Efficient and robust visible light photocatalytic H2 production based on CdSe quantum dots sensitized titania

In this work, cadmium selenide quantum dots (CdSe QDs) with different sizes were synthesized and employed as visible light sensitizers of titania, in comparison with other organic molecules based sensitizers, including the well-known ruthenium complex sensitizer, tris(4,4-dicarboxy-2,2-bipyridyl)ruthenium(II) chloride, phenolic-formaldehyde resin and poly (4-vinylphenol). The different sensitizers are linked to titania via different molecular linkages through self-assemble processes. CdSe QDs adsorbed onto titania via stabilization ligand (mercaptopropionic acid) are more stable and efficient in terms of photocatalytic H₂ generation and photocurrent generation. The CdSe QDs with a diameter of 2.5 nm exhibits a strong absorption peak centred at 500 nm (CdSe500) and shows the best photocatalytic performance than other QDs with larger size and organic sensitizers. The turnover number of CdSe500 QDs for H₂ generation reaches ca. 9000 after 96 h reaction, with a 0.6% quantum yield under irradiation at 450 nm (light intensity = 35 mW/cm²). During the initial 3.0 h reaction, the turnover numbers of different types of sensitizers are estimated about 4.3, 52.5, 323.2 and 16.5 for phenolic-formaldehyde resin, poly (4-vinylphenol), CdSe500 QDs and ruthenium complex, respectively. These results highlights the advantages of utilizing CdSe QDs as stable visible light sensitizers for solar energy conversion.     

Review: Dye sensitized solar cells based on natural photosensitizers

The performance of dye sensitized solar cells is mainly based on the dye as a sensitizer. Natural dyes have become a viable alternative to expensive and rare organic sensitizers because of its low cost, easy attainability, abundance in supply of raw materials and no environment threat. Various components of a plant such as the flower petals, leaves and bark have been tested as sensitizers. The nature of these pigments together with other parameters has resulted in varying performance. This review briefly discusses the emergence, operation and components of dye sensitized solar cells together with the work done on natural dye based dye sensitized solar cells over the years.     

Nanocrystalline TiO2 photosensitized with natural polymers with enhanced efficiency from 400 to 600 nm

TiO₂ sensitization for solar applications requires not only efficient but also stable and inexpensive sensitizers. Different condensed tannins extracted from bark wastes of tropical wood trees were studied as possible sensitizers of TiO₂. These natural polymers adhere strongly to the TiO₂ even from aqueous solutions. Absorption spectra are presented for 1 mM aqueous sensitizing solutions prepared with lyophilized condensed tannins which absorb light in the visible range. Spectral photocurrent measurements and I–V characterization show that no bias is required for electron injection to the TiO₂ from all studied condensed tannins. Incident photon to current efficiency (IPCE) analysis indicates that surface complexation originates absorption bands with different electron injection efficiencies. These play a dominant role in determining IPCE spectral shape. We propose that surface modification by the sensitizer changes the surface trap density, thereby decreasing recombination losses.     

Highly efficient photon-to-electron conversion with mercurochrome-sensitized nanoporous oxide semiconductor solar cells

Dye-sensitized solar cells based on nanoporous oxide semiconductor thin films such as TiO₂, Nb₂O₅, ZnO, SnO₂, and In₂O₃ with mercurochrome as the sensitizer were investigated. Photovoltaic performance of the solar cell depended remarkably on the semiconductor materials. Mercurochrome can convert visible light in the range of 400–600 nm to electrons. A high incident photon-to-current efficiency (IPCE), 69%, was obtained at 510 nm for a mercurochrome-sensitized ZnO solar cell with an I⁻/I₃⁻ redox electrolyte. The solar energy conversion efficiency under AM1.5 (99 mW cm⁻²) reached 2.5% with a short-circuit photocurrent density (J_sc) of 7.44 mA cm⁻², a open-circuit photovoltage (V_oc) of 0.52 V, and a fill factor (ff) of 0.64. The J_sc for the cell increased with increasing thickness of semiconductor thin films due to increasing amount of dye, while the V_oc decreased due to increasing of loss of injected electrons due to recombination and the rate constant for reverse reaction. Dependence of photovoltaic performance of mercurochrome-sensitized solar cells on semiconductor particles, light intensity, and irradiation time were also investigated. High performance of mercurochrome-sensitized ZnO solar cells indicate that the combination of dye and semiconductor is very important for highly efficient dye-sensitized solar cells and mercurochrome is one of the best sensitizers for nanoporous ZnO photoelectrode. In addition, a possibility of organic dye-sensitized oxide semiconductor solar cells has been proposed as well as one using metal complexes.     

