Visible Illumination Conditions Research Articles

Fabrication of a UV Photodetector Based on n-TiO2/p-CuMnO2 Heterostructures

The heterojunction based on n-TiO2 nanolayer/p-CuMnO2 thin film was achieved using an efficient two-step synthesis process for the fabrication of a UV photodetector. The first step consisted of obtaining the TiO2 nanolayer, which was grown on titan foil by thermal oxidation (Ti-TiO2). The second step consisted of CuMnO2 thin film deposition onto the surface of Ti-TiO2 using the Doctor Blade method. Techniques such as X-ray diffraction, UV-VIS analysis, SEM, and AFM morphologies were used for the investigation of the structural and morphological characteristics of the as-synthesized heterostructures. The Mott–Schottky analysis was performed in order to prove the n-TiO2/p-CuMnO2 junction. The I-V measurements of the n-TiO2 nanolayer/p-CuMnO2 thin film heterostructure confirm its diode characteristics under dark state, UV and visible illumination conditions. The obtained heterojunction, which is based on two types of semiconductors with different energy band structures, improves the separating results of charges, which is very important for high-performance UV photodetectors.

Stimulating photodiode characteristics of hybrid ZnPc-MWCNTs

The ability to functionalize multi-wall carbon nanotubes (MWCNTs) with organic semiconductors is a promising methodology to promote their use in multiple optoelectronic applications, especially at the visible region of the optical spectra. In this article, we investigate the structural, optical, and electrical properties of chemically synthesized MWCNTs hybridized with zinc phthalocyanine (ZnPc) molecules. Interestingly, the hybrid ZnPc-MWCNTs optical bandgap shifts towards the visible light at 2.6 eV instead of the near-infrared bandgap of the pristine sample at 1.2 eV. Besides, the ZnPc-MWCNTs hybrid exhibits enhanced p-n heterojunction photodiode characteristics that are significantly improved with temperature increase. Moreover, the I-V curves of the hybrid ZnPc-MWCNTs sample under visible illumination conditions express a current intensity three times higher than the pristine MWCNTs sample. This promotes the use of the ZnPc-MWCNTs hybrid in various optoelectronic applications working in the visible region of optical spectra.

Boosting Pt/TiO2 hydrogen photoproduction through Zr doping of the anatase structure: A spectroscopic and mechanistic study

The production of hydrogen from the photoreforming of methanol was essayed using an anatase-based system, having Pt nanoparticles and a Zr-doped anatase high surface area support. The production of hydrogen was quantitatively analyzed using the quantum efficiency parameter under UV and visible illumination conditions. The study showed that a 2.5 mol. % of Zr renders an outstanding material (4.6% quantum efficiency) for the fruitful use of sunlight as a green source of energy of the process. The outstanding promotion of activity orginated by Zr doping of the anatase structure was studied using in-situ infrared, photoluminescence and electron paramagnetic resonance spectroscopies. The multitechnique spectroscopic investigation of the reaction mechanism demonstrates that this occurs through a complex reforming plus water gas shift path. The study also unveils that optoelectronic properties of the solids derived from Zr incorporation drive activity of the system by controlling light absorption and, mainly, charge carrier recombination and interaction with the reactant molecules.

Hydrogen photogeneration using ternary CuGaS2-TiO2-Pt nanocomposites

In this contribution we synthesized ternary CuGaS2-TiO2-Pt materials. The semiconductor components were surface functionalized with mercapto-alifatic acids to drive their linking and were platinized prior to or after contact between the semiconductors. The corresponding samples were utilized in the photo-production of hydrogen using methanol as a sacrificial agent. The testing under UV and visible illumination conditions together with the calculation of the true quantum efficiency of the process demonstrate the outstanding performance of these ternary materials under sunlight operation. Optimum activity was achieved for samples having a 3 to 5 wt % of the chalcogenide and a selective interaction of the noble metal with the major oxide component. The physico-chemical characterization and particularly the use of photoluminescence spectroscopy showed that photo-activity is controlled by charge separation under illumination, which drives to charge location of electrons and holes in different components of the powders and the efficient use of charge carriers in the chemical reaction.

