Fengcai Lei

Oxygen Vacancies Confined in Ultrathin Indium Oxide Porous Sheets for Promoted Visible-Light Water Splitting

Finding an ideal model for disclosing the role of oxygen vacancies in photocatalysis remains a huge challenge. Herein, O-vacancies confined in atomically thin sheets is proposed as an excellent platform to study the O-vacancy-photocatalysis relationship. As an example, O-vacancy-rich/-poor 5-atom-thick In2O3 porous sheets are first synthesized via a mesoscopic-assembly fast-heating strategy, taking advantage of an artificial hexagonal mesostructured In-oleate complex. Theoretical/experimental results reveal that the O-vacancies endow 5-atom-thick In2O3 sheets with a new donor level and increased states of density, hence narrowing the band gap from the UV to visible regime and improving the carrier separation efficiency. As expected, the O-vacancy-rich ultrathin In2O3 porous sheets-based photoelectrode exhibits a visible-light photocurrent of 1.73 mA/cm(2), over 2.5 and 15 times larger than that of the O-vacancy-poor ultrathin In2O3 porous sheets- and bulk In2O3-based photoelectrodes.

Pits confined in ultrathin cerium(IV) oxide for studying catalytic centers in carbon monoxide oxidation

Finding ideal material models for studying the role of catalytic active sites remains a great challenge. Here we propose pits confined in an atomically thin sheet as a platform to evaluate carbon monoxide catalytic oxidation at various sites. The artificial three-atomic-layer thin cerium(IV) oxide sheet with approximately 20% pits occupancy possesses abundant pit-surrounding cerium sites having average coordination numbers of 4.6 as revealed by X-ray absorption spectroscopy. Density-functional calculations disclose that the four- and five-fold coordinated pit-surrounding cerium sites assume their respective role in carbon monoxide adsorption and oxygen activation, which lowers the activation barrier and avoids catalytic poisoning. Moreover, the presence of coordination-unsaturated cerium sites increases the carrier density and facilitates carbon monoxide diffusion along the two-dimensional conducting channels of surface pits. The atomically thin sheet with surface-confined pits exhibits lower apparent activation energy than the bulk material (61.7 versus 122.9 kJ mol(-1)), leading to reduced conversion temperature and enhanced carbon monoxide catalytic ability.

Metallic tin quantum sheets confined in graphene toward high-efficiency carbon dioxide electroreduction

Ultrathin metal layers can be highly active carbon dioxide electroreduction catalysts, but may also be prone to oxidation. Here we construct a model of graphene confined ultrathin layers of highly reactive metals, taking the synthetic highly reactive tin quantum sheets confined in graphene as an example. The higher electrochemical active area ensures 9 times larger carbon dioxide adsorption capacity relative to bulk tin, while the highly-conductive graphene favours rate-determining electron transfer from carbon dioxide to its radical anion. The lowered tin–tin coordination numbers, revealed by X-ray absorption fine structure spectroscopy, enable tin quantum sheets confined in graphene to efficiently stabilize the carbon dioxide radical anion, verified by 0.13 volts lowered potential of hydroxyl ion adsorption compared with bulk tin. Hence, the tin quantum sheets confined in graphene show enhanced electrocatalytic activity and stability. This work may provide a promising lead for designing efficient and robust catalysts for electrolytic fuel synthesis.

Single Unit Cell Bismuth Tungstate Layers Realizing Robust Solar CO2 Reduction to Methanol

Solar CO2 reduction into hydrocarbons helps to solve the global warming and energy crisis. However, conventional semiconductors usually suffer from low photoactivity and poor photostability. Here, atomically-thin oxide-based semiconductors are proposed as excellent platforms to overcome this drawback. As a prototype, single-unit-cell Bi2WO6 layers are first synthesized by virtue of a lamellar Bi-oleate intermediate. The single-unit-cell thickness allows 3-times larger CO2 adsorption capacity and higher photoabsorption than bulk Bi2WO6. Also, the increased conductivity, verified by density functional theory calculations and temperature-dependent resistivities, favors fast carrier transport. The carrier lifetime increased from 14.7 to 83.2 ns, revealed by time-resolved fluorescence spectroscopy, which accounts for the improved electron-hole separation efficacy. As a result, the single-unit-cell Bi2WO6 layers achieve a methanol formation rate of 75 μmol g(-1) h(-1), 125-times higher than that of bulk Bi2WO6. The catalytic activity of the single-unit-cell layers proceeds without deactivation even after 2 days. This work will shed light on designing efficient and robust photoreduction CO2 catalysts.

Atomically Thin Tin Dioxide Sheets for Efficient Catalytic Oxidation of Carbon Monoxide

The thinner the better: SnO2 sheets that are five atomic layers thick are an efficient catalyst for the oxidation of CO. These sheets, which have 40% surface atom occupancy and are fabricated by a scalable ethylenediamine-assisted pathway, show remarkably improved catalytic performances compared to other SnO2 species, with the apparent activation energy lowered to 59.2 kJ mol(-1) and the full-conversion-temperature lowered to 250 °C.

