Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications

Fujishima and Honda (1972) demonstrated the potential of titanium dioxide (TiO2) semiconductor materials to split water into hydrogen and oxygen in a photo-electrochemical cell. Their work triggered the development of semiconductor photocatalysis for a wide range of environmental and energy applications. One of the most significant scientific and commercial advances to date has been the development of visible light active (VLA) TiO2 photocatalytic materials. In this review, a background on TiO2 structure, properties and electronic properties in photocatalysis is presented. The development of different strategies to modify TiO2 for the utilization of visible light, including non metal and/or metal doping, dye sensitization and coupling semiconductors are discussed. Emphasis is given to the origin of visible light absorption and the reactive oxygen species generated, deduced by physicochemical and photoelectrochemical methods. Various applications of VLA TiO2, in terms of environmental remediation and in particular water treatment, disinfection and air purification, are illustrated. Comprehensive studies on the photocatalytic degradation of contaminants of emerging concern, including endocrine disrupting compounds, pharmaceuticals, pesticides, cyanotoxins and volatile organic compounds, with VLA TiO2 are discussed and compared to conventional UV-activated TiO2 nanomaterials. Recent advances in bacterial disinfection using VLA TiO2 are also reviewed. Issues concerning test protocols for real visible light activity and photocatalytic efficiencies with different light sources have been highlighted.

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A review on the visible light active titanium dioxide photocatalysts for environmental applications

We review the application of impedance spectroscopy in dye-sensitized solar cells, quantum dot-sensitized solar cells and organic bulk heterojunction solar cells. We emphasize the interpretation of the impedance parameters for determining the internal features of the device, concerning the carrier distribution, materials properties such as the density of states and/or doping of the semiconductors, and the match of energy levels for photoinduced charge generation and separation. Another central task is the determination of recombination mechanisms from the measured resistances, and the factors governing the device performance by combined analysis of resistances as a function of voltage and current–voltage curves.

Characterization of nanostructured hybrid and organic solar cells by impedance spectroscopy

Quantum-dot-sensitized solar cells (QDSCs) are a promising low-cost alternative to existing photovoltaic technologies such as crystalline silicon and thin inorganic films The absorption spectrum of quantum dots (QDs) can be tailored by controlling their size, and QDs can be produced by low-cost methods Nanostructures such as mesoporous films, nanorods, nanowires, nanotubes and nanosheets with high microscopic surface area, redox electrolytes and solid-state hole conductors are borrowed from standard dye-sensitized solar cells (DSCs) to fabricate electron conductor/QD monolayer/hole conductor junctions with high optical absorbance Herein we focus on recent developments in the field of mono- and polydisperse QDSCs Stability issues are adressed, coating methods are presented, performance is reviewed and special emphasis is given to the importance of energy-level alignment to increase the light to electric power conversion efficiency

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Quantum‐Dot‐Sensitized Solar Cells

Quantum dot sensitized solar cells (QDSCs) have attracted significant attention as promising third-generation photovoltaic devices. In the form of quantum dots (QDs), the semiconductor sensitizers have very useful and often tunable properties; moreover, their theoretical thermodynamic efficiency might be as high as 44%, better than the original 31% calculated ceiling. Unfortunately, the practical performance of these devices still lags behind that of dye-sensitized solar cells. In this Account, we summarize the strategies for depositing CdSe quantum dots on nanostructured mesoporous TiO2 electrodes and discuss the methods that facilitate improvement in the performance and stability of QDSCs. One particularly significant factor for solar cells that use polysulfide electrolyte as the redox couple, which provides the best performance among QDSCs, is the passivation of the photoanode surface with a ZnS coating, which leads to a dramatic increase of photocurrents and efficiencies. However, these solar cells us...

Recombination in Quantum Dot Sensitized Solar Cells

Here we present a CdS quantum dot sensitized solar cell based on a mesoporous TiO2 film with remarkable stability using I−/I3− electrolyte. Chemical Bath Deposition (CBD) was used to deposit the CdS quantum dots within the porous network. We show that a thin coating of the QD sensitized film with an amorphous TiO2 layer strongly improves the performance and photostability of the solar cell. We propose that the coating passivates QD surface states which act as hole traps and are responsible for photodegradation of the device. In addition, this coating decreases the recombination of electrons from the CdS quantum dots and the mesoporous TiO2 into the electrolyte solution. We obtain a significant improvement of all cell parameters resulting in a total light to electric power conversion efficiency of 1.24%.

