Platinum-based catalysts have been considered the most effective electrocatalysts for the hydrogen evolution reaction in water splitting. However, platinum utilization in these electrocatalysts is extremely low, as the active sites are only located on the surface of the catalyst particles. Downsizing catalyst nanoparticles to single atoms is highly desirable to maximize their efficiency by utilizing nearly all platinum atoms. Here we report on a practical synthesis method to produce isolated single platinum atoms and clusters using the atomic layer deposition technique. The single platinum atom catalysts are investigated for the hydrogen evolution reaction, where they exhibit significantly enhanced catalytic activity (up to 37 times) and high stability in comparison with the state-of-the-art commercial platinum/carbon catalysts. The X-ray absorption fine structure and density functional theory analyses indicate that the partially unoccupied density of states of the platinum atoms’ 5d orbitals on the nitrogen-doped graphene are responsible for the excellent performance. Downsizing platinum based nanocatalysts has the twin advantages of lower platinum usage and increased activity per platinum atom. Here, the authors report an atomic layer deposition technique for single platinum atom catalyst fabrication and assess their hydrogen evolution activity.

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Platinum single-atom and cluster catalysis of the hydrogen evolution reaction

The synthesis of semiconductor nanowires has been studied intensively worldwide for a wide spectrum of materials. Such low-dimensional nanostructures are not only interesting for fundamental research due to their unique structural and physical properties relative to their bulk counterparts, but also offer fascinating potential for future technological applications. Deeper understanding and sufficient control of the growth of nanowires are central to the current research interest. This Review discusses the various growth processes, with a focus on the vapor-liquid-solid process, which offers an opportunity for the control of spatial positioning of nanowires. Strategies for position-controlled and nanopatterned growth of nanowire arrays are reviewed and demonstrated by selected examples as well as discussed in terms of larger-scale realization and future prospects. Issues on building up nanowire-based electronic and photonic devices are addressed at the end of the Review, accompanied by a brief survey of recent progress demonstrated so far on the laboratory level.

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Semiconductor nanowires: from self-organization to patterned growth

Platinum-nanoparticle-based catalysts are widely used in many important chemical processes and automobile industries. Downsizing catalyst nanoparticles to single atoms is highly desirable to maximize their use efficiency, however, very challenging. Here we report a practical synthesis for isolated single Pt atoms anchored to graphene nanosheet using the atomic layer deposition (ALD) technique. ALD offers the capability of precise control of catalyst size span from single atom, subnanometer cluster to nanoparticle. The single-atom catalysts exhibit significantly improved catalytic activity (up to 10 times) over that of the state-of-the-art commercial Pt/C catalyst. X-ray absorption fine structure (XAFS) analyses reveal that the low-coordination and partially unoccupied densities of states of 5d orbital of Pt atoms are responsible for the excellent performance. This work is anticipated to form the basis for the exploration of a next generation of highly efficient single-atom catalysts for various applications.

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Single-atom Catalysis Using Pt/Graphene Achieved through Atomic Layer Deposition

Electrochemical conversion of nitrate (NO3-) into ammonia (NH3) recycles nitrogen and offers a route to the production of NH3, which is more valuable than dinitrogen gas. However, today's development of NO3- electroreduction remains hindered by the lack of a mechanistic picture of how catalyst structure may be tuned to enhance catalytic activity. Here we demonstrate enhanced NO3- reduction reaction (NO3-RR) performance on Cu50Ni50 alloy catalysts, including a 0.12 V upshift in the half-wave potential and a 6-fold increase in activity compared to those obtained with pure Cu at 0 V vs reversible hydrogen electrode (RHE). Ni alloying enables tuning of the Cu d-band center and modulates the adsorption energies of intermediates such as *NO3-, *NO2, and *NH2. Using density functional theory calculations, we identify a NO3-RR-to-NH3 pathway and offer an adsorption energy-activity relationship for the CuNi alloy system. This correlation between catalyst electronic structure and NO3-RR activity offers a design platform for further development of NO3-RR catalysts.

Enhanced Nitrate-to-Ammonia Activity on Copper-Nickel Alloys via Tuning of Intermediate Adsorption

Developing efficient and cost-effective electrocatalysts for hydrogen evolution reaction (HER) is highly desired for the hydrogen economy. In this study, we developed a facile microwave reduction method to synthesize single Pt atoms anchored on aniline-stacked graphene (Pt SASs/AG) with outstanding HER performance. Pt SASs/AG presents excellent HER activity with η = 12 mV at 10 mA cm−2 and a mass current density of 22 400 AgPt−1 at η = 50 mV, which is 46 times higher than that of commercial 20 wt% Pt/C. Moreover, the Pt SASs/AG catalyst is highly active and more stable than Pt/C. X-ray absorption fine spectroscopy and density functional theory calculations demonstrated that the coordination of atomically isolated Pt with the nitrogen of aniline optimized the electronic structure of Pt and the hydrogen adsorption energy, eventually promoting HER activity. This study provides a new avenue for the development of single-atom Pt electrocatalysts with high activity and stability.

