The increasing human need for clean and renewable energy has stimulated research in artificial photosynthesis, and in particular water photoelectrolysis as a pathway to hydrogen fuel. Nanostructured devices are widely regarded as an opportunity to improve efficiency and lower costs, but as a detailed analysis shows, they also have considerably disadvantages. This article reviews the current state of research on nanoscale-enhanced photoelectrodes and photocatalysts for the water splitting reaction. The focus is on transition metal oxides with special emphasis of Fe2O3, but nitrides and chalcogenides, and main group element compounds, including carbon nitride and silicon, are also covered. The effects of nanostructuring on carrier generation and collection, multiple exciton generation, and quantum confinement are also discussed, as well as implications of particle size on surface recombination, on the size of space charge layers and on the possibility of controlling nanostructure energetics via potential determining ions. After a summary of electrocatalytic and plasmonic nanostructures, the review concludes with an outlook on the challenges in solar fuel generation with nanoscale inorganic materials.

Inorganic nanostructures for photoelectrochemical and photocatalytic water splitting

Advanced electrodes with a high energy density at high power are urgently needed for high-performance energy storage devices, including lithium-ion batteries (LIBs) and supercapacitors (SCs), to fulfil the requirements of future electrochemical power sources for applications such as in hybrid electric/plug-in-hybrid (HEV/PHEV) vehicles. Metal sulfides with unique physical and chemical properties, as well as high specific capacity/capacitance, which are typically multiple times higher than that of the carbon/graphite-based materials, are currently studied as promising electrode materials. However, the implementation of these sulfide electrodes in practical applications is hindered by their inferior rate performance and cycling stability. Nanostructures offering the advantages of high surface-to-volume ratios, favourable transport properties, and high freedom for the volume change upon ion insertion/extraction and other reactions, present an opportunity to build next-generation LIBs and SCs. Thus, the development of novel concepts in material research to achieve new nanostructures paves the way for improved electrochemical performance. Herein, we summarize recent advances in nanostructured metal sulfides, such as iron sulfides, copper sulfides, cobalt sulfides, nickel sulfides, manganese sulfides, molybdenum sulfides, tin sulfides, with zero-, one-, two-, and three-dimensional morphologies for LIB and SC applications. In addition, the recently emerged concept of incorporating conductive matrices, especially graphene, with metal sulfide nanomaterials will also be highlighted. Finally, some remarks are made on the challenges and perspectives for the future development of metal sulfide-based LIB and SC devices.

Nanostructured metal sulfides for energy storage

In recent years, nanocrystals of metal sulfide materials have attracted scientific research interest for renewable energy applications due to the abundant choice of materials with easily tunable electronic, optical, physical and chemical properties. Metal sulfides are semiconducting compounds where sulfur is an anion associated with a metal cation; and the metal ions may be in mono-, bi- or multi-form. The diverse range of available metal sulfide materials offers a unique platform to construct a large number of potential materials that demonstrate exotic chemical, physical and electronic phenomena and novel functional properties and applications. To fully exploit the potential of these fascinating materials, scalable methods for the preparation of low-cost metal sulfides, heterostructures, and hybrids of high quality must be developed. This comprehensive review indicates approaches for the controlled fabrication of metal sulfides and subsequently delivers an overview of recent progress in tuning the chemical, physical, optical and nano- and micro-structural properties of metal sulfide nanocrystals using a range of material fabrication methods. For hydrogen energy production, three major approaches are discussed in detail: electrocatalytic hydrogen generation, powder photocatalytic hydrogen generation and photoelectrochemical water splitting. A variety of strategies such as structural tuning, composition control, doping, hybrid structures, heterostructures, defect control, temperature effects and porosity effects on metal sulfide nanocrystals are discussed and how they are exploited to enhance performance and develop future energy materials. From this literature survey, energy conversion currently relies on a limited range of metal sulfides and their composites, and several metal sulfides are immature in terms of their dissolution, photocorrosion and long-term durability in electrolytes during water splitting. Future research directions for innovative metal sulfides should be closely allied to energy and environmental issues, along with their advanced characterization, and developing new classes of metal sulfide materials with well-defined fabrication methods.

Recent advances in metal sulfides: from controlled fabrication to electrocatalytic, photocatalytic and photoelectrochemical water splitting and beyond

Plasmonic nanostructures may enhance solar energy collection. Here, the authors exploit both plasmon-induced resonant energy transfer and surface plasmon polaritons in a hematite–gold nano-array, leading to a tenfold increase in the photocurrent at a bias of 0.23 V in a photoelectrochemical cell.

/pdf/plasmon-induced-photonic-and-energy-transfer-enhancement-of-2akmnxeoua.pdf

Plasmon-induced photonic and energy-transfer enhancement of solar water splitting by a hematite nanorod array

Renewable energy technology has been considered as a "MUST" option to lower the use of fossil fuels for industry and daily life. Designing critical and sophisticated materials is of great importance in order to realize high-performance energy technology. Typically, efficient synthesis and soft surface modification of nanomaterials are important for energy technology. Therefore, there are increasing demands on the rational design of efficient electrocatalysts or electrode materials, which are the key for scalable and practical electrochemical energy devices. Nevertheless, the development of versatile and cheap strategies is one of the main challenges to achieve the aforementioned goals. Accordingly, plasma technology has recently appeared as an extremely promising alternative for the synthesis and surface modification of nanomaterials for electrochemical devices. Here, the recent progress on the development of nonthermal plasma technology is highlighted for the synthesis and surface modification of advanced electrode materials for renewable energy technology including electrocatalysts for fuel cells, water splitting, metal-air batteries, and electrode materials for batteries and supercapacitors, etc.

