bind stably to these Ti³⁺ states. Thicker TiO₂ films (15 nm) are crystalline and have poor charge transfer due to their insulating nature, while thinner amorphous TiO₂ films are conducting

Artificial Photosynthesis on TiO₂‑Passivated InP Nanopillars

Here, we report photocatalytic CO₂ reduction with water to produce methanol using TiO₂-passivated InP nanopillar photocathodes under 532 nm wavelength illumination. In addition to providing a stable photocatalytic surface, the TiO₂-passivation layer provides substantial enhancement in the photoconversion efficiency through the introduction of O vacancies associated with the nonstoichiometric growth of TiO₂ by atomic layer deposition. Plane wave-density functional theory (PW-DFT) calculations confirm the role of oxygen vacancies in the TiO₂ surface, which serve as catalytically active sites in the CO₂ reduction process. PW-DFT shows that CO₂ binds stably to these oxygen vacancies and CO₂ gains an electron (−0.897e) spontaneously from the TiO₂ support. This calculation indicates that the O vacancies provide active sites for CO₂ absorption, and no overpotential is required to form the CO₂^– intermediate. The TiO₂ film increases the Faraday efficiency of methanol production by 5.7× to 4.79% under an applied potential of −0.6 V vs NHE, which is 1.3 V below the <i>E</i>^o(CO₂/CO₂^–) = −1.9 eV standard redox potential. Copper nanoparticles deposited on the TiO₂ act as a cocatalyst and further improve the selectivity and yield of methanol production by up to 8-fold with a Faraday efficiency of 8.7%

Artificial Photosynthesis on TiO₂‑Passivated InP Nanopillars

Here, we report photocatalytic CO₂ reduction with water to produce methanol using TiO₂-passivated InP nanopillar photocathodes under 532 nm wavelength illumination. In addition to providing a stable photocatalytic surface, the TiO₂-passivation layer provides substantial enhancement in the photoconversion efficiency through the introduction of O vacancies associated with the nonstoichiometric growth of TiO₂ by atomic layer deposition. Plane wave-density functional theory (PW-DFT) calculations confirm the role of oxygen vacancies in the TiO₂ surface, which serve as catalytically active sites in the CO₂ reduction process. PW-DFT shows that CO₂ binds stably to these oxygen vacancies and CO₂ gains an electron (−0.897e) spontaneously from the TiO₂ support. This calculation indicates that the O vacancies provide active sites for CO₂ absorption, and no overpotential is required to form the CO₂^– intermediate. The TiO₂ film increases the Faraday efficiency of methanol production by 5.7× to 4.79% under an applied potential of −0.6 V vs NHE, which is 1.3 V below the <i>E</i>^o(CO₂/CO₂^–) = −1.9 eV standard redox potential. Copper nanoparticles deposited on the TiO₂ act as a cocatalyst and further improve the selectivity and yield of methanol production by up to 8-fold with a Faraday efficiency of 8.7%