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International Journal of Photoenergy
Volume 2014 (2014), Article ID 704729, 2 pages

Solar Energy Conversion by Nanostructured TiO2

1Institut für Technische Chemie, Leibniz Universität Hannover, Callinstraße 3, 30167 Hannover, Germany
2Department of Materials Science & Engineering, Faculty of Engineering, National University of Singapore, 5 Engineering Drive 2, Singapore 117576
3Laboratory of Eco-Materials and Sustainable Technology (LEMST), Xinjiang Technical Institute of Physics and Chemistry and Key Laboratory of Functional Materials and Devices for Special Environments, Chinese Academy of Sciences, Urumqi, Xinjiang 830011, China
4Department of Chemical Engineering, Monash University, Clayton, VIC 3800, Australia

Received 24 August 2014; Accepted 24 August 2014; Published 22 December 2014

Copyright © 2014 Jia Hong Pan et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Research in solar energy conversion and the associated photoactive materials has attracted continuous interest. Due to its proper electronic band structure, high quantum efficiency, and photonic and chemical innerness, TiO2 has been demonstrated as a versatile oxide semiconductor capable of efficiently utilizing sunlight to produce electrical and chemical energy. Its outstanding physicochemical performances have led to an array of advanced photocatalytic and photoelectrochemical applications including environmental photocatalysis, dye/semiconductor-sensitized solar cell, and solar fuel productions.

Scientific papers in this special issue have covered this highly developing field. Authors have submitted review and original research articles in the following topics:(i)kinetics and mechanism of TiO2-mediated environmental photocatalysis,(ii)TiO2-based dye/semiconductor-sensitized solar cells,(iii)photocatalytic organic synthesis over TiO2-mediated system,(iv)photocatalytic solar fuel production (water splitting and CO2 reduction),(v)novel TiO2-based nanomaterials for solar energy conversion.

We wish to express our sincere gratefulness to all the authors and referees whose contributions make this special issue possible. A brief overview of all nine accepted papers is below.

In “Recent progress in TiO2-mediated solar photocatalysis for industrial wastewater treatment,” the authors overview the major challenges in industrial wastewater treatment and analysed the merits of TiO2 photocatalysis over conventional water treatment technologies. Then a brief  but comprehensive review is carried out on the recent progress in their applications in several typical industrial wastewaters. The paper could advance the development of solar photocatalysis systems and promote their practical application.

In “Photocatalytic degradation of anthracene in closed system reactor,” the authors investigate the effect of operating parameters on the degradation efficiency of anthracene, one of the toxic persistent organic pollutants. The photodegradation products are identified by GC-MS to better understand the degradation path of anthracene on P25 TiO2 nanoparticles under UV irradiation. Optimum parameters have been systematically investigated to obtain a high photocatalytic degradation rate. The research reported here gives an excellent example how to design proper photocatalytic process for organic pollutants removal.

In “Sol-gel to prepare nitrogen doped TiO2 nanocrystals with exposed facets and high visible-light photocatalytic performance,” a facile synthetic method based on sol-gel process is developed to synthesize anatase TiO2 with dominant facets and nitrogen doping by simply hydrolysing tetrabutyl titanate in NH4F-containing alcoholic solution. NH4F not only acts as the N dopant source, but also guides the preferential growth along direction. The synthesized N-doped TiO2 nanoparticles outperform P25 referents in photocatalytic degradation of methylene blue under visible light irradiation.

In “One-dimensional nanostructured TiO2 for photocatalytic degradation of organic pollutants in wastewater,” current progress in the synthetic methods for one-dimensional (1D) TiO2 nanostructures including nanorods, nanotubes, nanowires/nanofibers, and nanobelts is reviewed. Modification of 1D TiO2 with metal oxide, metal ions, and anions in order to enhance the photocatalytic activity is discussed. Furthermore, photocatalytic degradations of organic pollutants in wastewater over 1D TiO2 are summarized, and the underlying mechanism is discussed. Finally, using 1D nanostructured TiO2 as building blocks to construct film or membrane is highlighted.

In “Layer-by-layer assembly and photocatalytic activity of titania nanosheets on coal fly ash microspheres,” an attempt to address the problem with nanoparticulate TiO2 distribution and recovery is made. Coal fly ash (CFA) microspheres are used as a substrate for the layer-by-layer assembly of Ti0.91O2 sheets. That is, the Ti0.91O2 nanosheets are immobilized on CFA by using sequential modification of cationic polyelectrolyte and Ti0.91O2 nanosheets. The resultant Ti0.91O2/CFA composites show considerable photocatalytic activity in degradation of methylene blue under UV irradiation. After photocatalysis, the Ti0.91O2/CFA can be easily separated and recycled from aqueous solution.

In “Effect of Mn doping on properties of CdS quantum dot-sensitized solar cells,” impurity Mn2+ ions are doped into the precursor solution for CdS deposition. By optimizing the experimental parameters, a significant improvement in photoelectric conversion efficiency can be achieved. After a successive ionic layer adsorption and reaction of a fixed number of six times, the photoelectric conversion efficiency shows the maximum value (1.51%) at the optimal doped ratio, which is much higher than that of the pure one (0.71%).

In “Theoretical study of one-intermediate band quantum dot solar cell,” the authors theoretically studied the influence of the new band on the power conversion efficiency for the structure of the quantum dots of intermediate band solar cell. The time-independent Schrödinger equation is used to determine the optimum width and location of the intermediate band. From their calculation results, the maximum power conversion efficiency is about 70.42% for simple cubic quantum dot crystal under full concentration light. It is strongly dependent on the width of quantum dots and barrier distances.

In “Equilibrium and kinetic aspects in the sensitization of monolayer transparent TiO2 thin films with porphyrin dyes for DSSC applications,” the adsorption of free base and Cu(II) and Zn(II) complexes of the 2,7,12,17-tetrapropionic acid of 3,8,13,18-tetramethyl-21H,23H porphyrin (CPI) on transparent monolayer TiO2 nanoparticle films was studied. The dye loading is found to be accordant with Langmuir isotherm, while kinetic data show significantly better fits to pseudo-first-order model and the evaluated rate constants linearly increase with the grow of initial dye concentrations. The stoichiometry of the adsorption of CPI-dyes into TiO2 and the influence of presence of coadsorbent (chenodeoxycholic acid) are established. This study paves a way for choosing the best experimental conditions for the adsorption of these dyes.

In “Steric and solvent effect in dye-sensitized solar cells utilizing phenothiazine-based dyes,” three novel phenothiazine-based dyes are prepared and utilized for dye-sensitized solar cells (DSSC). The dye-bath solvent presents a significant effect on the overall cell performance. The highest conversion efficiency of 3.78% is obtained using ethanol (EtOH) and 2.53% for tetrahydrofuran (THF) as solvent, respectively. The higher efficiency is related to the higher dye loading and coverage on the TiO2 surface. Meanwhile, phenothiazine-based dyes with longer and branched aliphatic chain increase the steric hindered effect which is mainly responsible for increasing the electron lifetime and decreases the dye aggregation as well as increasing the electron recombination resistance rate at the TiO2-dye-electrolyte interface, hence, enhancing the overall cell performance.

Jia Hong Pan
Detlef W. Bahnemann
Qing Wang
Chuanyi Wang
Xiwang Zhang