Solar light transmission of polymer optical fibers

Light transfer (10 m) has been shown in recent experiments that used large-core optical fibers. Theoretical models are not extensive, however, and a further correlation between the theory and experiments has not been given. In this paper, straight and bent fiber subsystem models are introduced with skew and meridional rays to predict the light transmission of POFs (plastic optical fibers). Such fibers have been realized, for example, in HSL (hybrid solar lighting) systems. The purpose of this paper is to combine the straight and bent fiber subsystems to estimate the light transmission of HSL systems. It is shown that meridional rays, for which the optical-loss parameters were estimated, better represent the experimental results compared to skew rays (±5.3% vs ±24.7% of %-difference). Model predictions were compared with the results of a commercial software. Sensitivity analysis on the subsystems indicated the most-to-least significant parameters in light transmission.     

Scalable optical fiber reactor for photocatalytic H2 production: Addressing scattering issues

Scattering is one of the main challenges in scaling up photocatalytic water splitting using the most prevalent powder catalysts. This can be overcome by decoupling the reaction medium from light transmission, as in the case of optical fibers. Here we explore utilizing optical fibers coated with 5 wt% CuO supported on TiO₂ for photocatlytic H₂ production from water-methanol mixtures. CuO/TiO₂ is a well studied photo catalyst in which photoreduced Cu species are known to act as sensitizers for inducing visible light activity. Lower activity of sequentially coated systems indicates that appropriate interfaces of active Cu and TiO₂ with water are desirable. The scalability of such optical fiber-based systems along with potential in non-potable turbid water media are demonstrated. Maximum activity of 22 μmoles of H₂ in 8 h was obtained with 50 mg of catalyst coated on optical fibers, which increases linearly with increase in fiber numbers, whereas, drastic reduction in activity is observed in powder catalyst upon increasing the catalyst quantity. A one-to-one comparison of 700 mg of catalyst in powder form and coated on optical fibers indicates more than one order enhancement in activity in the optical fiber based system. In addition, ∼70% retention in activity in highly turbid non-potable water was observed as compared to powdered system which shows complete reduction in the activity by 99.99%.     

A new design of luminescent solar concentrator and its trial run

Normal luminescent solar concentrators (LSC) have a great limitation in their light transportation because the light produced by the recently reported LSCs is not a point light source. In order to enable LSCs to use optical fibers as the effective remote light transport media, a new design of LSC, which uses three color luminescent fibers for solar absorption and uses clear optical fiber bundles to transport the absorbed sunlight into a remote place, is fabricated and tested. Radiation flux ratios with a mean value of 5.7%, the luminous flux up to 114.1 lumens, and the light efficiency of 0.56% have been achieved during the trial run. The luminous efficacy as 0.643 lm W⁻¹ is higher than that of combusting candles (0.3 lm W⁻¹) but lower than that of the incandescent light bulbs (16–40 lm W⁻¹). Further, since the sun light is free, different from electrical light sources, the luminous efficacy of the new LSC does not result in any electricity consumption when it is under operation. A color analysis and spectrum test proves that the light produced by the new LSC performs a great match to the direct sun light in color. Copyright © 2010 John Wiley & Sons, Ltd.     

Enhanced photocatalytic hydrogen evolution over semi-crystalline tungsten phosphide

Increasing the separation efficiency and transfer rate of photogenerated charges is the dominant factor for improving photocatalytic activity. Herein, we successfully prepared semi-crystalline WP (SC-WP) with good optical properties and as a cocatalyst to modify CdS nanorods (CdS NRs) to construct SC-WP/CdS (PD) composite catalyst by simple electrostatic self-assembly method for photocatalytic hydrogen evolution. Two high-efficiency and stable photocatalytic hydrogen evolution systems were constructed with 1.0 M ammonium sulfite solution and 10 vol% lactic acid solution as sacrificial agents, respectively. Surprisingly, the maximum photocatalytic H₂ production rate of 15446.21 μmol h⁻¹ g⁻¹ is obtained over 10PD composite, which is 10.58 times greater than that of pure CdS. The improved photocatalytic activity can be attributed to the fact that the SC-WP nanoparticles provides a large number of exposed active sites on the surface of CdS for hydrogen evolution reaction, which can efficiently capture photogenerated electrons from CdS nanorods and promotes the transport and separation of light-induced charges. And the introduction of SC-WP nanoparticles with excellent optical properties can efficiently improve the visible light absorption range and the utilization rate of the absorbed light of the PD composite. In addition, the SC-WP nanoparticles show semi-crystalline state, which is also conducive to enhancing the photocatalytic activity.     

