Ayyappan Ramakrishnan

Visible-light photocurrent response of TiO2–polyheptazine hybrids: evidence for interfacial charge-transfer absorption

We investigated photoelectrodes based on TiO(2)-polyheptazine hybrid materials. Since both TiO(2) and polyheptazine are extremely chemically stable, these materials are highly promising candidates for fabrication of photoanodes for water photooxidation. The properties of the hybrids were experimentally determined by a careful analysis of optical absorption spectra, luminescence properties and photoelectrochemical measurements, and corroborated by quantum chemical calculations. We provide for the first time clear experimental evidence for the formation of an interfacial charge-transfer complex between polyheptazine (donor) and TiO(2) (acceptor), which is responsible for a significant red shift of absorption and photocurrent response of the hybrid as compared to both of the single components. The direct optical charge transfer from the HOMO of polyheptazine to the conduction band edge of TiO(2) gives rise to an absorption band centered at 2.3 eV (540 nm). The estimated potential of photogenerated holes (+1.7 V vs. NHE, pH 7) allows for photooxidation of water (+0.82 V vs. NHE, pH 7) as evidenced by visible light-driven (λ > 420 nm) evolution of dioxygen on hybrid electrodes modified with IrO(2) nanoparticles as a co-catalyst. The quantum-chemical simulations demonstrate that the TiO(2)-polyheptazine interface is a complex and flexible system energetically favorable for proton-transfer processes required for water oxidation. Apart from water splitting, this type of hybrid materials may also find further applications in a broader research area of solar energy conversion and photo-responsive devices.

Visible‐Light Photooxidation of Water to Oxygen at Hybrid TiO2–Polyheptazine Photoanodes with Photodeposited Co‐Pi (CoOx) Cocatalyst

A cobalt oxide-based oxygen-evolving cocatalyst (Co-Pi) is photodeposited by visible-light irradiation onto nanocrystalline TiO(2)-polyheptazine (TiO(2)-PH) hybrid photoelectrodes in a phosphate buffer. The Co-Pi cocatalyst couples effectively to photoholes generated in the surface polyheptazine layer of the TiO(2)-PH photoanode, as evidenced by complete photooxidation of water to oxygen under visible-light (λ>420 nm) irradiation at moderate bias potentials. In addition, the presence of the cocatalyst also reduces significantly the recombination of photogenerated charges, particularly at low bias potentials, which is ascribed to better photooxidation kinetics resulting in lower accumulation of holes. This suggests that further improvements of photoconversion efficiency can be achieved if more effective catalytic sites for water oxidation are introduced to the surface structure of the hybrid photoanodes.

TiO2-Polyheptazine Hybrid Photoelectrodes: Dynamics of Photogenerated Holes

The dynamics of visible-light photogenerated holes in nanocrystalline photoelectrodes based on TiO2-polyheptazine (TiO2-PH) was investigated by polychromatic and wavelengthresolved photocurrent measurements performed at different potentials, different pH, and in the presence of various hole scavengers. The evaluation of the hole reactivity was addressed by direct comparison to photoelectrodes based on pristine TiO2. In aqueous electrolytes the TiO2-PH photoelectrodes exhibit IPCE values of ~ 2% at 450 nm. As compared to TiO2, the visible-light generated holes in TiO2-PH are located in the surface polyheptazine layer and possess a lower oxidation potential. Due to their slow water oxidation kinetics, they accumulate under irradiation at the surface. This leads to increased recombination unless sufficiently high positive bias is applied or a more readily oxidizable hole scavenger is present. The results suggest that highly efficient water splitting on TiO2-PH hybrid photoelectrodes can only be achieved after introducing additional catalytic sites into the polyheptazine component.