Plasma-Assisted Synthesis and Surface Modification of Electrode Materials for Renewable Energy

Our recent studies showed that nanowire based DSSCs exhibited over 250 mV higher open circuit potentials (VOC) compared to those using nanoparticles. In this study, the electron transport and surface properties of nanowires and nanoparticles are investigated to understand the reasons for the observed higher photovoltages with NW based solar cells. It was seen that, in addition to slow recombination kinetics, the lower work function of SnO2nanowires compared to the nanoparticle counterparts also significantly contributes to the high VOC observed for the nanowire based DSSCs.

Surface properties of SnO2nanowires for enhanced performance with dye-sensitized solar cells

Undoped hematite nanowire arrays grown using plasma oxidation of iron foils show significant photoactivity (?0.38?mA?cm?2 at 1.5?V versus reversible hydrogen electrode in 1?M KOH). In contrast, thermally oxidized nanowire arrays grown on iron exhibit no photoactivity due to the formation of a thick (>7??m?Fe1?xO) interfacial layer. An atmospheric plasma oxidation process required only a few minutes to synthesize hematite nanowire arrays with a 1?5??m interfacial layer of magnetite between the nanowire arrays and the iron substrate. An amorphous oxide surface layer on hematite nanowires, if present, is shown to decrease the resulting photoactivity of as-synthesized, plasma grown nanowire arrays. The photocurrent onset potential is improved after removing the amorphous surface on the nanowires using an acid etch. A two-step method involving high temperature nucleation followed by growth at low temperature is shown to produce a highly dense and uniform coverage of nanowire arrays.

http://eri.louisville.edu/papers_pres/Sunkara_Nanotechnology_2012.pdf

Photoelectrochemical activity of as-grown, α-Fe2O3 nanowire array electrodes for water splitting

We report the phase transformation of hematite (α-Fe2O3) single crystal nanowires to crystalline FeS nanotubes using sulfurization with H2S gas at relatively low temperatures. Characterization indicates that phase pure hexagonal FeS nanotubes were formed. Time-series sulfurization experiments suggest epitaxial growth of FeS as a shell layer on hematite. This is the first report of hollow, crystalline FeS nanotubes with NiAs structure and also on the Kirkendall effect in solid–gas reactions with nanowires involving sulfurization.

Iron Sulfide (FeS) Nanotubes Using Sulfurization of Hematite Nanowires

Here, we report direct band gap transition for Gallium Phosphide (GaP) when alloyed with just 1–2 at% antimony (Sb) utilizing both density functional theory based computations and experiments. First principles density functional theory calculations of GaSbxP1−x alloys in a 216 atom supercell configuration indicate that an indirect to direct band gap transition occurs at x = 0.0092 or higher Sb incorporation into GaSbxP1−x. Furthermore, these calculations indicate band edge straddling of the hydrogen evolution and oxygen evolution reactions for compositions ranging from x = 0.0092 Sb up to at least x = 0.065 Sb making it a candidate for use in a Schottky type photoelectrochemical water splitting device. GaSbxP1−x nanowires were synthesized by reactive transport utilizing a microwave plasma discharge with average compositions ranging from x = 0.06 to x = 0.12 Sb and direct band gaps between 2.21 eV and 1.33 eV. Photoelectrochemical experiments show that the material is photoactive with p-type conductivity. This study brings attention to a relatively uninvestigated, tunable band gap semiconductor system with tremendous potential in many fields.

/pdf/direct-band-gap-gallium-antimony-phosphide-gasb-x-p-1-x-3hrmtq54ni.pdf

Direct Band Gap Gallium Antimony Phosphide (GaSb x P 1-x ) Alloys

In this article, we present a large-area synthesis of carbon microtubes (CMTs) with internal diameters of ∼1.5 μm and wall thicknesses on the order of 50 nm and their lithium ion intercalation and deintercalation properties for the first time. The results show that CMTs exhibit a good capacity retention of 443 mAh g−1 after 20 cycles, higher than the theoretical capacity of graphite and ∼1.5 times higher than the capacity of multi-walled carbon nanotubes at similar conditions. The high capacity retention is attributed mostly to the presence of nanodomains of graphite in the thin walls of the microtubes providing multiple pathways for lithium ion intercalation and the open ends of the CMTs providing additional surface area for intercalation. The CMTs show good rate capability with a capacity retention of 135 mAh g−1 at a current density of 1.5 A g−1.

http://eri.louisville.edu/papers_pres/C-Microtubes.pdf

Harry B. Russell

Papers

Surface properties of SnO2nanowires for enhanced performance with dye-sensitized solar cells

Photoelectrochemical activity of as-grown, α-Fe2O3 nanowire array electrodes for water splitting

Iron Sulfide (FeS) Nanotubes Using Sulfurization of Hematite Nanowires

Direct Band Gap Gallium Antimony Phosphide (GaSb x P 1-x ) Alloys

Thin-Walled Carbon Microtubes as High-Capacity and High-Rate Anodes in Lithium-Ion Batteries