Preparation of floatable TiO2/poly(vinyl alcohol)-alginate composite for the photodegradation of ammonia wastewater

The use of floatable photocatalyst supports is an operable strategy of increasing photocatalytic performance in terms of improving light absorption. In this work, floatable organic support based on poly(vinyl alcohol) (PVA) was used to float commercial TiO₂ P25 nanoparticles. At first, by the use of CaCl₂ and boric acid at pH 3 the cross-linking of PVA blended with sodium alginate (PVA-Alg) was improved to enhance chemical and mechanical stability, and then it was chemically modified by trimethylchlorosilane to achieve floatable organic support (modified or MPVA-Alg). The prepared support and photocatalyst were characterized by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy, energy-dispersive X-ray spectroscopy (EDX)-elemental mapping, inductively coupled plasma optical emission spectroscopy, ultraviolet-visible (UV-vis) diffuse reflectance spectroscopy, photoluminescence (PL), thermalgravimetry, Brunauer-Emmett-Teller (BET), and water contact angle (WCA) analyses and applied for the ammonia degradation under UV to visible regions. Results of WCA and FTIR confirmed that chemical modification of PVA-Alg led to lowering hydrophilicity to be floated on the ammonia wastewater. UV-vis analysis indicated that the MPVA-Alg acts as a light absorbent in UV-vis ranges and makes TiO₂/MPVA-Alg a visible light active composite. Also, the PL analysis showed that the charge recombination process was strongly suppressed in the case of the TiO₂/MPVA-Alg sample. BET theory suggested that the textural properties were not the key factor in determining photocatalytic performance. The maximum photocatalytic ammonia removal was obtained to be 63% and 57% by the TiO₂/MPVA-Alg composite under UV-vis light irradiations, respectively. The MPVA-Alg and TiO₂ exhibited the ammonia degradation of 28.5% and 15.2% under visible and 29.4% and 40.0% under UV irradiation, respectively. The TiO₂/MPVA-Alg showed favorable reuse ability after five runs due to desirable chemical and mechanical resistance. This study provides an insight into the modification of polymeric structures to be used as both floatable photocatalyst support and light sensitizer.     

Copper modified-TiO2 catalysts for hydrogen generation through photoreforming of organics. A short review

An intense scientific activity was recorded during the last several years in the field of preparation, characterization and use of copper-based TiO₂ photocatalysts for hydrogen generation through photocatalytic reforming of organics. Different copper species were used dissolved in aqueous solution or incorporated on the TiO₂ surface as single co-catalyst or in the presence of a second catalyst (e.g., graphene, carbon fibers) to (1) effectively separate the electron–hole pairs, thus reducing the occurrence of the recombination reaction, and (2) extend the light absorption to the visible range of the solar spectrum. Many organic species (e.g., methanol, glycerol, formic acid) were proposed as sacrificial agents for hydrogen generation, although the prevailing idea is that of using organic compounds currently found in industrial wastewaters. The pH value was recognized as a fundamental variable in photocatalytic H₂ generation via copper modified-TiO₂ catalysts. A positive effect to promote hydrogen generation was associated to an increase in pH until moderate alkaline values. On the other hand, a release in the solution of cupric ions and a consequent decrease in photocatalytic activity were observed when decreasing pH. A relevant lack of information was recorded about the efficiencies of hydrogen generation which were reported only in few papers. Therefore, this critical literature review has been performed with the aim of providing a complete background to select the most efficient approaches and eventually promote new competitive systems for hydrogen generation via photoreforming for industrial applications.     

