Spectral sensitization of a TiO2 semiconductor electrode by CdS microcrystals and its photoelectrochemical properties

Graphite/carbon-doped TiO2 nanocomposite synthesized by ultrasound for the degradation of diclofenac

Graphite/C-doped TiO2 nanocomposite was synthesized at room temperature using a simple, impressive, and indirect sonication (20 kHz) by the cup horn system. Tetrabutyltitanate as the precursor of titanium and graphite (G) as the carbon source was used in the preparation of nanocomposite as a photocatalyst. The molar ratio of G/TiO2 as a key parameter was investigated in the synthesis of G/C-doped TiO2. The obtained materials were widely characterized using XRD, SEM, TEM, FTIR, XPS, and UV-Vis diffuse reflectance techniques. The UV-Vis diffuse reflectance spectroscopy results showed that the edge of light absorption of nanocomposite was distinctly red-shifted to the visible area via carbon doping. The XPS outcomes acknowledged the existence of the C, Ti, and O in the photocatalyst. The composite showed an enhancement in the dissociation efficiency of photoinduced charge carriers through the doping process. The photocatalytic activity of the synthesized nanocomposite was checked with diclofenac (DCF) as a pharmaceutical contaminant. The results displayed that G/C-doped TiO2 represented better photocatalytic performance for DCF than TiO2. This was due to the excellent crystallization, intense absorption of visible light, and the impressive separation of photoinduced charge carriers. Various active species such as •OH, •O2¯, h+, and H2O2 play a role in the degradation of DFC. Therefore, different scavengers were used and the role of each one in degradation was investigated. According to the obtained results, •O2¯ radical showed a major role in the photocatalytic process. This work not only proposes a deep insight into the photosensitization-like mechanism by using G-based materials but also develops new photocatalysts for the removal of emerging organic pollutants from waters using sunlight as available cheap energy.

Quantum dot-sensitized solar cells

A comprehensive overview of the development of quantum dot-sensitized solar cells (QDSCs) is presented.

An experimental and theoretical study on the electronic and structural properties of CdSe@TiO2 nanotube arrays

The effects of the structural and electronic parameters on the water splitting over CdSe@TiO₂NT were investigated using experimental and theoretical methods.

Mesoporous TiO₂-Based Photoanode Sensitized by BiOI and Investigation of Its Photovoltaic Behavior

We reported a novel BiOI/mesoporous TiO2 photoanode for solar cells, which was fabricated with BiOI attached onto a three-dimensional mesoporous TiO2 film by a chemical bath deposition (CBD) method. BiOI was revealed as an efficient and environmental friendly semiconductor sensitizer to make TiO2 respond to visible light. Based on this photoanode, mesoporous TiO2-based solar cell sensitized by BiOI exhibited promising photovoltaic performance. Meanwhile, the optimization of photovoltaic performance was also achieved by varying cycles of deposition immersions. The highest open circuit voltage and short circuit current of the solar cell can reach 0.5 V and 1.5 mA/cm(2), respectively.

Semiconductor-based photocatalysts and photoelectrochemical cells for solar fuel generation: a review

This perspective article describes the barrier, progress and future direction of research on the photocatalytic and photoelectrochemical solar fuel generation.

Carbon nanotube-based heterostructures for solar energy applications

One means of combining the unique physical and chemical properties of both carbon nanotubes and complementary material motifs (such as metal sulfide quantum dots (QDs), metal oxide nanostructures, and polymers) can be achieved by generating carbon nanotube (CNT)-based heterostructures. These materials can be subsequently utilized as novel and interesting constituent building blocks for the assembly of functional light energy harvesting devices and because of their architectural and functional flexibility, can potentially open up novel means of using and taking advantage of existing renewable energy sources. In this review, we present the reliable and reproducible synthesis of several unique model CNT-based heterostructured systems as well as include an accompanying discussion about the charge transfer and energy flow properties of these materials for their potential incorporation into a range of practical solar energy conversion devices.

