Crystal phase engineering of Ru for simultaneous selective photocatalytic oxidations and H2 production.

Noble metal nanoparticles are often used as cocatalysts to enhance the photocatalytic efficiency. While the effect of cocatalyst nanoparticle size and shape has widely been explored, the effect of the crystal phase is largely overlooked. In this work, we investigate the effect of Ru nanoparticle crystal phase, specifically regular hexagonal close-packed (hcp) and allotropic face-centered cubic (fcc) crystal phases, as cocatalyst decorated onto the surface of TiO2 photocatalysts. As reference photocatalytic reaction the simultaneous photocatalytic production of benzaldehyde (BAD) and H2 from benzyl alcohol was chosen. Both the fcc Ru/TiO2 and hcp Ru/TiO2 composites exhibit enhanced BAD and H2 production rates compared to pristine TiO2 due to the formation of a Schottky barrier promoting the photogenerated charge separation. Moreover, a 1.9-fold photoactivity enhancement of the fcc Ru/TiO2 composite is achieved as compared to the hcp Ru/TiO2 composite, which is attributed to the fact that the fcc Ru NPs are more efficient in facilitating the charge transfer as compared to hcp Ru NPs, thus inhibiting the recombination of electron-hole pairs and enhancing the overall photoactivity.

NaBH4 induces a high ratio of Ni3+/Ni2+ boosting OER activity of the NiFe LDH electrocatalyst.

Electrochemical water splitting is a promising way to produce hydrogen gas, but the sluggish kinetics of the oxygen evolution reaction (OER) extremely restrict the overall conversion efficiency of water splitting. Transition metal based LDHs (TM LDHs) are one of the most effective non-noble metal OER catalysts and have attracted wide interest, especially the nickel-iron LDH (NiFe LDH). The high valence Ni3+ species with a large coordination number play a vital role in OER catalysis. Herein, we report on a surprising discovery that reaction between NiFe LDH and NaBH4 with multi-hydrides induces vacancy formation around Fe3+ and enrichment in Ni3+, crucially activating the OER performance. The ratio of Ni3+/Ni2+ is found to be closely tied to the OER performance, nicely accounting for the leading role of Ni3+ ions in octahedral sites in electrocatalysis. Significantly, the NaBH4 treated NiFe LDH directly on nickel foam (NF), denoted as NaBH4-NiFe LDH@NF exhibited an outstanding OER performance with an overpotential of only 310 mV at 100 mA cm-2, and a Tafel slope of 47 mV dec-1. For the series of TM LDHs we studied with different metal combinations, the high valence metal ion is found to be positively related to OER performance.

NiMn compound nanosheets for electrocatalytic water oxidation: effects of atomic structures and oxidation states.

Although Mn-based oxygen evolution clusters in photosystem II show efficient activity in water oxidation, the catalytic performance of artificial Mn-based electrocatalysts is far from satisfactory, which is probably due to the undesirable atomic structure and electronic arrangement of their Mn ions. Aiming to systematically study the performance of two-dimensional (2D) catalysts, we designed and synthesized a series of nanosheets, including NiMn LDH and Ni-birnessite and their morphology-retained annealing products NiMnOx-L and NiMnOx-B, respectively. We comprehensively compared the OER performance of these 2D electrocatalysts in conjunction with the information on their crystalline phases, electronic conductivity, electrochemical surface area and oxidation states of their transition metal ions. It was found that the annealing-converted NiMnOx exhibited 3- and 5-times higher concentrations of catalytically active Mn(iii) than the corresponding NiMn LDH and Ni-birnessite precursors. Moreover, the layered atomic structure was beneficial for the charge transfer, leading to faster reaction kinetics. Among the nanosheets tested, NiMnOx-B showed the best alkaline OER performance with the lowest overpotential and the smallest Tafel slope because it not only retained the layered atomic structure and the 2D nanosheet morphology of the Ni-birnessite precursor, but also benefitted from the decreased interlayer distance and more Mn(iii) species. This work sheds light on the design of effective non-noble metal-based electrocatalysts towards water oxidation for hydrogen production.

Dispersing transition metal vacancies in layered double hydroxides by ionic reductive complexation extraction for efficient water oxidation.

Creating atomic defects in nanomaterials is an effective approach to promote the catalytic performance of a catalyst, but the defective catalysts are often prone to mechanical collapse if not properly synthesized. The uncontrollably formed defects also make it difficult to systematically investigate their effects on the catalytic performance. Herein, we report an efficient method of ionic reductive complexation extraction (IRCE) to fabricate atomic vacancies in a transition metal based nanomaterial without damaging its nanostructure, turning the otherwise catalytically inactive material to an advanced catalyst towards water oxidation in alkaline electrolyte. Here nickel based layered double hydroxide mixed with Cu(ii) is used to demonstrate the concept. With a tunable content and uniform dispersion of Cu(ii) on the brucite layer of the LDH, a suitable complexing agent could specifically combine with and remove the target Cu(ii), thereby creating the desired vacancies. The resulting vacancy rich TM LDH is found to be an excellent OER electrocatalyst with a low overpotential and small Tafel slope, due to the purposely modulated geometric and electronic structures of the active sites, and the greatly decreased charge transfer resistance.