Highly stable single Pt atomic sites anchored on aniline-stacked graphene for hydrogen evolution reaction

We report a soft x-ray excited optical luminescence (XEOL) and x-ray emission spectroscopy (XES) study of silicon nanowires (SiNW) with excitations at the silicon $K$ and ${L}_{3,2}$ edge, respectively. It is found that the XEOL of SiNW exhibits several luminescence bands at $\ensuremath{\sim}460$, $\ensuremath{\sim}530$, and $\ensuremath{\sim}630\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. These luminescence bands are broad and are sensitive to the $\mathrm{Si}\phantom{\rule{0.2em}{0ex}}1s$ excitation channel (Si versus $\mathrm{Si}{\mathrm{O}}_{2}$ whiteline). These chemical- and morphology-dependent luminescences are attributable to the emission from the encapsulating silicon oxide, the quantum-confined silicon crystallites of various sizes embedded in the oxide layer, and the silicon-silicon oxide interface. XES clearly shows the presence of a relatively thick oxide layer encapsulating the silicon nanowire and the densities of states tailing across the Fermi level. The implications of these findings to the electronic and optical properties of silicon nanowires are discussed.

Electronic structure and optical properties of silicon nanowires: A study using x-ray excited optical luminescence and x-ray emission spectroscopy

The Ca L3,2-edge XANES spectra of six calcium salts have been measured in both total electron and fluorescence yields using a high-resolution spherical grating monochromator. The compounds investigated were; CaF2, CaCO3, CaCl2 x 2H2O, calcium phosphate, calcium glycerophosphate and calcium gluconate. We find that the fine structure of the Ca L3,2-edges for each compound is unique and relates to the local structure of the Ca atom. The implications of these results to the study of the structure of calcium binding sites in systems of biologically interest will be discussed.

Calcium L‐edge XANES study of some calcium compounds

This work investigates charge redistribution in a series of Ni-Cu alloys using x-ray photoemission spectroscopy (XPS) and Ni/Cu ${L}_{3,2}$- and $K$-edge x-ray-absorption near-edge structure (XANES). XPS results show that the constituent $d$ bands are well separated and shifted to a slightly higher binding energy upon dilution into the other host, indicating that the atomic sites in the alloy are not as well screened relative to the pure metal. However, no significant $d$-band narrowing is observed, suggesting that there is modest $d\ensuremath{-}d$ interaction in the alloys. In contrast to the XPS observation, XANES results show a reduction in white-line intensity at both edges relative to the pure metal suggesting that both Ni and Cu sites gain $d$ charge. The unoccupied Ni $d$ band is far from fully occupied even at infinite dilution. The discrepancy between the implications of the XPS and XANES results is dealt with using a charge redistribution model in which $s\ensuremath{-}p\ensuremath{-}d$ rehybridization takes place at both sites within the framework of electroneutrality and electronegativity considerations. It appears that both Ni and Cu gain a small but measurable amount of $d$ charge in alloy formation through rehybridization (loss of non-$d$ conduction charge). Possible connection between these results and the disappearance of ferromagnetism in ${\mathrm{Ni}}_{1\ensuremath{-}x}{\mathrm{Cu}}_{x}$ alloys at $xg0.6$ is discussed.

Electronic structure of ni-cu alloys : the d-electron charge distribution

Preliminary synchrotron analysis of lead in hair from a lead smelter worker.

We report M3,2‐edge x‐ray absorption near‐edge structure (XANES) measurements for a series of 5d metals, Ta, W, Pt, and Au using a total electron yield technique. It is found that the M3,2 XANES absorption features and their systematics are comparable to their L3,2 counterparts. These results clearly indicate that M3,2‐edge XANES can be used as an alternative probe for the unoccupied densities of d states of the 5d metals and their compounds. The implication of these results is discussed.

M3,2‐edge x‐ray absorption near‐edge structure spectroscopy: An alternative probe to the L3,2‐edge near‐edge structure for the unoccupied densities of d states of 5d metals

S. J. Naftel

Papers

Electronic structure and optical properties of silicon nanowires: A study using x-ray excited optical luminescence and x-ray emission spectroscopy

Calcium L‐edge XANES study of some calcium compounds

Electronic structure of ni-cu alloys : the d-electron charge distribution

Preliminary synchrotron analysis of lead in hair from a lead smelter worker.

M3,2‐edge x‐ray absorption near‐edge structure spectroscopy: An alternative probe to the L3,2‐edge near‐edge structure for the unoccupied densities of d states of 5d metals