There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Two organolead halide perovskite nanocrystals, CH3NH3PbBr3 and CH3NH3PbI3, were found to efficiently sensitize TiO2 for visible-light conversion in photoelectrochemical cells. When self-assembled on mesoporous TiO2 films, the nanocrystalline perovskites exhibit strong band-gap absorptions as semiconductors. The CH3NH3PbI3-based photocell with spectral sensitivity of up to 800 nm yielded a solar energy conversion efficiency of 3.8%. The CH3NH3PbBr3-based cell showed a high photovoltage of 0.96 V with an external quantum conversion efficiency of 65%.

Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells

Single-layer metal dichalcogenides are two-dimensional semiconductors that present strong potential for electronic and sensing applications complementary to that of graphene.

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Electronics and optoelectronics of two-dimensional transition metal dichalcogenides.

Although gold is the subject of one of the most ancient themes of investigation in science, its renaissance now leads to an exponentially increasing number of publications, especially in the context of emerging nanoscience and nanotechnology with nanoparticles and self-assembled monolayers (SAMs). We will limit the present review to gold nanoparticles (AuNPs), also called gold colloids. AuNPs are the most stable metal nanoparticles, and they present fascinating aspects such as their assembly of multiple types involving materials science, the behavior of the individual particles, size-related electronic, magnetic and optical properties (quantum size effect), and their applications to catalysis and biology. Their promises are in these fields as well as in the bottom-up approach of nanotechnology, and they will be key materials and building block in the 21st century. Whereas the extraction of gold started in the 5th millennium B.C. near Varna (Bulgaria) and reached 10 tons per year in Egypt around 1200-1300 B.C. when the marvelous statue of Touthankamon was constructed, it is probable that “soluble” gold appeared around the 5th or 4th century B.C. in Egypt and China. In antiquity, materials were used in an ecological sense for both aesthetic and curative purposes. Colloidal gold was used to make ruby glass 293 Chem. Rev. 2004, 104, 293−346

Gold nanoparticles: assembly, supramolecular chemistry, quantum-size-related properties, and applications toward biology, catalysis, and nanotechnology.

To use solar irradiation or interior lighting efficiently, we sought a photocatalyst with high reactivity under visible light. Films and powders of TiO 2- x N x have revealed an improvement over titanium dioxide (TiO 2 ) under visible light (wavelength 2 has proven to be indispensable for band-gap narrowing and photocatalytic activity, as assessed by first-principles calculations and x-ray photoemission spectroscopy.

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Visible-Light Photocatalysis in Nitrogen-Doped Titanium Oxides

Two protonation equilibria (pKa at 3.7 and 7.4) have been identified for the dye, bis(2,4-dihydroxyphenyl)-squaraine in the ground state. These various protonation forms exhibit widely different absorption properties. The singlet excited-state behaviour of this dye has been characterized by picosecond laser flash photolysis. The fluorescence quantum yield and the excited singlet lifetime of the unprotonated dye (ϕf= 0.092, τs= 740 ps) are considerably higher than that of the neutral (ϕf= 0.037, τs= 130 ps) or the protonated (ϕf= 0.01, τs⩽ 100 ps) forms of the dye. The triplet excited state has been characterized by both direct excitation and T–T energy transfer methods. The triplet excited state has an absorption maximum around 430 nm and a lifetime of ca. 100 µs in ethanol. Photosensitization of a thin microporous TiO2 semiconductor film has been carried out by adsorbing the squaraine dye from an ethanolic solution and exciting it with visible light. An incident photon-to-current conversion efficiency of 0.05% at 500 nm has been obtained for the dye-modified TiO2 electrode.

Excited-state properties and photosensitization behaviour of bis(2,4-dihydroxyphenyl)squaraine

A new and simple approach of synthesizing size-quantized CdSe colloids in reverse micellar suspension is described. The room temperature reaction between Cd2+ and selenosulfate is carried out within the water pool of di-octyl sulphosuccinate (Aerosol-OT) reverse micelles. The size dependent absorption and emission properties of these small CdSe particles (3—5 nm) are described. The Q-sized CdSe nanoparticles exhibit an emission yield of 0.13. Up to a factor of two enhancement in the emission efficiency can be achieved following the surface functionalization of CdSe colloids with triethyl amine.

Tuning the properties of CdSe nanoparticles in reverse micelles

Unique electronic and chemical properties of metal nanoparticles have drawn the attention of chemists, physicists, biologists, and engineers who wish to use them for the development of new generation nanodevices. Metal nanoparticles such as gold and silver show noticeable photoactivity under UV−visible irradiation as is evident from the photoinduced fusion and fragmentation processes. Binding a photoactive molecule (e.g., pyrene) to metal nanoparticle enhances the photochemical activity and renders the organic−inorganic hybrid nanoassemblies suitable for light-harvesting and optoelectronic applications. The nature of charge-transfer interaction of fluorophore with gold surface dictates the pathways with which the excited-state deactivates. Obtaining insight into energy and electron-transfer processes is important to improve the charge separation efficiencies in metal−fluorophore nanoassemblies and photocatalytic activity of metal−semiconductor composites.

Photophysical, Photochemical and Photocatalytic Aspects of Metal Nanoparticles

Having gained prominence in the design of high-efficiency solar cells, metal halide perovskites have taken center stage in photocatalysis. Despite the demonstrations of photocatalytic activity in inducing reduction and oxidation processes through visible-light excitation, the long-term stability of CsPbBr3 and other perovskite nanocrystals and nanostructures still poses a challenge. This Perspective discusses how factors such as instability arising from labile ligands, ease of halide exchange, halide mobility under photoirradiation, chemical and morphological transformations in polar solvents, and difficulty in designing heterostructures pose major hurdles in developing halide perovskites as photocatalysts. The key examples of organic and inorganic transformations as well as interfacial charge-transfer processes discussed here present some unique features of perovskite photocatalysts and highlight the complexity in their utilization for future chemical conversion processes. Strategies to protect the perovskite surface are needed to maintain the required long-term stability in practical applications. 

Prashant V. Kamat

Papers

Excited-state properties and photosensitization behaviour of bis(2,4-dihydroxyphenyl)squaraine

Tuning the properties of CdSe nanoparticles in reverse micelles

Photophysical, Photochemical and Photocatalytic Aspects of Metal Nanoparticles

Efficacy of Perovskite Photocatalysis: Challenges to Overcome

Dominance of Metal Oxides in the Era of Nanotechnology