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.

/pdf/electronics-and-optoelectronics-of-two-dimensional-2jbqr1unvw.pdf

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.

/pdf/visible-light-photocatalysis-in-nitrogen-doped-titanium-np6nklr9gh.pdf

Visible-Light Photocatalysis in Nitrogen-Doped Titanium Oxides

The emergence of semiconductor nanocrystals as the building blocks of nanotechnology has opened up new ways to utilize them in next generation solar cells. This paper focuses on the recent developments in the utilization of semiconductor quantum dots for light energy conversion. Three major ways to utilize semiconductor dots in solar cell include (i) metal−semiconductor or Schottky junction photovoltaic cell (ii) polymer−semiconductor hybrid solar cell, and (iii) quantum dot sensitized solar cell. Modulation of band energies through size control offers new ways to control photoresponse and photoconversion efficiency of the solar cell. Various strategies to maximize photoinduced charge separation and electron transfer processes for improving the overall efficiency of light energy conversion are discussed. Capture and transport of charge carriers within the semiconductor nanocrystal network to achieve efficient charge separation at the electrode surface remains a major challenge. Directing the future resear...

http://wiki.phy.queensu.ca/shughes/images/d/d1/Quantum_Dot_Solar_Cells._Semiconductor_Nanocrystals_as_Light_Harvesters.pdf

Quantum Dot Solar Cells. Semiconductor Nanocrystals as Light Harvesters

Graphene oxide suspended in ethanol undergoes reduction as it accepts electrons from UV-irradiated TiO2 suspensions. The reduction is accompanied by changes in the absorption of the graphene oxide, as the color of the suspension shifts from brown to black. The direct interaction between TiO2 particles and graphene sheets hinders the collapse of exfoliated sheets of graphene. Solid films cast on a borosilicate glass gap separated by gold-sputtered terminations show an order of magnitude decrease in lateral resistance following reduction with the TiO2 photocatalyst. The photocatalytic methodology not only provides an on-demand UV-assisted reduction technique but also opens up new ways to obtain photoactive graphene-semiconductor composites.

http://www.dstuns.iitm.ac.in/grouppresentations/2008/TiO2-Graphene_Nanocomposites_UV-Assisted_Photocatalytic_Reduction_of_Graphene_Oxide.pdf

TiO2-Graphene Nanocomposites. UV-Assisted Photocatalytic Reduction of Graphene Oxide

The increasing energy demand in the near future will force us to seek environmentally clean alternative energy resources. The emergence of nanomaterials as the new building blocks to construct light energy harvesting assemblies has opened up new ways to utilize renewable energy sources. This article discusses three major ways to utilize nanostructures for the design of solar energy conversion devices:  (i) Mimicking photosynthesis with donor−acceptor molecular assemblies or clusters, (ii) semiconductor assisted photocatalysis to produce fuels such as hydrogen, and (iii) nanostructure semiconductor based solar cells. This account further highlights some of the recent developments in these areas and points out the factors that limit the efficiency optimization. Strategies to employ ordered assemblies of semiconductor and metal nanoparticles, inorganic-organic hybrid assemblies, and carbon nanostructures in the energy conversion schemes are also discussed. Directing the future research efforts toward utiliza...

Meeting the Clean Energy Demand: Nanostructure Architectures for Solar Energy Conversion

Photoexcited semiconductor nanoparticles undergo charge equilibration when they are in contact with metal nanoparticles. Such a charge distribution has direct influence in dictating the energetics of the composite by shifting the Fermi level to more negative potentials. The transfer of electrons to Au nanoparticles has now been probed by exciting TiO2 nanoparticles under steady-state and laser pulse excitation. Equilibration with the C60/C60- redox couple provides a means to determine the apparent Fermi level of the TiO2−Au composite system. The size-dependent shift in the apparent Fermi level of the TiO2−Au composite (20 mV for 8-nm diameter and 40 mV for 5-nm and 60 mV for 3-nm gold nanoparticles) shows the ability of Au nanoparticles to influence the energetics by improving the photoinduced charge separation. Isolation of individual charge-transfer steps from UV-excited TiO2 → Au → C60 has provided mechanistic and kinetic information on the role of metal in semiconductor-assisted photocatalysis and siz...

Catalysis with TiO2/Gold Nanocomposites. Effect of Metal Particle Size on the Fermi Level Equilibration

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.

https://pubs.acs.org/doi/pdf/10.1021/jp0209289

Prashant V. Kamat

Papers

Quantum Dot Solar Cells. Semiconductor Nanocrystals as Light Harvesters

TiO2-Graphene Nanocomposites. UV-Assisted Photocatalytic Reduction of Graphene Oxide

Meeting the Clean Energy Demand: Nanostructure Architectures for Solar Energy Conversion

Catalysis with TiO2/Gold Nanocomposites. Effect of Metal Particle Size on the Fermi Level Equilibration

Photophysical, photochemical and photocatalytic aspects of metal nanoparticles