Status and outlook of sensitizers/dyes used in dye sensitized solar cells (DSSC): a review

Dye‐sensitized solar cells (DSSCs) have become a topic of significant research in the last two decades because of their scientific importance in the area of energy conversion. Currently, DSSC is using inorganic ruthenium (Ru)‐based, metal‐free organic dyes, quantum‐dot sensitizer, perovskite‐based sensitizer, and natural dyes as sensitizer. The use of metal‐free, quantum‐dot sensitizer, perovskite‐based sensitizer, and natural dyes has become a viable alternative to expensive and rare Ru‐based dyes because of low cost, ease of preparation, easy attainability, and environmental friendliness. Most of the alternatives to Ru‐based dyes have so far proved inferior to the Ru‐based dyes because of their narrow absorption bands (Δλ ≈ 100–250 nm), adverse dye aggregation, and instability. This review highlights the recent research on sensitizers for DSSC, including ruthenium complexes, metal‐free organic dyes, quantum‐dot sensitizer, perovskite‐based sensitizer, mordant dyes, and natural dyes. It also details and tabulates all types of sensitizer with their corresponding efficiencies. Plot of progress in efficiency (η) of DSSC till date based on different types of sensitizers is also presented. Copyright © 2016 John Wiley & Sons, Ltd. Copyright © 2016 John Wiley & Sons, Ltd.     

Calculation of the photocurrent-potential characteristic for regenerative, sensitized semiconductor electrodes

The steady state anodic photocurrent for sensitized semiconductor electrodes, where the sensitizer is regenerated by a redox electrolyte, has been modeled taking into account the rate of light absorption by the sensitizer S, the rate of electron injection from the excited state S^* of the sensitizer to the conduction band of the semiconductor, the rate of decay of S^* (radiative and non-radiative) and the rate of reductive regeneration of the sensitizer by the redox electrolyte. In this model the rate of recombination between the conduction band electron and the oxidized sensitizer, S⁺, and the reactions between S^* and the redox couple have been assumed to be negligible. The rate constant for injection, k_inj, the injection efficiency, φ_inj, the photocurrent density, J_P, and the steady state concentrations of S^* and S⁺, have been calculated as a function of light intensity, Helmholtz potential, λ_max and halfwidth, ΔE_0.5, of the sensitizer absorption spectrum, and the semiconductor band gap and electron affinity both for monochromatic light and for AM1.5 sunlight simulated by radiation from a 5600K black body. For the calculation of k_inj as a function of Helmholtz potential, the Gurney -Gerischer-Marcus (GGM) model has been used. Allowance for the distribution of electrode potential between the semiconductor and the electrolyte has been introduced in principle. The steady state concentrations of S^* and S⁺ were used in the Nernst equation to calculate the S^*/S⁺ quasi-reversible potential. It is shown that the short-circuit current density of the cell is a maximum for a sensitizer with a λ_max of about 1550 nm. Inter-relationships between variables involving the sensitizer have been used to show that only four sensitizer parameters are needed when considering the effects of changing the sensitizer. These parameters are the reorganization energies and the standard reduction potentials for the couples S⁺/S^* and S⁺/S. For a related series of sensitizers, such as obtained by changing a substituent group, only the two standard reduction potentials are required to predict the effects of changing the sensitizer.     

Panchromatic response composed of hybrid visible-light absorbing polymers and near-IR absorbing dyes for nanocrystalline TiO2-based solid-state solar cells

In pursuit of panchromatic sensitizers for mesoporous TiO₂-based solid-state solar cells, a near-IR absorbing zinc phthalocyanine dye (coded TT1) was firstly adsorbed over relatively thin (∼1 μm) TiO₂ mesoporous films and then a visible-light absorbing polymer [regioregular poly(3-hexylthiophene), P3HT] was incorporated into the mesopores as both a second sensitizer and a solid hole conductor. After optimizing some experimental parameters, these hybrid solid-state cells exhibited a clear panchromatic response, and an overall conversion efficiency of around 1% at full sun intensity.     

IMMOBILIZED PHOTOSENSITIZERS FOR SOLAR PHOTOCHEMICAL APPLICATIONS

New hydrophilic immobilized photosensitizers (heterogeneous phase) were synthesized that overcome some disadvantages of the use of homogeneous phase sensitizers for detoxification and disinfection of water. The chosen sensitizers, based on porphyrin moieties, were bound on poly(methyl methacrylate) (PMMA). The measured production rate of singlet oxygen is significantly higher than that of the well-known rose bengal immobilized on Merrifield polymer. The sensitive polymer can be used for detoxification and disinfection of polluted water.     