Role of P in improving V-doped SrTiO3 visible light photocatalytic activity for water splitting: A first – Principles study

In this work, the +3-valent P-doped SrTiO₃ has been shown for the first time to be theoretically achievable and to improve the photocatalytic performance of SrTiO₃ in co-doping with metal ions. The effects of P and V doping on the electronic structure, optical properties and photocatalytic activity of the SrTiO₃ system were systematically investigated using a first-principles calculation method. The results show that when V is singly doped with SrTiO₃, although the band gap width is greatly reduced and the optical absorption performance is significantly enhanced, it is difficult to photocatalytic split water for hydrogen production due to the high VBM. When one electron provided by +3-valent P as a donor is transferred near +5-valent V, the orbital hybridization between V-3d and O-2p will lead to a suitable reduction in the band gap width of SrTiO₃. The band edge alignment respect to water redox potentials proves that its band edges match well with the redox potentials of water. It was also found that P and V co-doping could reduce the complexation of photogenerated carriers and create a new optical absorption peak in the visible range. The calculated results imply that the P and V co-doping could improve the visible light photocatalytic activity of perovskite SrTiO₃.     

Structural,elastic, phononic,optical and electronic properties investigation of two-dimensional XIS (X=Al,Ga, In) for photocatalytic water splitting

Two-dimensional (2D) materials have been widely developed due to their attractive properties. Here, by using density functional theory (DFT) calculations, for the first time, we explore potential applications of the novel XIS (X = Al, Ga, In) monolayer 2D materials on photocatalytic water splitting. A series of simulations were carried out to predict and study the structural, elastic, phononic, optical and electronic properties of 2D XIS materials. The results show that GaIS and InIS demonstrate low thermal conductivity. For optical properties, AlIS shows strong light absorption coefficients and refractive index only under ultraviolet (UV) light, while GaIS and InIS show stronger performance under both visible light and UV light with the band edge positions spanned the redox potential of water. The reasonable band positions and bandgaps make them promising photocatalysts for water splitting. This work reveals the potential applications of monolayer 2D XIS in thermal, electronic, and photocatalytic water splitting.     

SiI2 monolayer as a promising photocatalyst for water splitting hydrogen production under the irradiation of solar light

The feasibility of SiI₂ monolayer as the candidate for photocatalytic water splitting for hydrogen generation under the irradiation of the solar light is explored. The geometrical structure, the electronic and optical properties, the mobility of carrier and strain engineering of the monolayer are investigated based on the first-principles calculations. The results demonstrate SiI₂ monolayer possesses an indirect gap of 2.33 eV (HSE06), and both the band edge and the bandgap match the redox potential conditions of the water splitting for hydrogen generation. There is an obvious optical absorption in the visible light and near-ultraviolet region and can be enhanced by the compressive strain. Moreover, the mobility of the electron is significantly different from that of the hole, implicating that the effective spatial charge separation is expectable and the ratio of the recombination of the photogenerated charge pairs is low. The primary adsorption site of the water molecule is identified. The Gibbs free energy and the adsorption energies are calculated to demonstrate the H₂ generation from the water molecule splitting on the monolayer. All the considered properties support that SiI₂ monolayer can be achieved as a promising candidate for the photocatalytic water splitting for hydrogen production under the irradiation of the solar light.     

Fabrication of porous titanium dioxide fibers and their photocatalytic activity for hydrogen evolution

TiO₂ semiconductor is one of the important photocatalysts for solar light conversion. The challenge is how to improve their efficiency. Creation of porous structures on/in the fibers could favor them a higher surface area as compared to the conventional solid counterparts, which thus could make the achievement for the desired high efficiency. In present work, we report the fabrication of porous TiO₂ fibers with high purity via electrospinning of butyl titanate (TBOT) and polyvinylpyrrolidone (PVP) combined with the subsequent calcination in air. It is found that the TBOT content in the spinning solution plays a profound effect on the growth of the fibers, enabling the synthesis of porous TiO₂ fibers with tunable structures and high purity. The photocatalytic activity for hydrogen evolution of the as-fabricated TiO₂ nanostrcutres has been investigated, suggesting that porous TiO₂ nanomaterials with a high purity and well-defined one-dimensional fiber shape could be an excellent candidate to be utilized as the photocatalyst for hydrogen evolution.     

