Photoinduced H₂ Evolution by Hexaniobate Sheets Grafted with Metal Ions: The Fate of Photogenerated Carriers

Abstract

Layered niobates are well-known photocatalysts for H2 evolution with a rich surface chemistry. Their photoactivity, however, is limited by their wide band gap energy (∼3.5 eV) and partial deactivation due to surface poisoning by photogenerated H2O2. In this way, a surface modification method able to induce novel electronic processes without changing the bulk properties can improve their performance. In this work, the surface of exfoliated hexaniobate (K4–x H x Nb6O17) layers was modified by grafting with metal ions such as Co­(II) and Fe­(III) and their photocatalytic properties were fully investigated. Morphological characterization showed that grafting ions are attached to the niobate surface forming amorphous clusters. These species induce an additional absorption feature in the UV-A region, which is attributed to an interfacial charge transfer from the niobate valence band to the metal ion centers. Enhanced UV-driven photoactivity was observed for 0.1% grafted samples, especially for those modified with Co­(II) ions, while smaller H2 evolution rates are observed as the concentration of the grafting ions increases. When Pt was added to the photocatalyst, the H2 evolution rate for the 0.1% Co-grafted sample in plain water was 70% higher than that observed for the nongrafted Pt-hexaniobate. Full characterization by electron paramagnetic resonance, transient absorption spectroscopy, and photoelectrochemical measurements reveals that grafted ions can work as both electron and hole acceptors. In the presence of Pt as a preferential electron acceptor, Co­(II) ions act as hole acceptors forming Co­(III) centers, favoring the formation of OH• radicals from water and avoiding surface poisoning. At higher Co­(II) concentrations and in the absence of Pt clusters, electrons are trapped at the Co centers, decreasing the H2 evolution rate. Thus, grafted Co­(II) ions act as active redox shuttles in the hexaniobate sheets, contributing to more efficient charge separation.

Efficient Photocatalytic H2 Evolution by Hexaniobate Nanosheets Grafted with Copper Nanoclusters

“…Additionally, amorphous clusters could be observed on the surface of the grafted sample, as highlighted by the arrows, which are not present on the pristine hexaniobate. Such nanoclusters have been observed by our group for Co(II) and Fe(III)-grafted hexaniobates [12] and also for TiO 2 nanoparticles grafted with Cu(II) ions. [22,23] Therefore, it seems most of Cu + 2 ions remains at the surface to form amorphous clusters of around 10 nm length.…”

“…As described previously, [12,18] potassium hexaniobate prepared by solid state reaction exhibits a structure corresponding to K 4 Nb 6 O 17 .3H 2 O (JCPDS 21-1297). Following exfoliation with tetrabutylammonium hydroxide, an increase in the surface area is observed, but the crystallinity is lost as the well-ordered stacked layers became randomly organized.…”

“…Following exfoliation with tetrabutylammonium hydroxide, an increase in the surface area is observed, but the crystallinity is lost as the well-ordered stacked layers became randomly organized. The exfoliated layers remain in suspension and were precipitated by acidification as reported elsewhere, [12,18] resulting in hexaniobate sheets free of ammonium ions. Exfoliated hexaniobate nanosheets (Bare Nb) and the sample modified by grafting with 1 wt % Cu 2 + have the same XRD profile and no additional peaks related to the Cu 2 + species were observed (Figure S1).…”

“…Such limitations can be overcome by chemical modification of its interlayer spaces as well as by surface modification. Previously, we have reported hexaniobate nanosheets grafted with Co 2+ and Fe 3+ ions aiming at improving their optical and surface properties [12] . The presence of 0.1 % Co 2+ on the hexaniobate surface could support the H 2 production in water when coupled with Pt as cocatalyst, thus avoiding self‐poisoning.…”

Flux-Assisted Low Temperature Synthesis of SnNb2O6 Nanoplates with Enhanced Visible Light Driven Photocatalytic H2-Production

“…Recently, layered niobates have gained a great deal of attention in photocatalytic water splitting owning to their various electronic and structural features and high stability. − Foordite SnNb 2 O 6 as a typical layered niobate has gained great attention in photocatalytic applications due to its suitable band gap (∼2.3 eV) and proper band location. − To date, various methodologies have been adopted to prepare nanostructured SnNb 2 O 6 , such as the hydrothermal method, , the microwave-assisted hydrothermal method, the solvothermal method, and the flux-assisted method. , Wu et al demonstrated that ultrathin SnNb 2 O 6 nanosheets prepared by a microwave-assisted hydrothermal method exhibited excellent visible light driven photocatalytic performance for degradation of organic pollution . Unfortunately, the aforementioned hydrothermal method toward the synthesis of SnNb 2 O 6 nanosheets required the use of the environmentally unfriendly solvent hydrofluoric acid (HF).…”