Parallel-plate solar photocatalytic reactor for air purification: Semi-empirical correlation, modeling, and optimization

This study focused on modeling and optimization of a photoreactor packed with parallel glass plates coated with sol–gel TiO₂ thin films on both sides. The photoreactor design has great potential for solar photocatalytic air purification. Borosilicate glass substrate was selected because its high transmissivity facilitated the transmission and distribution of exterior solar radiation onto interior immobilized photocatalyst. Flat-plate configuration was adopted because a TiO₂-coated plate could be easily set up for effective ion implantation treatment to extend the activating spectral range from UV to visible light for a higher solar photonic efficiency. In the analytical study of this research, a semi-empirical correlation model was formulated for prediction of the photoreactor performance. The correlation coefficients were obtained based on the results of photocatalytic air purification experiments conducted. Additional experimental tests were carried out for validation of the model. In the photoreactor design optimization analysis, the semi-empirical correlation model was used as an optimizer to determine the number of parallel TiO₂-coated plates needed for efficient photocatalytic air purification. It was found that the optimal number of plates was dependent of the incident irradiance. The modeling and optimization methods developed for solar photocatalytic air purification are equally applicable for solar photocatalytic water purification.     

TiO2-fixed-bed reactor for water decontamination using solar light

A photocatalytic reactor using immobilized TiO₂ (Degussa P25) on a glass plate was studied on a bench scale using solar light as the source of radiation. The influence of parameters such as the slope of the plate, solar light intensity, flow rate and molar flow rate, as well as the geometry of the reactor, was studied using dichloroacetic acid (DCA) as a model compound. A linear dependence of degradation with solar light intensity, measured at 365 nm, was observed. Experiments with recirculation as well as a single pass of solution suggested no mass transfer limitations in this system. The mineralization of DCA resulted in the production of quantitative amounts of chloride ions. An initial concentration of 5 mmol/L of DCA decayed to 2 mmol/L in about 2 min of irradiation. An exponential decay of degradation was observed with an increase of the molar flow rate, achieving saturation around 1.5 mmol DCA/min.     

Photoelectrochemical studies of nanocrystalline TiO2 co-sensitized by novel cyanine dyes

Pentamethylcyanine derivative (A), trimethylcyanine derivative (B) and their mixtures (AB) were used as sensitizers in nanocrystalline TiO₂ solar cells to improve photoelectric conversion efficiency. It was found that the aggregates of the cyanine dyes were efficient in light harvesting and that the mixture of A and B could be employed to sensitize the solar cell over the entire visible spectrum. The sensitizing properties of three mixed dyes with different A-to-B ratio were systematically studied, and it was found that A1B3 (A:B=1:3 V/V)-sensitized solar cell generated the highest photoelectric conversion yield of 3.4%. The effect of co-adsorption of A and B on their aggregation behavior and photosensitization was also investigated. Co-sensitization was found to suppress the aggregation and affect the sensitization performance profoundly.     

Fenton degradation of 4-chlorophenol contaminated water promoted by solar irradiation

The degradation of 4-chlorophenol (4-CP) contaminated water by Fenton process with or without solar irradiation assistance were investigated. It was found that the COD degradation and mineralization efficiency of 4-CP were more than 90% when a 30 min treatment of solar photo-Fenton oxidation process was applied and under an optimum [H₂O₂]₀/[Fe²⁺]₀ ratio of 40, the COD degradation and mineralization efficiency increased 65% as compared to Fenton oxidation. Meanwhile, the AOS values increased from −0.33 to 2.13 in solar photo-Fenton process while no significant improvement for AOS values was found in Fenton process, implying a higher degree of oxidation for 4-CP in solar photo-Fenton process. In addition, increasing the intensity of solar irradiation seemed to be beneficial for treatment of 4-CP contaminated water. Formation of chloride ion as a result of mineralization of organically bounded chlorine was identified during the treatment of 4-CP solution. Near-stoichiometric accumulation of chlorine was observed during the degradation of 4-CP in both Fenton and solar photo-Fenton processes. However, accumulation rate of chloride ions were much faster in solar photo-Fenton process. The degradation of 4-CP was found to obey a pseudo-first-order reaction kinetics. As compared to Fenton process, the presence of solar light in photo-Fenton process increases the reaction rate by a factor of 6.5 and 15.8 for COD and TOC degradation, respectively. In other words, during the treatment of 4-CP contaminated water, solar photo-Fenton process possesses notably higher mineralization efficiency in a relatively short radiation time as compared to Fenton process, and could enhance the degradation treatment of refractory organic wastewater such as 4-CP in a cost-effective approach.     

