Pt Single Atoms on TiO 2 Polymorphs—Minimum Loading with a Maximized Photocatalytic Efficiency

For more than 20 years, Pt/TiO$_2$ represents the benchmark photocatalyst/co-catalyst platform for photocatalytic hydrogen (H$_2$) generation. Here, single atom (SA) Pt is decorated on different polymorphs of TiO$_2$ (anatase, rutile, and the mixed phase of P25) using a simple immersion anchoring approach. On P25 and anatase, Pt SAs act as highly effective co-catalyst for pure water splitting with a photocatalytic H$_2$ evolution activity (4600 µmol h$^{−1}$ g$^{−1}$)—on both polymorphs, SA deposition yields a significantly more active photocatalyst than those decorated with classic Pt nanoparticles or conventional SA deposition approaches. On rutile, Pt SAs provide hardly any co-catalytic effect. Most remarkable, for P25, the loading of Pt SAs from precursor solution with a very low concentration (<1 ppm Pt) leads already to a maximized co-catalytic effect. This optimized efficiency is obtained at 5.3 × 10$^{5}$ atoms µm$^{−2}$ (at macroscopic loading of 0.06 at%)—for a higher concentration of Pt (a higher density of SAs), the co-catalytic efficiency is significantly reduced due to H$_2$/O$_2$ recombination. The interactions of the SA Pt with the different polymorphs that lead to this high co-catalytic activity of SA Pt at such low concentrations are further discussed

Ultrathin hierarchical porous carbon nanosheets for high-performance supercapacitors and redox electrolyte energy storage

ICN2 is funding from the CERCA Programme/Generalitat de CatalunyaThe design of advanced high-energy-density supercapacitors requires the design of unique materials that combine hierarchical nanoporous structures with high surface area to facilitate ion transport and excellent electrolyte permeability. Here, shape-controlled 2D nanoporous carbon sheets (NPSs) with graphitic wall structure through the pyrolysis of metal-organic frameworks (MOFs) are developed. As a proof-of-concept application, the obtained NPSs are used as the electrode material for a supercapacitor. The carbon-sheet-based symmetric cell shows an ultrahigh Brunauer-Emmett-Teller (BET)-area-normalized capacitance of 21.4 µF cm (233 F g), exceeding other carbon-based supercapacitors. The addition of potassium iodide as redox-active species in a sulfuric acid (supporting electrolyte) leads to the ground-breaking enhancement in the energy density up to 90 Wh kg, which is higher than commercial aqueous rechargeable batteries, maintaining its superior power density. Thus, the new material provides a double profits strategy such as battery-level energy and capacitor-level power densit

Metastable Ni(I)-TiO _2-x Photocatalysts: Self-Amplifying H₂ Evolution from Plain Water without Noble Metal Co-Catalyst and Sacrificial Agent

Decoration of semiconductor photocatalysts with cocatalysts is generally done by a step-by-step assembly process. Here, we describe the self-assembling and self-activating nature of a photocatalytic system that forms under illumination of reduced anatase TiO2 nanoparticles in an aqueous Ni2+ solution. UV illumination creates in situ a Ni+/TiO2/Ti3+ photocatalyst that self-activates and, over time, produces H-2 at a higher rate. In situ X-ray absorption spectroscopy and electron paramagnetic resonance spectroscopy show that key to self-assembly and self-activation is the light-induced formation of defects in the semiconductor, which enables the formation of monovalent nickel (Ni+) surface states. Metallic nickel states, i.e., Ni-0, do not form under the dark (resting state) or under illumination (active state). Once the catalyst is assembled, the Ni+ surface states act as electron relay for electron transfer to form H-2 from water, in the absence of sacrificial species or noble metal cocatalysts.Web of Science14548261322612

Conductive Cu doped TiO2 nanotubes for enhanced photoelectrochemical methanol oxidation and concomitant hydrogen generation