Novel dual-layer photoelectrode prepared by RF magnetron sputtering for photocatalytic water splitting

Visible-light absorbing TiO₂ and WO₃ photocatalytic thin films were prepared by radio-frequency (RF) magnetron sputtering. The effects of sputtering condition on the structural, optical, as well as photocatalytic properties of the prepared thin films were explored. In addition, a novel dual-layer photocatalytic thin film that combines both visible-light TiO₂ and WO₃ was prepared by the same deposition technique to further enhance the photocatalytic performance. Instrumental analyses such as XRD, SEM-EDX, and UV–visible absorption spectrometry were performed to reveal the crystallinity, surface morphology, chemical composition, and light absorption of the prepared photocatalytic thin films. The activities of the prepared photocatalytic thin films under both UV and visible-light irradiations were evaluated by conducting photovoltammetry and water-splitting reaction in an H-type reactor. The enhanced photocurrent of dual-layer photocatalytic thin film was proved to be resulted from the improved charge separation of the dual-layer structure. The H₂ and O₂ yields obtained from the water-splitting reactions were consistent with the photocurrent results, showing dual-layer photocatalyst with higher photoactivity than mono-layer photocatalyst.     

Constructing a new 2D Janus black phosphorus/SMoSe heterostructure for spontaneous wide-spectral-responsive photocatalytic overall water splitting

Janus MoSSe monolayer with built-in electric dipole, as another emerging two-dimensional (2D) material after MoS₂, is predicted to be an ideal photocatalyst for overall water splitting. However, in spite of the excellent hydrogen evolution reaction (HER) activity of Se-surface, the extremely poor oxygen evolution reaction (OER) activity of S-surface hinders the achievement of photocatalytic overall water splitting. Herein, we construct a new 2D van der Waals heterostructure consisting of high-OER-active black phosphorus (BP) and Janus MoSSe monolayer, and demonstrate a new strategy of Janus BP/SMoSe heterostructure to achieve wide-spectral-responsive photocatalytic overall water splitting. The electronic structures and optical properties of two different heterostructures, BP/SMoSe and BP/SeMoS, are systematically investigated via first principles density, exhibiting a type-II band arrangement. Unlike BP/SeMoS, the BP/SMoSe heterostructure shows excellent optical properties, such as a large dielectric constant of 8.14 and a small optical absorption boundary of 0.10 eV. Furthermore, BP/SMoSe heterostructure possesses greater light absorption intensity and a broader light absorption range. It is found that the BP/SMoSe heterostructure exhibits proper band alignment and enhanced intrinsic dipole, which is favorable to obtain high electron-hole separation efficiency. This work provides a feasible strategy of 2D Janus BP/SMoSe heterostructure for approaching almost perfect overall water-splitting photocatalysis.     

Integrating optimal amount of carbon dots in g-C3N4 for enhanced visible light photocatalytic H2 evolution

Carbon dots (CDs) hold great promise for photocatalytic application owning to their low production cost, unique optical properties, as well as excellent stability and conductivity. Integrating CDs in graphite carbon nitride (g-C₃N₄) nanosheets helps to broaden visible light absorption, retard charge recombination and promote photoelectrons transport. Herein, we demonstrated a simple strategy to introduce CDs on g-C₃N₄ nanosheets by hydrothermal treatment of ginkgo leaves followed by thermal polymerization of urea. We found that there was were two volcano-trends in the photocatalytic H₂ evolution rate with the increase of CDs loading. As a result, the optimized CDs/g-C₃N₄ nanocomposites demonstrated a superior hydrogen evolution rate as high as 3.12 mmol g^?1·h^?1 and 14.4% apparent quantum efficiency (AQE) was achieved at 420 nm visible light irradiation. The contribution of CDs towards the photocatalytic hydrogen evolution enhancement was discussed in depth via experiment characterization and Density functional theory (DFT) calculation. This work may shed light on the rational design and bottom-up synthesis of eco-friendly energy conversion materials with high-performance and low cost.     