CdS and CdTeS quantum dot decorated TiO2 nanowires. Synthesis and photoefficiency

An easy process was developed to synthesize TiO(2) nanowires sensitized with CdS and CdTeS quantum dots (QDs) requiring no pretreatment of the TiO(2) nanowires prior to nanoparticle generation. CdS and CdTeS nanoparticles were firstly grown by an in situ colloidal method directly onto the TiO(2) surface, hence not requiring subsequent functionalization of the QDs. The resulting nanostructure assembly and composition was confirmed by transmission electron microscopy (TEM), x-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Successful decoration of the TiO(2) nanowires by the QDs was observed by TEM, while XPS spectra provided clear evidence for the coexistence of CdS and CdTeS QDs and TiO(2) nanowires. The electronic structure of the TiO(2) nanowires was preserved as indicated by Raman spectroscopy. Preliminary photocurrent measurements showed that inclusion of Te in CdS QDs improved the photocurrent efficiency. Compared to bare TiO(2) nanowires, CdS/TiO(2) nanoassemblies showed an enhancement in photocurrent efficiency of 300% while CdTeS/TiO(2) presented an improvement of 350%. This study indicates that the generation of strongly anchored CdS and CdTeS QDs on a TiO(2) nanowire surface is achievable without introduction of a linker molecule, whose presence is known to decrease the electron injection efficiency.

Tailored TiO2-SrTiO3 heterostructure nanotube arrays for improved photoelectrochemical performance

TiO(2) nanotube arrays formed on Ti substrate by electrochemical anodization have been converted into TiO(2)-SrTiO(3) heterostructures by controlled substitution of Sr under hydrothermal conditions. The growth of SrTiO(3) crystallites on the nanotube array electrode was probed by electron microscopy and X-ray diffraction. As the degree of Sr substitution increases with the duration of hydrothermal treatment, an increase in the size of SrTiO(3) crystallites was observed. Consequently, with increasing SrTiO(3) fraction in the TiO(2)-SrTiO(3) nanotube arrays, we observed a shift in the flat band potential to more negative potentials, thus confirming the influence of SrTiO(3) in the modification of the photoelectrochemical properties. The TiO(2)-SrTiO(3) composite heterostructures obtained with 1 h or less hydrothermal treatment exhibit the best photoelectrochemical performance with nearly 100% increase in external quantum efficiency at 360 nm. The results presented here provide a convenient way to tailor the photoelectrochemical properties of TiO(2)-SrTiO(3) nanotube array electrodes and employ them for dye- or quantum-dot-sensitized solar cells and/or photocatalytic hydrogen production.

Photoeletrocatalytic activity of a Cu2O-loaded self-organized highly oriented TiO2 nanotube array electrode for 4-chlorophenol degradation

Differently sized Cu2O nanoparticles have been assembled photocatalytically on the surface of self-organized highly oriented TiO2 nanotubes obtained by anodization of a Ti sheet in fluoride-containing electrolytes. X-ray diffraction analysis identifies an anatase structure and fine preferential orientation of (101) planes. The UV-vis absorption edge of the TiO2 nanotube arrays shift to lower energy after Cu2O loading. The composite array electrode exhibits a higher photovoltage response than the TiO2 powders directly deposited on a Ti sheet. The highest photoconversion efficiencies observed for the Cu2O-loaded electrode are 17.2% and 0.82% under UV light and visible light irradiation, respectively. Especially, the composite array electrode shows a higher efficiency than the nonloaded one for the photoelectrocatalytic decomposition of 4-chlorophenol. The improved photoeletrocatalytic activity of the TiO2/Cu2O composite array electrode is attributed to the synergistic effect of Cu2O nanoparticles and TiO2 nanotube arrays. The Cu2O nanoparticles could enhance the efficiency of photon harvesting and reduce the chances of electron-hole recombination by sending the electrons to the conduction band of TiO2 nanotubes. The accumulated electrons in the conduction band of TiO2 nanotubes would reduce oxygen to form peroxides for enhanced advanced oxidation. The byproducts were identified by high-performance liquid chromatography.

