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Hybrid structures combining a photo-active material and non noble metal oxide co-catalyst nanoparticles are of strong interest to produce efficient photoelectrodes for hydrogen production while limiting the cost due to noble metals (Pt, Ru …). In order to get some insight on the mechanism change induced by such co-catalyst nanoparticles (NPs), we conducted a comparative study between cobalt and nickel oxide nanoparticles deposited onto TiO2 nanotubes (NTs) prepared through anodization. The TiO2-NTs were decorated with CoO and NiO nanoparticles using the reactive pulsed laser deposition (PLD) method. The oxygen atmosphere used during the PLD process, ensures the in-situ oxidation of the metal nanoparticles into CoO and NiO. The surface loadings of CoO or NiO NPs were controlled by the number of laser ablation pulses (NLP). The efficiency of CoO and NiO NPs as co-catalysts for photo-electrochemical water splitting was studied by cyclic voltammetry, as a function of NLP, under both simulated sunlight and visible light illuminations and by external quantum efficiency (EQE) measurements. We determined an optimal catalyst loading at NLP = 2000, allowing to almost triple the photoconversion efficiency (PCE) for both TiO2-NTs/CoO-NPs and TiO2-NTs/NiO-NPs nanohybrid photoanodes, in comparison with bare TiO2-NTs. Moreover, CoO-NPs were found to be the best co-catalyst under visible light illumination, with a PCE almost 10 times higher than for TiO2. This significant enhancement of the visible light activity is ascribed to the narrower band gap of CoO and to the formation of a heterojunction with TiO2 that is more favorable for charge transfer than in the case of NiO co-catalyst.

Co-alloyed (M,N) titanium dioxide nanotubes were synthesized by a simple anodization process with M-Ti alloys (M = Ta or Nb), followed by a thermal treatment in ammonia to introduce nitrogen in those nanostructure. The photo-electrochemical performances of these co-alloyed samples was compared with the ones of N doped and undoped TiO 2 NTs. Different conditions of thermal treatment under ammonia were studied in order to control the amount of nitrogen introduced in the TiO 2 structure and tend to achieve a balance of charges between Ta 5+ /Nb 5+ cations and N 3- anions. The structure and composition of these materials were characterized by X-Ray Diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS) and Transmission Electron Microscopy (TEM) EDX mapping. This combination of methods confirm the successful introduction of doping atoms, and allowed us to estimate the amount of M 5+ cations and N 3- anions in the TiO 2 NTs. TEM EDX mapping indicates a heterogeneous distribution of Nb 5+ and Ta 5+ cations in the nanotubes that can originate from the use of highly concentrated alloys. External quantum efficiency measurements were used to determine the photo-electrochemical activity of our samples in different spectral domains. A significant improvement of activity in the visible region of the solar spectra was observed for co-alloyed TiO 2 samples when compared to undoped nanotubes but also when compared to N doped TiO 2 . The obtained results indicate that the parameters of the NH 3 thermal treatments should be finely controlled to improve the conversion efficiency in the visible domain for the co-alloyed samples by introducing a precise amount of N 3- in substitution of oxygen in the TiO 2 lattice and to avoid the damaging of the nanotubular structure.

The pulsed laser deposition (PLD) technique has been used to decorate TiO2 nanotubes (NTs) with cobalt-nickel (CoNi) nanoparticles (NPs). The TiO2 NTs were produced beforehand through the controlled anodic oxidation of titanium substrates. The effect of the nature of the PLD background gas (Vacuum, O2 and He) on the microstructure, composition and chemical bondings of the CoNi-NPs deposited onto the TiO2-NTs has been investigated. We found that the PLD CoNi-NPs have a core/shell (oxide/metal) structure when deposited under vacuum, while they are fully oxidized when deposited under O2. On the other hand, by varying the CoNi-NPs loading of the TiO2-NTs (through the increase of the number of laser ablation pulses (NLP)), we have systematically studied their photocatalytic effect by means of cyclic-voltammetry (CV) measurements under both AM1.5 simulated solar light and filtered visible light. We show that depositing CoNi-NPs on the substrate under vacuum and He increases the photo-electrochemical conversion effectiveness (PCE) by 600% (at NLP = 10,000) in the visible light domain, while their overall PCE degrades with NLP under solar illumination. In contrast, the fully oxidized CoNi-NPs (deposited under O2) are found to be the most effective catalyst under sunlight with an overall increase of more than 50% of the PCE at the optimum loading around NLP ~1000. Such catalytic enhancement is believed to result from both an enhanced light absorption by CoO (of which bandgap is of ~2.4 eV) and the formation of a heterojunction between NiO/CoO nanoparticles and TiO2 nanotubes.