Bimetallic Pt-Pd co-catalyst Nb-doped TiO2 materials for H2 photo-production under UV and Visible light illumination

In this work we synthesized a series of binary PtPd co-catalysts supported on a Nb-doped TiO2 support. The catalytic solids and corresponding monometallic reference systems are characterized using X-ray diffraction, X-ray photoelectron, and UV–vis spectroscopies, together with microscopy and porosimetry tools. Such characterization was able to show the formation of PtPd alloy particles in the bimetallic catalysts. The mono and bimetallic TiO2-based powders were tested in the photo-production of hydrogen from methanol:water mixtures under UV and visible illumination conditions. Analysis of catalytic properties was carried out through the measurement of the optical properties of the materials and the calculation of the true quantum efficiency parameter. Results indicate that the PtPd co-catalysts have superior performance that the Pt and Pd monometallic counterparts both under UV and visible illumination conditions. Optimum performance was achieved with a material having a Pt:Pd 1:1 atomic ratio. A remarkable increase in the use of the visible range and thus in sunlight utilization is achieved with the 1:1 Pt:Pd bimetallic system with respect to the monometallic counterparts. The evolution of the bimetallic co-catalysts under reaction conditions as well as their key properties to interpret photo-activity were analyzed with the help of the above mentioned techniques as well as photoluminescence spectroscopy and an in-situ infrared analysis of the materials under reaction conditions. Results point out the critical role that both PtPd alloying and the metal-support interface play in the reaction.

Enhancing photocatalytic performance of TiO2 in H2 evolution via Ru co-catalyst deposition

A series of ruthenium catalysts supported into pure anatase titania were tested in the photo-production of hydrogen from methanol:water mixtures under UV and visible illumination conditions. Catalysts containing 1, 2, 3 and 5 wt.% of ruthenium were subjected to a characterization study with the help of X-ray diffraction, Raman, X-ray photoelectron spectroscopy, photoluminescence, morphology as well as scanning and transmission electron microscopy. Through the measurement of the optical properties of the suspension of the catalysts and the hydrogen photo-production reaction rate we calculate the true quantum efficiency. Optimum activity is presented by the catalyst with a 3 wt.% of Ru component, which shows quantum efficiency values of ca. 3.1 and 0.6% under, respectively, UV and visible light illumination. Careful examination of the physico-chemical properties of the solid allows to establish a correlation between the ruthenium surface exposed and the quantum efficiency. The implications of such result to justify chemical activity of the ruthenium supported samples are discussed both for UV and visible illumination conditions.

Characterization of the High-Spin Co(II) Intermediate Species of the O2-Evolving Co4O4 Cubic Molecules.

An artificial oxygen-evolving complex (OEC) contains a tetranuclear CoIII4O4 cubic cluster ligated with acetate and pyridine molecules, a light-activated Ru(bpy)32+ moiety (bpy = 2,2'-bipyridine), and the sacrificial electron acceptor S2O82- side. A recent EPR investigation of this system showed the formation of the high-spin Co(II) ion under visible-illumination conditions. It has been supported that this center originates from the cluster and is involved in the oxygen-evolving process. The present report is focused on the further characterization of the high-spin d7 configuration of the Co(II) species. The measurement of the EPR spectrum at a wider magnetic field range in comparison to that reported recently shows the presence of an additional signal that contributes to the spectrum of the Co(II) center. The theoretical simulation of this spectrum reveals that an isotropic g value and considerably small zero-field splitting parameters describe the high-spin Co(II) ion in a unique way, which supports a tetrahedral crystal field symmetry. On the basis of the spin-Hamiltonian parameters, the looping transitions that lead to the experimental EPR signals are determined. Additionally, a possible role of the symmetry of the Co(II) species and a proposed model that explains its formation during the O2-evolving process of the Co4O4 cubic molecules are discussed.

