Robert J. Hamers

Highly active hydrogen evolution catalysis from metallic WS2 nanosheets

We report metallic WS2 nanosheets that display excellent catalytic activity for hydrogen evolution reaction (HER) that is the best reported for MX2 materials. They are chemically exfoliated from WS2 nanostructures synthesized by chemical vapour deposition, including by using a simple and fast microwave-assisted intercalation method. Structural and electrochemical studies confirm that the simultaneous conversion and exfoliation of semiconducting 2H-WS2 into nanosheets of its metallic 1T polymorph result in facile electrode kinetics, excellent electrical transport, and proliferation of catalytically active sites.

Determination of the local electronic structure of atomic‐sized defects on Si(001) by tunneling spectroscopy

Tunneling spectroscopy and voltage‐dependent scanning tunneling microscopy have been used to study the geometry and electronic properties of atomic‐sized defects on the Si(001) surface. Individual dimer vacancies are shown to be semiconducting, consistent with the π‐bonded defect model of Pandey. Another type of characteristic defect is found which gives rise to strongly metallic tunneling I–V characteristics, demonstrating that it has a high density of states at the Fermi level and is likely active in Fermi level pinning on Si(001). Spatially dependent I–V measurements and tunneling barrier height measurements also directly reveal the spatial extent of this metallic character and provide direct measures of the ‘‘size’’ of the defects.

Efficient photoelectrochemical hydrogen generation using heterostructures of Si and chemically exfoliated metallic MoS2.

We report the preparation and characterization of highly efficient and robust photocathodes based on heterostructures of chemically exfoliated metallic 1T-MoS2 and planar p-type Si for solar-driven hydrogen production. Photocurrents up to 17.6 mA/cm(2) at 0 V vs reversible hydrogen electrode were achieved under simulated 1 sun irradiation, and excellent stability was demonstrated over long-term operation. Electrochemical impedance spectroscopy revealed low charge-transfer resistances at the semiconductor/catalyst and catalyst/electrolyte interfaces, and surface photoresponse measurements also demonstrated slow carrier recombination dynamics and consequently efficient charge carrier separation, providing further evidence for the superior performance. Our results suggest that chemically exfoliated 1T-MoS2/Si heterostructures are promising earth-abundant alternatives to photocathodes based on noble metal catalysts for solar-driven hydrogen production.

Solution Growth of Single Crystal Methylammonium Lead Halide Perovskite Nanostructures for Optoelectronic and Photovoltaic Applications

Understanding crystal growth and improving material quality is important for improving semiconductors for electronic, optoelectronic, and photovoltaic applications. Amidst the surging interest in solar cells based on hybrid organic-inorganic lead halide perovskites and the exciting progress in device performance, improved understanding and better control of the crystal growth of these perovskites could further boost their optoelectronic and photovoltaic performance. Here, we report new insights on the crystal growth of the perovskite materials, especially crystalline nanostructures. Specifically, single crystal nanowires, nanorods, and nanoplates of methylammonium lead halide perovskites (CH3NH3PbI3 and CH3NH3PbBr3) are successfully grown via a dissolution-recrystallization pathway in a solution synthesis from lead iodide (or lead acetate) films coated on substrates. These single crystal nanostructures display strong room-temperature photoluminescence and long carrier lifetime. We also report that a solid-liquid interfacial conversion reaction can create a highly crystalline, nanostructured MAPbI3 film with micrometer grain size and high surface coverage that enables photovoltaic devices with a power conversion efficiency of 10.6%. These results suggest that single-crystal perovskite nanostructures provide improved photophysical properties that are important for fundamental studies and future applications in nanoscale optoelectronic and photonic devices.

Facile Solution Synthesis of α-FeF3·3H2O Nanowires and Their Conversion to α-Fe2O3 Nanowires for Photoelectrochemical Application

We report for the first time the facile solution growth of α-FeF(3)·3H(2)O nanowires (NWs) in large quantity at a low supersaturation level and their scalable conversion to porous semiconducting α-Fe(2)O(3) (hematite) NWs of high aspect ratio via a simple thermal treatment in air. The structural characterization by transmission electron microscopy shows that thin α-FeF(3)·3H(2)O NWs (typically <100 nm in diameter) are converted to single-crystal α-Fe(2)O(3) NWs with internal pores, while thick ones (typically >100 nm in diameter) become polycrystalline porous α-Fe(2)O(3) NWs. We further demonstrated the photoelectrochemical (PEC) application of the nanostructured photoelectrodes prepared from these converted hematite NWs. The optimized photoelectrode with a ~400 nm thick hematite NW film yielded a photocurrent density of 0.54 mA/cm(2) at 1.23 V vs reversible hydrogen electrode potential after modification with cobalt catalyst under standard conditions (AM 1.5 G, 100 mW/cm(2), pH = 13.6, 1 M NaOH). The low cost, large quantity, and high aspect ratio of the converted hematite NWs, together with the resulting simpler photoelectrode preparation, can be of great benefit for hematite-based PEC water splitting. Furthermore, the ease and scalability of the conversion from hydrated fluoride NWs to oxide NWs suggest a potentially versatile and low-cost strategy to make NWs of other useful iron-based compounds that may enable their large-scale renewable energy applications.

