Electron acceptor of Ni decorated porous carbon nitride applied in photocatalytic hydrogen production.

Nickel, a non-noble metal, is one of the most promising candidates for photocatalysis because it is inexpensive and an earth-abundant metal. Herein, Ni/CM-C3N4 nanocomposites with Ni as a cocatalyst were synthesized by a simple solvothermal method. Field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD) confirmed that Ni nanoparticles were loaded onto the surface of CM-C3N4. The prepared Ni/CM-C3N4 nanocomposites exhibited an enhanced hydrogen evolution activity. The most active catalyst contained 10% Ni and produced H2 at a rate of 313.2 µmol h-1 g-1, which was obviously higher than that of pure CM-C3N4. The results of photoluminescence (PL) and photoacoustics (PA) studies indicated that the recombination efficiency of photo-induced electron-hole pairs was decreased for CM-Ni10 as compared to that for unmodified CM-C3N4. The transient photovoltage (TPV) measurements directly demonstrated that the recombination time of electron-hole pairs in CM-Ni10 was prolonged. More importantly, the reversed surface photovoltage (SPV) and the declined surface photocurrent (SPC) response of CM-Ni10 revealed that the photogenerated electrons could be trapped by Ni, leading to a better separation efficiency and a superior hydrogen production. Finally, the possible mechanism is proposed to illuminate the photogenerated charge behavior between CM-C3N4 and Ni, which might provide a theoretical basis to develop efficient cocatalysts for photocatalytic water splitting.

A Ni(OH)2-modified Ti-doped α-Fe2O3 photoanode for improved photoelectrochemical oxidation of urea: the role of Ni(OH)2 as a cocatalyst.

For semiconductor-based PEC systems, loading an appropriate cocatalyst on a semiconductor (such as a solar-active material) can significantly improve the PEC activity due to the suppression of photogenerated charge recombination. But there is little direct information about the role of a cocatalyst in the spatial separation of photogenerated charge carriers. In our work, a combination of surface photovoltage spectroscopy (SPS), transient photovoltage (TPV) technique, photoelectrochemical impedance spectroscopy (PEIS) and transient photocurrent measurements was used to study the real role of Ni(OH)2 as a cocatalyst for the enhanced PEC performance of Ni(OH)2-modified Ti-doped α-Fe2O3. It was found that Ni(OH)2 as a hole storage layer enhances the separation of photogenerated charge carriers and increases the lifetime of holes, which contributed to the enhanced photocurrent. In addition, Ni(OH)2 is a good cocatalyst for urea oxidation which suppresses the over-potential, resulting in a negative shift of the onset potential.

Photoelectrochemical Performance for Water Oxidation Improved by Molecular Nickel Porphyrin-Integrated WO3 /TiO2 Photoanode.

A WO3 /TiO2 heterojunction photoanode was prepared by in situ growth of WO3 on a mesoporous TiO2 electrode. The photoinduced charge-transfer properties and chargeseparation improvement in this kind of type-II heterojunction were characterized by transient surface photovoltage spectra. By using sulfite oxidation as a hole scavenger, we demonstrated that 72 % of the photo-generated holes are reaching the surface of the photoanode, but the efficiency of hole injection (ηox ) into the electrolyte was only 48 %. For the first time, a NiII meso-tetra(4-carboxyphenyl)porphyrin (NiTCPP) was incorporated as a water oxidation catalyst into the WO3 /TiO2 heterojunction photoanode, which promoted the value of ηox to 81 %. The maximum applied bias photon-to-current efficiency for the WO3 /TiO2 /NiTCPP photoanode was determined to be 0.2 % at 1.01 V vs. the reversible hydrogen electrode (RHE), under which condition a Faradic efficiency of 89 % for water oxidation was achieved (averaged over 1 h of photolysis).