Enhancing Solar Cell Efficiencies through 1-D Nanostructures

The current global energy problem can be attributed to insufficient fossil fuel supplies and excessive greenhouse gas emissions resulting from increasing fossil fuel consumption. The huge demand for clean energy potentially can be met by solar-to-electricity conversions. The large-scale use of solar energy is not occurring due to the high cost and inadequate efficiencies of existing solar cells. Nanostructured materials have offered new opportunities to design more efficient solar cells, particularly one-dimensional (1-D) nanomaterials for enhancing solar cell efficiencies. These 1-D nanostructures, including nanotubes, nanowires, and nanorods, offer significant opportunities to improve efficiencies of solar cells by facilitating photon absorption, electron transport, and electron collection; however, tremendous challenges must be conquered before the large-scale commercialization of such cells. This review specifically focuses on the use of 1-D nanostructures for enhancing solar cell efficiencies. Other nanostructured solar cells or solar cells based on bulk materials are not covered in this review. Major topics addressed include dye-sensitized solar cells, quantum-dot-sensitized solar cells, and p-n junction solar cells.

A CdS-modified TiO(2) nanocrystalline photoanode for efficient hydrogen generation by visible light

Cadmium sulfide (CdS) was in situ synthesized onto mesoporous TiO(2) films and used as a sensitizer to fabricate a photoelectrode for hydrogen generation in visible light. The incorporation of CdS extends the optical absorption threshold of a TiO(2) electrode to visible light, enhancing the visible-light-induced photocurrent. A maximum photoconversion efficiency of 3.67% was achieved for this CdS-sensitized TiO(2) electrode under visible light illumination of 100 mW cm(-2). The hydrogen generation rate obtained at conditions of maximum efficiency is 95.5 µmol cm(-2) h(-1). To the best of our knowledge, the hydrogen generation rate is the highest among those reported for a photoelectrochemical cell under the illumination of visible light.

Quantum dot solar cells. Tuning photoresponse through size and shape control of CdSe-TiO2 architecture

Different-sized CdSe quantum dots have been assembled on TiO2 films composed of particle and nanotube morphologies using a bifunctional linker molecule. Upon band-gap excitation, CdSe quantum dots inject electrons into TiO2 nanoparticles and nanotubes, thus enabling the generation of photocurrent in a photoelectrochemical solar cell. The results presented in this study highlight two major findings: (i) ability to tune the photoelectrochemical response and photoconversion efficiency via size control of CdSe quantum dots and (ii) improvement in the photoconversion efficiency by facilitating the charge transport through TiO2 nanotube architecture. The maximum IPCE (photon-to-charge carrier generation efficiency) obtained with 3 nm diameter CdSe nanoparticles was 35% for particulate TiO2 and 45% for tubular TiO2 morphology. The maximum IPCE observed at the excitonic band increases with decreasing particle size, whereas the shift in the conduction band to more negative potentials increases the driving force and favors fast electron injection. The maximum power-conversion efficiency </=1% obtained with CdSe-TiO2 nanotube film highlights the usefulness of tubular morphology in facilitating charge transport in nanostructure-based solar cells. Ways to further improve power-conversion efficiency and maximize light-harvesting capability through the construction of a rainbow solar cell are discussed.

Enhanced solar water-splitting efficiency using core/sheath heterostructure CdS/TiO2 nanotube arrays

A novel fabrication route for core/sheath heterostructure CdS/TiO(2) nanotube arrays is proposed using ac electrodeposition for application in photoelectrochemical cells. The morphologies of the CdS/TiO(2) electrodes, which were prepared by electrochemically depositing CdS directly into anodic titanium nanotubes from an electrolyte containing Cd(2+) and S in dimethyl sulfoxide, were characterized by a field emission scanning electron microscope (FESEM). The deposited material was found to be in a hexagonal CdS structure by x-ray diffraction (XRD). The synthesized CdS/TiO(2) electrodes showed much higher photocurrent density in the visible wavelength region than pure TiO(2) nanotube arrays. We demonstrate that ac deposition voltage and time can control the CdS/TiO(2) composite architecture, which is crucial in determining the overall efficiency of the water-splitting reaction. The maximum photocurrent density was obtained with the core/sheath heterostructure CdS/TiO(2) nanotube arrays, which were fabricated by deposition of CdS at 5 V for 30 min with 2.5 microm tube length.