Photo-illuminated diamond as a solid-state source of solvated electrons in water for nitrogen reduction

The photocatalytic reduction of N₂ to NH₃ is typically hampered by poor binding of N₂ to catalytic materials and by the very high energy of the intermediates involved in this reaction. Solvated electrons directly introduced into the reactant solution can provide an alternative pathway to overcome such limitations. Here we demonstrate that illuminated hydrogen-terminated diamond yields facile electron emission into water, thus inducing reduction of N₂ to NH₃ at ambient temperature and pressure. Transient absorption measurements at 632 nm reveal the presence of solvated electrons adjacent to the diamond after photoexcitation. Experiments using inexpensive synthetic diamond samples and diamond powder show that photocatalytic activity is strongly dependent on the surface termination and correlates with the production of solvated electrons. The use of diamond to eject electrons into a reactant liquid represents a new paradigm for photocatalytic reduction, bringing electrons directly to reactants without requiring molecular adsorption to the surface.

Scanning tunneling microscopy study of low‐temperature epitaxial growth of silicon on Si(111)‐(7×7)

Scanning tunneling microscopy is used to investigate nucleation and growth phenomena in the molecular‐beam epitaxial (MBE) growth of silicon on Si(111)‐(7×7) from the submonolayer range up to a few monolayers. At room temperature small amorphous clusters form which grow in locally ordered arrays on the (7×7) lattice. Deposition at a higher substrate temperature produces triangular islands of epitaxial silicon which have preferred step propagation in the [112] direction. Preferred nucleation of Si islands is found to occur along boundaries between (7×7) superstructure translational domains of the substrate. The preferred nucleation which arises from defects in the epilayer accounts for the formation of a second epitaxial layer long before the first layer is completed. A variety of metastable reconstructions which differ from (7×7) are also found in the epitaxial islands and are discussed.

Geomicrobiology of Pyrite (FeS2) Dissolution: Case Study at Iron Mountain, California

Geomicrobiology of pyrite weathering at Iron Mountain, CA, was investigated by molecular biological, surface chemical, surface structural, and solution chemical methods in both laboratory and field-based studies. Research focused at sites both within and peripheral to the ore-body. The acid-generating areas we have examined thus far at Iron Mountain (solution pH 35 C) were populated by species other than Thiobacillus ferrooxidans . 16S rDNA bacterial sequence analysis and domain- and specieslevel oligonucleotide probe-based investigations confirmed the presence of planktonic Leptospirillum ferrooxidans and indicated the existence of other species apparently related to other newly described acidophilic chemolithotrophs. T. ferrooxidans was confined to relatively moderate environments (pH 2-3, 20-30 C) that were peripheral to the orebody. Dissolution rate measurements indicated that, per cell, attached and planktonic species contributed comparably in acid release. Surface colonizati...

Facile post-growth doping of nanostructured hematite photoanodes for enhanced photoelectrochemical water oxidation

We report a facile approach to perform post-growth doping of hematite (α-Fe2O3) nanostructures by depositing titanium (Ti) precursor solution and subsequent annealing in air. Using hematite nanowire photoanodes on fluorine doped tin oxide (FTO) glass substrates as a model system, the doping conditions were carefully optimized and highly photoactive hematite photoanodes were prepared at a more practically acceptable temperature of 650–700 °C than the ≥800 °C commonly used in previous works. A combination of microstructural characterization, elemental analysis, photoelectrochemical (PEC) measurements, and electrochemical impedance spectroscopy (EIS) analysis were employed to confirm the distribution of Ti atoms in hematite nanostructures and the role of Ti dopants in enhancing the photocurrent of hematite photoanodes. It was found that the Ti-treatment increases the donor concentration of hematite by about 10 fold and facilitates majority carrier transport and collection, which may account for the performance enhancement. Moreover, EIS measurements under illumination and Mott–Schottky analysis clearly showed that Ti dopants interact with the surface trap states of hematite, suggesting that surface passivation may also contribute to the improved PEC performance. This facile post-growth doping method can be applied to other hematite nanostructures such as electrochemically deposited hematite films and expanded to other dopants such as zirconium (Zr).

Electronic and geometric structure of Si(111)-(7 × 7) and Si(001) surfaces

Abstract The atomic origins of the intrinsic surface states of the Si(111)-(7 × 7) and Si(001) surfaces have been identified using the recently developed method of current imaging tunneling spectroscopy (CITS). On Si(111)-(7 × 7) three filled and two empty surface states are found and directly identified with atomic features of the dimer-adatom-stacking fault model. On Si(001) one filled and one empty state are observed and identified with atomic features of a dimer model. The STM images of Si(001) are shown to be dominated by the surface electronic structure rather than geometric structure.