Decoration of Pt on the metal free RGO-TiO2 composite photocatalyst for the enhanced photocatalytic hydrogen evolution and photocatalytic degradation of pharmaceutical pollutant β blocker

Easy synthesis of graphene based composite photocatalyst with the incorporation of minimal quantity of noble metals for the enhanced photocatalytic hydrogen evolution as well as photocatalytic degradation and mineralization of recalcitrant pollutants under solar irradiation is an urgent requirement from the clean energy and environment point of view all over the globe. Herein, we demonstrate the decoration of Pt by photodeposition method on the hydrothermally synthesized RGO-TiO₂ nanocomposite. The various photocatalysts synthesized were successfully characterized by XRD, FTIR, Raman, UV–visible absorption spectra, XPS, SEM and TEM techniques. The well characterized photocatalysts were further investigated for the photocatalytic hydrogen evolution studies of methanol water mixtures under UV as well as simulated solar light irradiation. The optimized Pt-RGO-TiO₂ (1 wt % Pt and 10 wt % RGO) composite was found to show 14 fold increase in the photocatalytic hydrogen evolution efficiency under UV light irradiation and 20 fold increase under simulated solar light irradiation as compared to bare TiO₂ under UV light irradiation. The ternary photocatalyst showed very good recycle and reuse capability up to 4 cycles. The optimized Pt-RGO-TiO₂ was further tested for the enhanced photocatalytic degradation and mineralization of pharmaceutical pollutant namely β blocker Propranolol under UV as well as simulated solar light irradiation. The obtained results showed 79% and 94% reduction in COD of Propranolol under UV and simulated solar light irradiation respectively. The appreciable enhancement in the photocatalytic activity of the Pt decorated RGO-TiO₂ photocatalyst as compared to bare TiO₂ under UV and simulated solar light can be attributed to the use of maximum range of solar spectrum along with their excellent properties of charge separation by RGO and Pt.     

Energy-absorption-based explanation of the photocatalytic activity enhancement mechanism of TiO2 nanofibers

As is reported, the photocatalytic activity will increase significantly when TiO₂ nanoparticles are agglomerated into TiO₂ nanofibers (NFs), but the photocatalytic activity enhancement mechanisms are still not fully understood. As is widely accepted, the optical absorption process plays a key role in photocatalysis, and it can even be said that the optical absorption capability of the photocatalyst directly determines its photocatalytic activity, while the influence of the structure on the optical absorption characteristics of TiO₂ has largely been ignored in the existing explanations. In this paper, optical simulations are introduced into analyzing optical characteristics of TiO₂ Nanofibers with which, the photocatalytic activity enhancement mechanism is further discussed, and a photocatalytic activity enhancement mechanism of TiO₂ Nanofibers is proposed.     

Electronic and optical properties of nitrogen and sulfur doped strontium titanate as efficient photocatalyst for water splitting: A DFT study

One of the most effective option of photocatalysts for water splitting is doped strontium titanate, SrTiO₃. It has a high rate of photo-generated charge transfer and limited photocatalytic activity for water splitting. The search of an appropriate photocatalyst having a high visible light absorption as well as fast charge transportation is extremely needed, however it is a difficult task. The structural, electronic and optical properties of sulfur-doped SrTiO₃ and nitrogen-doped SrTiO₃ are investigated using calculations based on density functional theory (DFT). According to the band structure calculations, the O-2p states represented the higher levels of the valence band of pure SrTiO₃. When S and N atoms are introduced into the SrTiO₃ structure on the O site, electronic structure findings indicate that doping the Sulfur (S) atoms reduced the band gap significantly, whereas doping of N atoms increased the bandgap of SrTiO₃. According to our results, the N-doped SrTiO₃ has a sufficient band gap of 2.03 eV, as well as suitable high visible light absorption and charge carrier transportation. The optical properties showed that N-doped SrTiO₃ has good photosensitivity for visible light. In addition, we have found a significant impurity state that differs from O 2p-states, which can increase photocatalytic efficiency. The results of studies of electronic band structure showed that electron-hole transportation was well consistent with the experimental data. Thus, the N-doped SrTiO₃ in this study is indeed an attractive candidate for hydrogen evolution throughout the visible light range, providing a logical base for the establishment of innovative photocatalysts.     