XPS studies and photocurrent applications of alkali-metals-doped ZnO nanoparticles under visible illumination conditions

The present work is a study about a relationship between X-ray photoelectron spectrometer (XPS) results and photocurrent intensity of alkali-metals-elements doped ZnO nanoparticles, which is carried out under visible illumination conditions. The nanoparticles were synthesized by a simple sol–gel method. Structure and morphology studies of the NPs were carried out by X-ray diffraction analysis (XRD) and transmission electron microscopy (TEM). The effect of doping on the optical band-gap was investigated by using UV–visible spectrometer. The absorption peak of the doped ZnO NPs was red-shifted with respect to that of the undoped ZnO NPs. After that, the photocurrent application of the products was examined under a white light source at 2V bias. The photocurrent results showed that, the current intensity of the ZnO NPs was increased by doping materials. However, K-doped ZnO NPs showed the highest photocurrent intensity. Finally, a discussion was carried out about the obtained photocurrent results by the O-1s spectra of the XPS of the samples. Our results suggest that the alkali-metals-doped ZnO NPs exhibit considerable promise for highly sensitive visible-light photodetectors.

Synthesis of mesoporous functional hematite nanofibrous photoanodes by electrospinning

Iron(III) oxide (hematite, Fe2O3) nanofibers, as visible light-induced photoanode for water oxidation reaction of a water splitting process, were fabricated through electrospinning method followed by calcination treatment. The prepared samples were characterized with scanning electron microscopy, and three-electrode galvanostat/potentiostat for evaluating their photoelectrochemical (PEC) properties. The diameter of the as-spun fibers is about 300 nm, and calcinated fibers have diameter less than 110 nm with mesoporous structure. Optimized multilayered electrospun α-Fe2O3 nanostructure mats showed photocurrent density of 0.53 mA/cm2 under dark and visible illumination conditions at voltage 1.23 V and constant intensity (900 mW/cm2). This photovoltaic performance of nanostructure mats makes it suitable choice for using in the PEC water splitting application as an efficient photoanode. This method, if combined with appropriate flexible conductive substrate, has the potential for producing flexible hematite solar fuel generators. Copyright © 2015 John Wiley & Sons, Ltd.

Disorder engineering of undoped TiO2 nanotube arrays for highly efficient solar-driven oxygen evolution.

The trade-off between performance and complexity of the device manufacturing process should be balanced to enable the economic harvest of solar energy. Here, we demonstrate a conceptual, yet practical and well-regulated strategy to achieve efficient solar photocatalytic activity in TiO2 through controlled phase transformation and disorder engineering in the surface layers of TiO2 nanotubes. This approach enabled us to fine-tune the bandgap structure of undoped TiO2 according to our needs while simultaneously obtaining robust separation of photo-excited charge carriers. Introduction of specific surface defects also assisted in utilization of the visible part of sunlight to split water molecules for the production of oxygen. The strategy proposed here can serve as a guideline to overcome the practical limitation in the realization of efficient, non-toxic, chemically stable photoelectrochemical systems with high catalytic activity at neutral pH under visible illumination conditions. We also successfully incorporated TiO2 nanotube arrays (TNTAs) with free-based porphyrin affording a pathway with an overall 140% enhanced efficiency, an oxygen evolution rate of 436 μL h(-1) and faradic efficiencies over 100%.

Abatement of organics and Escherichia coli using CeO2-TiO2 composite oxides: Ultraviolet and visible light performances

A series of CeO2-TiO2 composite samples were obtained by a single-pot microemulsion method followed by calcination. The composite samples contain fluorite (CeO2) and anatase (TiO2) nanometric phases with a ceria content ranging from 1 to 25mol%. Using these samples as photocatalysts, toluene mineralization and Escherichia coli 1337-H inactivation were carried out under UV and visible illumination conditions. To interpret the photo-catalytic results and provide quantitative bases to compare the performance of the composite samples with respect to titania and ceria single oxide nano-references the study focuses on analyzing kinetic differences. The kinetic analysis of both types of degradation processes provides conclusive evidence that the composite Ce-Ti system outperforms the Ti reference nano-materials (including P25) by a factor exceeding 3 under UV conditions and 1.7 for sunlight-type or visible light. Together with the low activity of the Ce single reference, the results show that a synergistic effect occurs between the two oxide components of the Ce-Ti system. Maximum activity, irrespective of the chemical or biological target nature as well as the energy of the excitation, was always achieved with samples having low Ce loadings, below 5mol%. Such a general enhanced activity indicates that the Ce-Ti system can be a “universal” candidate to provide a highly active material with improved performance with respect to nano-anatase.