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.

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.

No abstract

The dynamics of visible-light photogenerated holes in nanocrystalline TiO 2 -polyheptazine (TiO 2 -PH) hybrid photoelectrodes for water photooxidation was investigated by polychromatic and wavelength-resolved photocurrent measurements. The evaluation of the hole reactivity was addressed by direct comparison to photoelectrodes based on pristine TiO 2 . The visible-light generated holes in TiO 2 -PH are located in the thin polyheptazine ("graphitic carbon nitride") layer at the surface of TiO 2 and possess a lower oxidation potential (by ∼0.9 V) as compared to UV light-photogenerated holes in pristine TiO 2 . Due to their slow water oxidation kinetics, the photoholes accumulate at the surface, which leads to negligible oxygen evolution and increased recombination. This problem can be overcome by introducing a suitable co-catalyst (IrO 2 nanoparticles), as evidenced by dioxygen evolution under visible light (λ > 420 nm) irradiation.

The first catalytic photosulfoxidation of alkanes is accomplished in the presence of titanium dioxide and visible light (λ≥400 nm) under an atmosphere of SO2/O2. For n‐heptane and cyclohexane the reaction is performed in the neat liquid, for adamantane in glacial acetic acid. Charge‐transfer (CT) complexation of sulfur dioxide by the titania surface generates a CT band at 400–420 nm responsible for the visible‐light activity of otherwise only UV light absorbing titania. The primary charges generated upon optical electron transfer produce alkyl radicals by dissociative electron transfer from the alkane and by hydrogen abstraction by OH radicals produced from oxygen reduction. Once formed, the alkyl radicals initiate a radical chain reaction as known from the classical UV‐induced sulfoxidation in the absence of a catalyst. The reaction exhibits features characteristic for product inhibition by strong adsorption. Accordingly, the initial photocatalytic activity is fully restored after washing the catalyst with methanol. Time‐resolved photovoltage measurements indicate that photocatalyst deactivation is connected with a change from n‐type to p‐type titanium dioxide. Small amounts of water and radical scavengers inhibit product formation. The reaction proceeds with high chemoselectivity because only traces of expected by‐products like sulfates, ketones, and alcohols are formed. Thus, in addition to its basic role in visible‐light‐induced charge generation, the surface of titania enables also a chemoselective CS bond formation.