Scattering phase function of fractal aggregates of TiO2 particulate photocatalyst simulated with discrete dipole approximation

The aggregate structures of photocatalyst particles are critical for their light absorption and scattering properties and thus significantly impact the photocatalytic performance toward solar hydrogen production. Herein, taking P25 TiO₂ nanoparticles as model photocatalysts, we calculate the optical properties of the aggregates of nanoparticles with the Discrete Dipole Approximation (DDA). A scattering phase function, which can reveal and predict the structural information of photocatalyst particles, is accordingly obtained. Specifically, it is found that the forward scattering is dominant when light is scattered on the particles, and this scattering mode becomes more intense with the increment of the particle size. Our results show that the scattering phase function is also in agreement with the Rayleigh-Gans-Debye (RGD) approximation.     

MoS2-mesoporous LaFeO3 hybrid photocatalyst: Highly efficient visible-light driven photocatalyst

Perovskite type materials have high potential photocatalytic application towards both hydrogen energy generation and organic dye degradation due to their high stability and good reusability. Here, it is the first analysis of photocatalytic degradation of RhB and hydrogen energy evolution under visible light over MoS₂/LaFeO₃ nanocomposite. The physicochemical properties of the materials were characterized using a range of techniques such as XRD, TEM, XPS, FTIR, PL, photocurrent, etc. The optical properties of the nanocomposite show good absorption in UV-Vis spectra as compared to the bare LaFeO₃. In this study, MoS₂/LaFeO₃ nanocomposite was synthesized through single step in situ hydrothermal processes with a narrow bandgap, enhanced photocatalytic application under visible light. This novel MoS₂/LaFeO₃ nanocomposite is an efficient and promising photocatalyst for both hydrogen energy evolution and organic dye degradation.     

Enhanced photocatalytic H2 evolution and phenol degradation over sulfur doped meso/macroporous g-C3N4 spheres with continuous channels

S-doped meso/macroporous g-C₃N₄ spheres (SMCN) were successfully synthesized via an in situ novel method utilizing millimeter-scale porous silica spheres as template and thiourea as precursor and S source. Such SMCN possessed millimeter-scale spherical morphology with continuous channels at 20–80 nm in the interior of the spheres, and exhibited increased H₂ generation rate (15 times) and phenol degradation rate (5 times) under visible light irradiation compared with that over pristine g-C₃N₄, mainly due to the enlarged surface area, enhanced mass transfer and improved efficiency of charges separation all stemming from the synergetic effects of the S doping and pore creating. Notably, density functional theory (DFT) calculations were employed to further understand the mechanism of the photocatalytic enhancement with regard to the optical absorption property at atomic level. Combined with the finite difference time domain (FDTD) simulations aiming at evaluating the effect of the nanoscale pore architecture on the optical absorption ability, it was revealed that not only the S doping but also the meso/macroporous structure resulted in the enhancement of the optical absorption, which was considered to be an essential role for the enhanced photocatalytic performances over SMCN.     

First-principles screening visible-light active delafossite ABO2 structures for photocatalytic application

Delafossite structures with composition ABO₂ have attracted great attention for various applications owing to their adjustable electric and optical properties. However, the photocatalytic usage of ABO₂ materials is still limited, owing to the various composition of chemical elements which makes it hard to be studied and fabricated. In this work, the stabilities, electronic, and optical properties of 40 kinds of Ag and Cu based delafossite ABO₂ materials have been investigated to tap the potential of photocatalytic applications by performing first-principle calculations. The bulk structure of ABO₂ are derived by minimizing the total energy. The electronic properties have been studied through the calculations of band structure, density of states (DOS), and work functions by using the method of GGA + U. The calculated results indicate that 4 kinds of them has superior stability, as well as appropriate electronic and optical properties for photocatalytic applications. Among them Cu₃Zn₂TaO₆ has the highest visible light absorption coefficient, indicating its immense potential for photocatalytic applications. This work presented some based physical properties of Ag and Cu based ABO₂ structures, and will promote the applications of ABO₂ materials.