Enabling practical electrocatalyst-assisted photoelectro- chemical water splitting with earth abundant materials

Sustainable development and continued prosperity of humanity hinge on the availability of renewable energy sources on a terawatts scale. In the long run, solar energy is the only source that can meet this daunting demand. Widespread utilization of solar energy faces challenges as a result of its diffusive （hence low energy density） and intermittent nature. How to effectively harvest, concentrate, store and redistribute solar energy constitutes a fundamental challenge that the scientific community needs to address. Photoelectrochemical （PEC） water splitting is a process that can directly convert solar energy into chemical energy and store it in chemical bonds, by producing hydrogen as a clean fuel source. It has received significant research attention lately. Here we provide a concise review of the key issues encountered in carrying out PEC water splitting. Our focus is on the balance of considerations such as stability, earth abundance, and efficiency. Particular attention is paid to the combination of photoelectrodes with electrocatalysts, especially on the interfaces between different components.

Free-standing porous carbon electrodes derived from wood for high-performance Li-O2 battery applications

Porous carbon materials are widely used in particulate forms for energy applications such as fuel cells, batteries, and （super） capacitors. To better hold the particles together, polymeric additives are utilized as binders, which not only increase the weight and volume of the devices, but also cause adverse side effects. We developed a wood-derived, free-standing porous carbon electrode and successfully applied it as a cathode in Li-O2 batteries. The spontaneously formed hierarchical porous structure exhibits good performance in facilitating the mass transport and hosting the discharge products of Li202. Heteroatom （N） doping further improves the catalytic activity of the carbon cathode with lower overpotential and higher capacity. Overall, the Li-O2 battery based on the new carbon cathode affords a stable energy efficiency of 65% and can be operated for 20 cycles at a discharge depth of 70%. The wood-derived free-standing carbon represents a new, unique structure for energy applications.

Development of Ultra-High Density Silicon Nanowire Arrays for Electronics Applications

This article reviews our recent progress on ultra-high density nanowires(NWs)array-based electronics.The superlattice nanowire pattern transfer(SNAP)method is utilized to produce aligned,ultra-high density Si NW arrays.We fi rst cover processing and materials issues related to achieving bulk-like conductivity characteristics from 1020 nm wide Si NWs.We then discuss Si NW-based fi eld-effect transistors(FETs).These NWs&NW FETs provide terrifi c building blocks for various electronic circuits with applications to memory,energy conversion,fundamental physics,logic,and others.We focus our discussion on complementary symmetry NW logic circuitry,since that provides the most demanding metrics for guiding nanofabrication.Issues such as controlling the density and spatial distribution of both p-and n-type dopants within NW arrays are discussed,as are general methods for achieving Ohmic contacts to both p-and n-type NWs.These various materials and nanofabrication advances are brought together to demonstrate energy effi cient,complementary symmetry NW logic circuits.

Mechanisms of water oxidation on heterogeneous catalyst surfaces

Water oxidation,an essential step in photosynthesis,has attracted intense research attention.Understanding the reaction pathways at the electrocatalyst/water interface is of great importance for the development of water oxidation catalysts.How the water is oxidized on the electrocatalyst surface by the positive charges is still an open question.This review summarizes current advances in studies on surface chemistry within the context of water oxidation,including the intermediates,reaction mechanisms,and their influences on the reaction kinetics.The Tafel analyses of some electrocatalysts and the rate-laws relative to charge consumption rates are also presented.Moreover,how the multiple charge transfer relies on the intermediate coverage and the accumulated charge numbers is outlined.Lastly,the intermediates and rate-determining steps on some water oxidation catalysts are discussed based on density functional theories.

Kinetically-Induced Hexagonality in Chemically Grown Silicon Nanowires

Various silicon crystal structures with different atomic arrangements from that of diamond have been observed in chemically synthesized nanowires.The structures are typified by mixed stacking mismatches of closely packed Si dimers.Instead of viewing them as defects,we define the concept of hexagonality and describe these structures as Si polymorphs.The small transverse dimensions of a nanowire make this approach meaningful.Unique among the polymorphs are cubic symmetry diamond and hexagonal symmetry wurtzite structures.Electron diffraction studies conducted with Au as an internal reference unambiguously confirm the existence of the hexagonal symmetry Si nanowires.Cohesive energy calculations suggest that the wurtzite polymorph is the least stable and the diamond polymorph is the most stable.Cohesive energies of intermediate polymorphs follow a linear trend with respect to their structural hexagonality.We identify the driving force in the polymorph formations as the growth kinetics.Fast longitudinal elongation during the growth freezes stacking mismatches and thus leads to a variety of Si polymorphs.The results are expected to shed new light on the importance of growth kinetics in nanomaterial syntheses and may open up ways to produce structures that are uncommon in bulk materials.

High Performance Ring Oscillators from 10-nm Wide Silicon Nanowire Field-Effect Transistors

We explore 10-nm wide Si nanowire （SiNW） field-effect transistors （FETs） for logic applications, via the fabrication and testing of SiNW-based ring oscillators. We report on SiNW surface treatments and dielectric annealing, for producing SiNW FETs that exhibit high performance in terms of large on/off-state current ratio （-10s）, low drain-induced barrier lowering （-30 mV） and low subthreshold swing （-80 mV/decade）. The performance of inverter and ring-oscillator circuits fabricated from these nanowire FETs are also explored. The inverter demonstrates the highest voltage gain （-148） reported for a SiNW-based NOT gate, and the ring oscillator exhibits near rail-to-rail oscillation centered at 13.4 MHz. The static and dynamic characteristics of these NW devices indicate that these SiNW-based FET circuits are excellent candidates for various high-performance nanoelectronic applications.

Dependence of interface energetics and kinetics on catalyst loading in a photoelectrochemical system

Solar hydrogen production by the photoelectrochemical method promises a means to store solar energy.While it is generally understood that the process is highly sensitive to the nature of the interface between the semiconductor and the electrolyte,a detailed understanding of this interface is still missing.For instance,few prior studies have established a clear relationship between the interface energetics and the catalyst loading amount.Here we aim to study this relationship on a prototypical Si-based photoelectrochemical system.Two types of interfaces were examined,one with GaN nanowires as a protection layer and one without.It was found that when GaN was present,higher Pt loading (> 0.1 μg/cm2) led to not only better water reduction (and,hence,hydrogen evolution) kinetics but also more favorable interface energetics for greater photovoltages.In the absence of the protection layer,by stark contrast,increased Pt loading exhibited no measurable influence on the interface energetics,and the main difference was observed only in the hydrogen evolution kinetics.The study sheds new light on the importance of interface engineering for further improvement of photoelectrochemical systems,especially concerning the role of catalysts and protection layers.