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Core/CdS Quantum Dot/Shell Mesoporous Solar Cells with Improved Stability and Efficiency Using an Amorphous TiO2 Coating

Cathodic electrophoretic deposition (EPD) of commercially available TiO 2 nanocrystals (P-25 and P-90, Degussa) for the formation of nanoporous electrodes for dye-sensitized solar cells (DSSCs) is reported. Thick, uniform, adherent TiO 2 films were obtained from alcoholic suspensions prepared in two steps: first, nanoparticles were treated in an alcoholic suspension with a small amount of acetylacetone; second, the obtained suspension was mixed with a EPD “charging solution” with small additives of iodine, acetone and water. A modified mechanical compression technique was employed to the electrophoretically deposited films to vary the porous structure and thickness of TiO 2 layers. By applying non-polar volatile organic liquids to fill the pores of dry TiO 2 films before compression, we obtained a substantial improvement in the quality of the pressed films. The fabrication of multilayer electrodes enabled us to obtain TiO 2 films with a thickness up to 25 μm and the photoelectric conversion efficiency up to 8.5% under 1 sun illumination.

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Electrophoretic deposition and compression of titania nanoparticle films for dye-sensitized solar cells

Here, a new method based on sol–gel electrophoretic deposition to produce uniform high-quality inorganic conformal coatings on mesoporous nanoparticulate films is presented. This novel sol preparation method allows for very fine control of the coating properties, thus inducing new adjustable functionalities to these electrodes. It is shown that the deposition of an amorphous TiO2 and/or MgO shell onto photoanodes used in dye-sensitized solar cells (DSSCs) improves their light-to-electric-power conversion efficiency without the need for sintering. It is proposed that the amorphous TiO2 coating improves the electronic inter-particle connection and passivates the surface states. The insulating MgO coating further reduces the electron transfer from the conduction band into the electrolyte while the electron injection from the excited dye state remains unperturbed for thin coatings. Using a low-temperature method for DSSC production on plastic substrates, a maximum efficiency of 6.2% applying pressure together with an optimized TiO2 coating is achieved. For systems that cannot be pressed a conversion efficiency of 5.1% is achieved using a double shell TiO2/MgO coating.

Conformal Nano‐Sized Inorganic Coatings on Mesoporous TiO2 Films for Low‐Temperature Dye‐Sensitized Solar Cell Fabrication

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The influence of suspension composition and deposition mode on the electrophoretic deposition of TiO2 nanoparticle agglomerates

Quantum dot sensitized solar cells (QDSSCs) present a promising technology for next generation photovoltaic cells, having exhibited a considerable leap in performance over the last few years. However, recombination processes occurring in parallel at the TiO2–QDs–electrolyte triple junction constitute one of the major limitations for further improvement of QDSSCs. Reaching higher conversion efficiencies necessitates gaining a better understanding of the mechanisms of charge recombination in these kinds of cells; this will essentially lead to the development of new solutions for inhibiting the described losses. In this study we have systematically examined the contribution of each interface formed at the triple junction to the recombination of the solar cell. We show that the recombination of electrons at the TiO2/QDs interface is as important as the recombination from TiO2 and QDs to the electrolyte. By applying conformal MgO coating both above and below the QD surface, recombination rates were significantly reduced, and an improvement of more than 20% in cell efficiency was recorded.

Larissa Grinis

Papers

Core/CdS Quantum Dot/Shell Mesoporous Solar Cells with Improved Stability and Efficiency Using an Amorphous TiO2 Coating

Electrophoretic deposition and compression of titania nanoparticle films for dye-sensitized solar cells

Conformal Nano‐Sized Inorganic Coatings on Mesoporous TiO2 Films for Low‐Temperature Dye‐Sensitized Solar Cell Fabrication

The influence of suspension composition and deposition mode on the electrophoretic deposition of TiO2 nanoparticle agglomerates

The importance of the TiO2/quantum dots interface in the recombination processes of quantum dot sensitized solar cells