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.

The efficient coupling between light-harvesting absorbers and cocatalysts allowing for chemical transformation along multielectron pathways is of fundamental importance for the development of solar-fuel-producing photochemical systems. Herein we demonstrate that IrO x nanoparticles acting as efficient cocatalyst for water oxidation can be photoelectrochemically deposited from hexahydroxoiridate solutions into the porous structure of TiO 2 -PH (polyheptazine, "graphitic carbon nitride") hybrid photoanodes for water photooxidation. As compared to photoanodes loaded with IrO x by the conventional colloidal deposition method, hybrid photoanodes with photodeposited IrO x exhibit significantly enhanced dioxygen evolution under long-term irradiation with visible light (λ > 420 nm). Photocurrent transient measurements show that the undesired accumulation of holes in the TiO 2 -PH absorber is significantly reduced due to improved coupling between the absorber and the photodeposited cocatalyst. This decreases significantly the recombination rate, leads to more efficient dioxygen evolution, and improves the stability against photocorrosion. Photocurrent measurements under potentiodynamic conditions revealed that at low bias potentials (<0.6 V vs RHE) the photoconversion efficiency of hybrid photoanodes is limited by fast primary back electron transfer and by reduction of Ir(IV) to Ir(III). The performance and stability of hybrid photoanodes are also found to be drastically influenced by the solution chemistry (electrolyte composition and pH). The highest photoconversion efficiency was observed in sulfate-based electrolytes at pH ∼6.

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.

N-doped titanium dioxide (TiO 2 ) thin films are grown on Si(100) and indium tin oxide (ITO)-coated borosilicate glass substrates by metal-organic (MO)CVD. The intrinsic doping of TiO 2 thin films is achieved using all-nitrogen-coordinated Ti precursors in the presence of oxygen. The titanium amide-guanidinate complex, [Ti(NMe 2 ) 3 (guan)] (guan ¼ N,N 0 -diisopropyl-2-dimethylamidoguanidinato) has been developed to compensate for the thermal instability of the parent alkylamide [Ti(NMe 2 ) 4 ]. Both of these amide-based compounds are tested and compared as precursors for intrinsically N-doped TiO 2 at various deposition temperatures in the absence of additional N sources. The structure and morphology of TiO 2 thin films are characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM). Rutherford back scattering (RBS), nuclear reaction analysis (NRA), and secondary ion mass spectrometry (SIMS) analyses are performed to determine N content and distribution in the films. The optical and photoelectrochemical properties of TiO 2 thin films on ITO substrates are also examined. N-doped TiO 2 thin films, grown from [Ti(NMe 2 ) 3 (guan)] at 600 8C, exhibit the lowest optical absorption edge (3.0 eV) and the highest visible light photocurrent response. When compared to undoped TiO 2 , while in UV light photoconversion efficiency decreases significantly, the intrinsically N-doped TiO 2 shows enhanced photocurrents under visible light irradiation.