Quantum Dot Solar Cells. Harvesting Light Energy with CdSe Nanocrystals Molecularly Linked to Mesoscopic TiO2 Films

By using bifunctional surface modifiers (SH-R-COOH), CdSe quantum dots (QDs) have been assembled onto mesoscopic TiO(2) films. Upon visible light excitation, CdSe QDs inject electrons into TiO(2) nanocrystallites. Femtosecond transient absorption as well as emission quenching experiments confirm the injection from the excited state of CdSe QDs into TiO(2) nanoparticles. Electron transfer from the thermally relaxed s-state occurs over a wide range of rate constant values between 7.3 x 10(9) and 1.95 x 10(11) s(-1). The injected charge carriers in a CdSe-modified TiO(2) film can be collected at a conducting electrode to generate a photocurrent. The TiO(2)-CdSe composite, when employed as a photoanode in a photoelectrochemical cell, exhibits a photon-to-charge carrier generation efficiency of 12%. Significant loss of electrons occurs due to scattering as well as charge recombination at TiO(2)/CdSe interfaces and internal TiO(2) grain boundaries.

Photocatalytic activity of a hierarchically macro/mesoporous titania

Light-harvesting macroporous channels have been successfully incorporated into a mesoporous TiO(2) framework to increase its photocatalytic activity. This bimodal porous material was characterized by X-ray diffractometry in both low-angle and wide-angle ranges, N(2) adsorption-desorption analysis, scanning and transmission electron microscopy, FT-IR, and diffuse reflectance spectroscopy. Ethylene photodegradation in gas-phase medium was employed as a probe reaction to evaluate the photocatalytic reactivity of the catalysts. The results reveal that sintering temperature significantly affects the structural stability and photocatalytic activity of titania. The catalyst which calcined at 350 degrees C possessed an intact macro/mesoporous structure and showed photocatalytic reactivity about 60% higher than that of commercial P25 titania. When the sample was calcined at 500 degrees C, the macroporous structure was retained but the mesoporous structure was partly destroyed. Further heating at temperatures above 600 degrees C destroyed both macro- and mesoporous structures, accompanied by a loss in photocatalytic activity. The high photocatalytic performance of the intact macro/mesoporous TiO(2) may be explained by the existence of macrochannels that increase photoabsorption efficiency and allow efficient diffusion of gaseous molecules.

Composite tin and zinc oxide nanocrystalline particles for enhanced charge separation in sensitized degradation of dyes

Composite ZnO/SnO2 catalyst has been studied for the sensitized degradation of dyes e.g. Eosin Y (2', 4', 5', 7'-tetrabromofluorescein disodium salt) in relation to efficient charge separation properties of the catalyst. Improved photocatalytic activity was observed in the case of ZnO/SnO2 composite catalyst compared to the catalytic activity of ZnO, SnO2 or TiO2 powder. The suppression of charge recombination in the composite ZnO/SnO2 catalyst led to higher catalytic activity for the degradation of Eosin Y. Degradation of Eosin follows concomitant formation of CO2 and formation of CO2 followed a pseudo-first-order rate. Photoelectrochemical cells constructed using SnO2, ZnO, ZnO/SnO2 sensitized with Eosin Y showed V(oc) of 175, 306, 512 mV/cm2 and I(sc) of 50, 70, 200 microA/cm2 respectively. A higher irreversible degradation of Eosin Y and higher V(oc) observed on composite ZnO/SnO2 than ZnO and SnO2 separately can be considered as a proof of enhanced charge separation of ZnO/SnO2 catalyst. Eosin Y showed a higher emission decreases on ZnO/SnO2 composite than on individual ZnO, SnO2 or TiO2 indicating dominance of the charge injection process. Photoinjected electrons are tunneled from ZnO to SnO2 particles accumulating injected electrons in the conduction bands allowing wider separation of excited carriers.

Interparticle Charge Transfer in Dye-Sensitized Films Composed of Two Kinds of Semiconductor Crystallites

Interparticle charge transfer between different types of semiconductor crystallites in contact on band gap excitation or dye-sensitization is documented. The general consensus had been that electrons always transfer from particles of higher conduction band position to those with lower conduction band position. Observation on dye-sensitizated photoelectrochemical cells made from SnO(2)/ZnO films sensitized with different dyes suggests that the electron transfer could occur in either direction, that is from semiconductor of high band position to the semiconductor of the low band position or vice versa, depending on which surface adsorbs the dye more strongly. Copyright 2001 Academic Press.