Cu-doping in titania is usually detrimental to the material’s photoconductivity which prevents the use of this combination in photoanodes. In this work, we produce TiO2 nanotube arrays intrinsically doped with copper and establish sufficient conductivity to use them as efficient photoanodes for methanol oxidation in a photoelectrochemical hydrogen generation setting. Firstly, Cu doped TiO2 nanotubes were produced by anodizing a Ti-Cu binary alloy. By subsequent thermal reduction of the structure in an Ar/H2 environment, conductive copper doped TiO2 nanotubes (TiCuTN-Ar/H2) can be achieved with an approximately 103 times higher conductivity than the non-reduced material. When these reduced Cu doped TiO2 nanotubes are used as photoanode, copper species embedded in the TiO2 wall catalyze the methanol oxidation reaction. As a result of the combined effect of conductivity and catalytic effect of Cu, such a reduced Cu:TiO2 nanotubes can generate a photocurrent of 0.76 mA.cm-2 at 1 V vs. RHE, under AM1.5 (100 mW/Cm2) irradiation – in a 50:50 MeOH/water solution – this is 33 times higher than for pristine Cu:TiO2 nanotubes

Intrinsic Cu nanoparticle decoration of TiO2 nanotubes: A platform for efficient noble metal free photocatalytic H2 production

In this work, we grow intrinsically Cu-doped TiO2 nanotubes (TiNTs) by self-organizing anodization of Ti–Cu binary alloys. We demonstrate that up to a copper concentration of 1.5 at.% in the alloy, self-ordered Cu2+-doped nanotubes can be grown. Under UV illumination the Cu2+ ion-doped oxide structures can be converted to nanotubes that carry metallic nanoparticles (NPs) uniformly decorated on top of the TiNTs. We investigate the formation of these metallic nanoparticles under UV illumination by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR). The resulting intrinsic copper-doped and decorated TiNTs have a strongly enhanced photocatalytic activity for H2 evolution in comparison to pristine TiNTs. Key is the light-induced conversion of the intrinsic Cu dopant to metallic copper nanoparticles that act as a stable co-catalyst for H2 generation. Keywords: TiO2, Cu doped TiO2, Photocatalytic activity, Intrinsic copper decoratio

An Operando X-ray Absorption Spectroscopy Study of a NiCu−TiO2_{2} Photocatalyst for H2_{2} Evolution

Cu- or Ni-decorated semiconductors represent a potential low-cost alternative to noble-metal-modified photocatalysts. Even more effective are bimetallic NiCu nanoparticles, which can provide a remarkable photocatalytic H$_{2}$ evolution enhancement compared to single-element Cu or Ni systems. The main concern of such alloyed co-catalysts is their activity with respect to alteration of their elemental composition and oxidation state over reaction time. Ex situ characterization techniques provide controversial interpretations of the co-catalytic role of the individual elements. Hypotheses such as the in situ reduction of “native” Ni or Cu species during photocatalysis, the oxidation of metallic Cu or Ni into oxides or hydroxides, or the formation of p–n junctions or core/shell structures have been proposed. Herein, we present an operando X-ray absorption spectroscopy study of a NiCu–TiO$_{2}$ system under UV light illumination in ethanol–water solutions, i.e., under photocatalytic H$_{2}$ evolution conditions. The experimental approach allows for monitoring in real time chemical changes that take place in the co-catalyst under intermittent illumination, i.e., under light on–off cycles. We show that while Ni and Cu are partially oxidized in the as-formed NiCu co-catalyst (air-formed surface oxides or hydroxides) and undergo partial dissolution in the liquid phase under dark conditions, such Ni and Cu oxidized and dissolved species are reduced/redeposited as a bimetallic NiCu phase at the TiO$_{2}$ surface under illumination. The dissolution/redeposition mechanism is triggered by TiO$_{2}$ conduction band electrons. We not only prove a UV-light-induced healing of the NiCu co-catalyst but also unambiguously demonstrate that the species responsible for the strongly enhanced photocatalytic H$_{2}$ evolution of NiCu nanoparticles are the metallic states of Ni and Cu