Metallic Single‐Unit‐Cell Orthorhombic Cobalt Diselenide Atomic Layers: Robust Water‐Electrolysis Catalysts

The bottleneck in water electrolysis lies in the kinetically sluggish oxygen evolution reaction (OER). Herein, conceptually new metallic non-metal atomic layers are proposed to overcome this drawback. Metallic single-unit-cell CoSe2 sheets with an orthorhombic phase are synthesized by thermally exfoliating a lamellar CoSe2 -DETA hybrid. The metallic character of orthorhombic CoSe2 atomic layers, verified by DFT calculations and temperature-dependent resistivities, allows fast oxygen evolution kinetics with a lowered overpotential of 0.27 V. The single-unit-cell thickness means 66.7 % of the Co(2+) ions are exposed on the surface and serve as the catalytically active sites. The lowered Co(2+) coordination number down to 1.3 and 2.6, gives a lower Tafel slope of 64 mV dec(-1) and higher turnover frequency of 745 h(-1) . Thus, the single-unit-cell CoSe2 sheets have around 2 and 4.5 times higher catalytic activity compared with the lamellar CoSe2 -DETA hybrid and bulk CoSe2 .

Atomic-Layer-Confined Doping for Atomic-Level Insights into Visible-Light Water Splitting.

A model of doping confined in atomic layers is proposed for atomic-level insights into the effect of doping on photocatalysis. Co doping confined in three atomic layers of In2S3 was implemented with a lamellar hybrid intermediate strategy. Density functional calculations reveal that the introduction of Co ions brings about several new energy levels and increased density of states at the conduction band minimum, leading to sharply increased visible-light absorption and three times higher carrier concentration. Ultrafast transient absorption spectroscopy reveals that the electron transfer time of about 1.6 ps from the valence band to newly formed localized states is due to Co doping. The 25-fold increase in average recovery lifetime is believed to be responsible for the increased of electron-hole separation. The synthesized Co-doped In2S3 (three atomic layers) yield a photocurrent of 1.17 mA cm(-2) at 1.5 V vs. RHE, nearly 10 and 17 times higher than that of the perfect In2S3 (three atomic layers) and the bulk counterpart, respectively.

Free-floating ultrathin tin monoxide sheets for solar-driven photoelectrochemical water splitting

Solar-driven photoelectrochemical water splitting represents one of the most challenging tasks for solar-energy utilization. In this study, free-floating ultrathin SnO sheets with different thicknesses were successfully synthesized via a convenient liquid exfoliation strategy, with efforts to disclose the thickness-dependent solar water splitting efficiency in p-type semiconductors. The thinner thickness and larger surface area afford a higher fraction of surface atoms to serve as active sites, while the calculated increased density of states near the Fermi level ensures rapid carrier transport/separation efficiency along the two-dimensional conducting paths of the thinner SnO sheets. As expected, the 3 nm thick SnO sheet-based photocathode shows an incident photon-to-current conversion efficiency of up to 20.1% at 300 nm, remarkably higher than 10.7% and 4.2% for the 5.4 nm thick SnO sheet- and bulk SnO-based electrodes. This work discusses the thickness-dependent solar water splitting efficiency in ultrathin p-type semiconductor sheets, thus opening new opportunities in the field of solar cells and photocatalysts.

Partially amorphous nickel–iron layered double hydroxide nanosheet arrays for robust bifunctional electrocatalysis

Bifunctional electrocatalysts that can boost energy-related reactions are urgently in demand for pursual of dual and even multiple targets towards practical applications such as energy conversion, clean fuel production and pollution treatment. Herein, we highlight that an in situ grown nickel–iron layered double hydroxide (NiFe LDH) nanosheet array catalyst with partially amorphous characteristics, rich native Ni3+ ions and an optimal Ni : Fe ratio can exhibit robust performances on both the oxygen evolution reaction (OER) and the urea oxidation reaction (UOR). Benefitting from the partially amorphous feature, the catalytically active high-valence species are easy to generate and stabilize, thus further realizing enhanced electrooxidation activity with the aid of an internal 2D charge transfer pathway and native Ni3+ ions. As expected, the partially amorphous catalyst exhibits a higher OER current of 284.4 mA cm−2 at an overpotential of 500 mV, which shows 2.2–10.0 times enhancement than the counterparts with various Ni : Fe ratios. In addition, the UOR current density of the partially amorphous catalyst at 1.8 V vs. RHE shows 1.6 and 2.4 times increment relative to fully amorphous and highly crystalline catalysts, and 2.7–9.4 fold larger than the catalysts with other Ni : Fe ratios. The optimization strategy of designing the partially amorphous bifunctional catalyst in this work may broaden the way of searching for advanced electrocatalysts for simultaneous waste water treatment and clean energy production.

Removal of toxic metal ions using chitosan coated carbon nanotube composites for supercapacitors

Environmental pollution and energy crisis are two major global challenges to human beings. Recovering energy from wastewater is considered to be one of the effective approaches to address these two issues synchronously. As the main pollutants in wastewater, toxic heavy metal ions are the potential candidates for energy storage devices with pseudocapacitive behaviors. In this study, toxic metal ions of Cr(VI) and Cu(II) are removed efficiently by chitosan coated oxygen-containing functional carbon nanotubes, and the corresponding equilibrium adsorption capacity is 142.1 and 123.7 mg g−1. Followed by carbonization of metal ions-adsorbed adsorbents, Cu- and CrN-loaded carbon composites can be obtained. Electrochemical measurements show that the supercapacitor electrodes based on Cu- and CrN-loaded carbon composites have specific capacitance of 144.9 and 114.9 F g−1 at 2 mV s−1, with superior electrochemical properties to pure chitosan coated carbon nanotubes after carbonization. This work demonstrates a new strategy for the resource-utilization of other heavy metal ions for energy devices, and also provides a new way to turn environmental pollutants into clean energy.