Shicheng Xu

Electrochemical tuning of vertically aligned MoS2 nanofilms and its application in improving hydrogen evolution reaction

Significance The electronic structures of two-dimensional materials can be tuned for a variety of applications by guest species intercalation into the van der Waals gaps. Using Li electrochemical intercalated MoS2 as an example here, we correlate the continuously tuned electronic structure of lithiated MoS2 with the corresponding enhanced hydrogen evolution reaction activity, and thus construct the electronic structure–catalytic activity relationship. This work offers a unique thinking of tuning the electronic structures of layered materials by guest species intercalation for optimizing different kinds of catalysis on the basis of the strong correlation between the electronic structures and catalytic activities of the catalysts. The ability to intercalate guest species into the van der Waals gap of 2D layered materials affords the opportunity to engineer the electronic structures for a variety of applications. Here we demonstrate the continuous tuning of layer vertically aligned MoS2 nanofilms through electrochemical intercalation of Li+ ions. By scanning the Li intercalation potential from high to low, we have gained control of multiple important material properties in a continuous manner, including tuning the oxidation state of Mo, the transition of semiconducting 2H to metallic 1T phase, and expanding the van der Waals gap until exfoliation. Using such nanofilms after different degree of Li intercalation, we show the significant improvement of the hydrogen evolution reaction activity. A strong correlation between such tunable material properties and hydrogen evolution reaction activity is established. This work provides an intriguing and effective approach on tuning electronic structures for optimizing the catalytic activity.

Direct and continuous strain control of catalysts with tunable battery electrode materials.

Tuning nanoparticle strain The catalytic activity of metals in heterogeneous catalysts can be altered by applying strain, which changes the crystalline lattice spacing and modifies the metals electronic properties. Wang et al. show how particles of cobalt oxide, a positive electrode for lithium batteries, can expand or contract with charging and transfer strain to adsorbed platinum nanoparticles. For the oxygen reduction reaction used in fuel cells, compressive strain boosted activity by 90%, and tensile strain decreased it by 40%. Science, this issue p. 1031 The expansion or contraction of lithium electrode particles with charging transfers strain to platinum nanoparticles. We report a method for using battery electrode materials to directly and continuously control the lattice strain of platinum (Pt) catalyst and thus tune its catalytic activity for the oxygen reduction reaction (ORR). Whereas the common approach of using metal overlayers introduces ligand effects in addition to strain, by electrochemically switching between the charging and discharging status of battery electrodes the change in volume can be precisely controlled to induce either compressive or tensile strain on supported catalysts. Lattice compression and tension induced by the lithium cobalt oxide substrate of ~5% were directly observed in individual Pt nanoparticles with aberration-corrected transmission electron microscopy. We observed 90% enhancement or 40% suppression in Pt ORR activity under compression or tension, respectively, which is consistent with theoretical predictions.

Quantifying Geometric Strain at the PbS QD-TiO2 Anode Interface and Its Effect on Electronic Structures

Quantum dots (QDs) show promise as the absorber in nanostructured thin film solar cells, but achieving high device efficiencies requires surface treatments to minimize interfacial recombination. In this work, lead sulfide (PbS) QDs are grown on a mesoporous TiO2 film with a crystalline TiO2 surface, versus one coated with an amorphous TiO2 layer by atomic layer deposition (ALD). These mesoporous TiO2 films sensitized with PbS QDs are characterized by X-ray and electron diffraction, as well as X-ray absorption spectroscopy (XAS) in order to link XAS features with structural distortions in the PbS QDs. The XAS features are further analyzed with quantum simulations to probe the geometric and electronic structure of the PbS QD-TiO2 interface. We show that the anatase TiO2 surface structure induces PbS bond angle distortions, which increases the energy gap of the PbS QDs at the interface.

Self-limiting atomic layer deposition of barium oxide and barium titanate thin films using a novel pyrrole based precursor

Barium oxide (BaO) is a critical component for a number of materials offering high dielectric constants, high proton conductivity as well as potential applicability in superconductivity. For these properties to keep pace with continuous device miniaturization, it is necessary to study thin film deposition of BaO. Atomic layer deposition (ALD) enables single atomic layer thickness control, conformality on complex shaped substrates, and the ability to precisely tune stoichiometry. Depositing multicomponent BaO containing ALD films in a self-limiting manner at low temperatures may extend the favorable bulk properties of these materials into the ultrathin film regime. Here we report the first temperature and dose independent thermal BaO deposition using a novel pyrrole based Ba precursor (py-Ba) and water (H2O) as the co-reactant. The growth per cycle (GPC) is constant at 0.45 A with excellent self-terminating behavior. The films are smooth (root mean squared (RMS) roughness 2.1 A) and contain minimal impurities at the lowest reported deposition temperatures for Ba containing films (180–210 °C). We further show conformal coating of non-planar substrates (aspect ratio ∼ 1:2.5) at step coverages above 90%. Intermixing TiO2 ALD layers, we deposited amorphous barium titanate with a dielectric constant of 35. The presented approach for infusing self-terminating BaO in multicomponent oxide films may facilitate tuning electrical and ionic properties in next-generation ultrathin devices.

Variation of Energy Density of States in Quantum Dot Arrays due to Interparticle Electronic Coupling

Subnanometer-resolved local electron energy structure was measured in PbS quantum dot superlattice arrays using valence electron energy loss spectroscopy with scanning transmission electron microscopy. We found smaller values of the lowest available transition energies and an increased density of electronic states in the space between quantum dots with shorter interparticle spacing, indicating extension of carrier wave functions as a result of interparticle electronic coupling. A quantum simulation verified both trends and illustrated the wave function extension effect.

Extending the limits of Pt/C catalysts with passivation-gas-incorporated atomic layer deposition

Controlling the morphology of noble metal nanoparticles during surface depositions is strongly influenced by precursor–substrate and precursor–deposit interactions. Depositions can be improved through a variety of means, including tailoring the surface energy of a substrate to improve precursor wettability, or by modifying the surface energy of the deposits themselves. Here, we show that carbon monoxide can be used as a passivation gas during atomic layer deposition to modify the surface energy of already deposited Pt nanoparticles to assist direct deposition onto a carbon catalyst support. The passivation process promotes two-dimensional growth leading to Pt nanoparticles with suppressed thicknesses and a more than 40% improvement in Pt surface-to-volume ratio. This approach to synthesizing nanoparticulate Pt/C catalysts achieved high Pt mass activities for the oxygen reduction reaction, along with excellent stability likely facilitated by strong catalyst–support interactions afforded by this synthetic technique.The synthesis of nanocatalysts with small dimensions and high surface-to-volume ratios is of great interest to lower catalyst costs and exploit catalytic performance enhancements through size effects. Now, Prinz and colleagues show that two-dimensional growth of platinum nanoparticles with suppressed thicknesses can be promoted with passivation-gas-incorporated atomic layer deposition.