The double-walled nature of TiO2 nanotubes and formation of tube-in-tube structures – a characterization of different tube morphologies

In the present work we show how to achieve different nanotube morphologies (namely, not only single- and double-walled tubes, but also a tube-in-tube configuration) by combining specific anodization parameters. We characterize as-grown tube layers in terms of morphology, chemical composition and properties. As-grown tubes exhibit a double-walled morphology, that is, they consist of an inner and an outer shell. The low quality inner shell can be removed by an optimized chemical treatment thus leading to nanotubes of a higher quality oxide and with a single-walled nature. Double-walled tubes grown at low electrolyte temperature provide a thick inner shell that after adequate annealing can form a unique tube-in-tube morphology

Připevnění ultramalých nanočástic greigitu na grafen pro účinné superkondenzátory přechodného kovu a sulfidu v iontovém kapalném elektrolytu

To meet the future demands for off-grid power, high-performance electrochemical energy storage based on earth-abundant materials is essential. Supercapacitors are attractive in this sense due to their sustainable carbon-based architecture, rapid charging/discharging, and long cycle-life in comparison to battery chemistries. However, hybridizing carbon electrodes with inorganic phases is intensively explored in supercapacitor research to mitigate their low energy content. Iron sulfides are attractive because they are non-toxic and composed of earth-abundant elements, but, despite their hydrophobic nature, they have only been studied in aqueous electrolytes, limiting the energy content due to the narrow voltage stability window of water. Here, exploiting a rapid growth method and a highly functionalized graphene support, we strongly immobilized greigite (Fe3S4) nanoparticles with an ultrasmall size which could not be attained in the absence of graphene. The respective supercapacitor cell was found significantly more electroactive in the ionic liquid electrolyte than in water, boosting the energy content. Furthermore, greigite has high conductivity and fast surface faradaic reactions due to the enzyme-mimicking triple redox state of its thiocubane basic structural unit. Thus, fully reversible and fast redox processes in the expanded voltage-window of the ionic liquid also endowed excellent rate capability, cycling stability, and power. The work demonstrates a pathway, not previously explored, whereby greigite/graphene hybrids can surpass in these aspects top-rated supercapacitor materials.Ke splnění budoucích požadavků na energii mimo síť je nezbytné vysoce výkonné skladování elektrochemické energie založené na materiálech bohatých na Zemi. Superkondenzátory jsou v tomto smyslu atraktivní díky své udržitelné architektuře založené na uhlíku, rychlému nabíjení / vybíjení a dlouhé životnosti ve srovnání s chemickými bateriemi. Hybridizace uhlíkových elektrod s anorganickými fázemi se však intenzivně zkoumá ve výzkumu superkapacitorů, aby se zmírnil jejich nízký obsah energie. Sulfidy železa jsou atraktivní, protože jsou netoxické a skládají se z prvků bohatých na Zemi, ale navzdory své hydrofobní povaze byly studovány pouze ve vodných elektrolytech, což omezuje obsah energie v důsledku úzkého okna stability napětí vody. Zde jsme s využitím metody rychlého růstu a vysoce funkcionalizované podpory grafenu silně imobilizovali nanočástice greigite (Fe3S4) s ultravysokou velikostí, které by nebylo možné dosáhnout při absenci grafenu. Příslušný superkondenzátorový článek byl v iontovém kapalném elektrolytu nalezen významně více elektroaktivně než ve vodě, což zvyšuje energetický obsah. Kromě toho má greigit vysokou vodivost a rychlé povrchové faradaické reakce v důsledku trojitého redoxního stavu napodobujícího enzymy své základní strukturní jednotky thiocubanu. Plně reverzibilní a rychlé redoxní procesy v rozšířeném napěťovém okně iontové kapaliny tedy také poskytly vynikající schopnost rychlosti, stabilitu cyklování a výkon. Práce demonstruje cestu, která dosud nebyla prozkoumána, přičemž hybridy greigit / grafen mohou v těchto aspektech překonat špičkové superkondenzátorové materiály