The past several decades have seen a significant rise in atmospheric carbon dioxide levels resulting from the combustion of hydrocarbon fuels. A solar energy based technology to recycle carbon dioxide into readily transportable hydrocarbon fuel (i.e., a solar fuel) would help reduce atmospheric CO2 levels and partly fulfill energy demands within the present hydrocarbon based fuel infrastructure. We review the present status of carbon dioxide conversion techniques, with particular attention to a recently developed photocatalytic process to convert carbon dioxide and water vapor into hydrocarbon fuels using sunlight.

Toward Solar Fuels: Photocatalytic Conversion of Carbon Dioxide to Hydrocarbons

CuO nanostructures: Synthesis, characterization, growth mechanisms, fundamental properties, and applications

It is well-known that inorganic nanocrystals are a benchmark model for nanotechnology, given that the tunability of optical properties and the stabilization of specific phases are uniquely possible at the nanoscale. Copper (I) oxide (Cu(2)O) is a metal oxide semiconductor with promising applications in solar energy conversion and catalysis. To understand the Cu/Cu(2)O/CuO system at the nanoscale, we have developed a method for preparing highly uniform monodisperse nanocrystals of Cu(2)O. The procedure also serves to demonstrate our development of a generalized method for the synthesis of transition metal oxide nanocrystals. Cu nanocrystals are initially formed and subsequently oxidized to form highly crystalline Cu(2)O. The volume change during phase transformation can induce crystal twinning. Absorption in the visible region of the spectrum gave evidence for the presence of a thin, epitaxial layer of CuO, which is blue-shifted, and appears to increase in energy as a function of decreasing particle size. XPS confirmed the thin layer of CuO, calculated to have a thickness of approximately 5 A. We note that the copper (I) oxide phase is surprisingly well-stabilized at this length scale.

Copper oxide nanocrystals.

In this article, we present recent advances that we have achieved toward improving the properties of anodically formed semiconducting TiO2 nanotubes as well as nanowire arrays as electrodes for oxidative photoelectrochemistry. The morphology, crystallinity, composition, and illumination geometry of nanotube or nanowire arrays are critical factors in their performance as photoelectrodes. We discuss the key aspects relating to each factor and the advances achieved in improving each. With respect to the more fully investigated nanotube arrays, the ability to control the morphological parameters such as pore size, tube length, and wall thickness of the nanotube architecture has enabled high performance in applications such as water photoelectrolysis, photocatalysis, dye-sensitized solar cells, and heterojunction TiO2−polymer hybrid solar cells. We begin by reviewing the photoelectrochemical performance of state-of-the-art nanotube arrays fabricated on planar substrates. We then present more recent results rel...

Recent Advances in the Use of TiO2 Nanotube and Nanowire Arrays for Oxidative Photoelectrochemistry

Photoanodes consisting of nanostructured hematite prepared by atmospheric pressure chemical vapor deposition (APCVD) have previously set a benchmark for solar water splitting. Here, we fully investigate this promising system by varying critical synthetic parameters and probing the photoanode performance to determine the major factors that influence operation. By varying the film thickness, we show film growth to be linear with an incubation time. We find no concern with electron transport for films up to 600 nm, but a higher recombination rate of photogenerated carriers in the hematite near the interface with the fluorine-doped tin oxide, as compared to the bulk section of the film. The mechanism for the formation of the thin film’s nanoporous dendritic structure is discussed on the basis of the results from varying the substrate growth temperate. The observed feature sizes of the film are found to depend strongly on this temperature and the presence of silicon dopant precursor (TEOS). Raman and Mossbauer...

Influence of Feature Size, Film Thickness, and Silicon Doping on the Performance of Nanostructured Hematite Photoanodes for Solar Water Splitting

Lithium‐doped p‐type CuO is a low mobility semiconductor with an indirect band gap of 1.35 eV and a flatband potential of +0.55 V, with respect to the saturated calomel electrode (SCE), when in contact with an electrolyte at a pH of 9.4. Its valence band lies 5.42 eV below the vacuum level and is made up mainly from the Cu2+−3d wavefunctions. An oxygen‐2p type band is at 7.33 eV, in agreement with a semi‐empirical estimate. Its performance as a photoelectrode for the solar photoelectrolysis of water is rather poor, due to the presence of recombination centers in the band gap and its chemical instability. As photoelectrolysis electrodes, oxides with 3d‐type valence bands may have advantages over the more common oxides with 2p‐type valence bands.

F. P. Koffyberg

Papers

A photoelectrochemical determination of the position of the conduction and valence band edges of p‐type CuO

Opto-electronic properties of p- and n-type delafossite, CuFeO2

Quantum efficiency and optical transitions of CdO photoanodes

Optical bandgaps and electron affinities of semiconducting Rh2O3(I) and Rh2O3(III)

Preparation and opto-electronic properties of semiconducting CuCrO2