Destruction of phenol in water with sun, sand, and photocatalysis

The photocatalytic degradation and oxidation to carbon dioxide of aqueous phenol solutions have been studied using natural sunlight in geometries simulating shallow ponds. The photocatalyst was titanium dioxide freely suspended in the solution or immobilized on sand or silica gel. Photodegradation rates were approximately three times faster with the free suspension than with the immobilized catalyst under the same conditions, and were dependent on the time of the year and the time of the day. The seasonal variation correlated roughly with seasonal solar irradiance tabulations for the UV component of the spectrum. For 10 ppm of phenol the maximum rate of solar degradation resulted in a decrease in concentration to 10 ppb in less than 80 min with total mineralization in 110 min. Maintaining the same geometry, 30 L of solution covering 1 m² would degrade to the same extent.     

Photochemical synthesis of fluorescent Ag nanoclusters and enhanced fluorescence by ionic liquid

Fluorescent silver nanoclusters with a size of about 2 nm were synthesized within 90 s by irradiating a mixture of silver precursor and polymethacrylic acid sodium salt solution under Xe lamp. The proposed method was well optimized by a systematical investigation of experimental parameters, such as irradiation time, initial concentration, pH of the solution. The obtained nanoclusters were well dispersed in water and exhibited bright emission upon visible light excitation. This approach can be further extended to use solar light for the large scale synthesis. Strikingly, the emission intensity of as-prepared silver nanoclusters could be enhanced up to ∼60% by the addition of 1-(2′-hydroxylethyl)-3-methylimidazolium tetrafluoroborate ([HOEtMIm][BF₄]) ionic liquid. Meanwhile, these nanoclusters, in between molecules and normal nanoparticles, intensely absorb light around 510 nm covering the most intense part of solar light, should be a promising visible light sensitizer in solar cell and water splitting applications.     

Photo-hydrogen production rate of a PVA-boric acid gel granule containing immobilized photosynthetic bacteria cells

The polyvinyl alcohol (PVA)-boric acid gel granule facilitates the light penetration and mass transport as it has the features of the transparency and adequate porous structure. In this work, a hydrogen production bioreactor with the indigenous photosynthetic bacteria (PSB) Rhodopseudomonas palustris CQK 01 immobilized in a PVA-boric acid gel granule is developed to enhance the rate of photo-hydrogen production. Particular attention is paid to exploring the effects of illumination wavelength and intensity, as well as the effects of concentration, flow rate, pH, and temperature of influent substrate solution on the hydrogen production rate. The immobilized PSB gel granule exhibited the maximum hydrogen production rate of 3.6 mmol/g cell dry weight/h in all tests. The experimental results show that the hydrogen production rate of an immobilized PSB granule illuminated at 590 nm is distinctly higher than that at 470 and 630 nm. Photo-inhibition of the gel granule occurs as the long-wavelength illumination intensity exceeds 7000 lux. In addition, there exists an optimal pH of 7.0 and temperature of 30 °C for PSB immobilized in the granule to produce hydrogen. More importantly, the feasibility of PSB immobilized in the PVA-boric acid gel granule for the enhancement of the photo-hydrogen production is demonstrated.     

A solar-powered adsorption cooling system using a silica gel–water mixture

Solar-powered adsorption cooling is an attractive solar energy application. Metallic solar collectors with fins have been used to increase the thermal conductivity in solar collectors. This approach has a negative effect due to solar energy loss by reflection and heat loss resulting from the sensible heat of the metal. For these reasons, a direct-radiation absorption collector is proposed here. The effects of the wavelength of the absorbed light, types of silica gel used and additives to improve the absorptivity have been investigated. We have verified that blue silica gel has a better absorptivity in the near-infrared region than white silica gel. The addition of activated carbon to the silica gel improves the desorption rate and regeneration temperature of the packed bed.     

Visible-light-assisted photocatalytic degradation of gaseous formaldehyde by parallel-plate reactor coated with Cr ion-implanted TiO2 thin film

Sol–gel nano titanium dioxide (TiO₂) thin film can be activated by the ultraviolet (UV) radiation available in sunlight to perform solar photocatalysis. The useful spectral range can be extended from UV to visible light by implantation of metal ion into the TiO₂ lattice. As a result, the solar visible light can be utilized more efficiently to enhance the solar photocatalysis. In this study, visible-light-assisted photocatalytic glass reactors were built by parallel borosilicate glass plates coated on the upper surfaces with sol–gel TiO₂ thin films implanted with chromium (Cr) ion. The properties of the Cr/TiO₂ thin films were fully characterized by X-ray diffraction (XRD), differential scanning calorimetry (DSC), thermal gravity (TG) analysis, scanning-electron microscopy (SEM), and energy dispersive X-ray (EDX) analysis. In the performance tests, a metal halide lamp was used as an external light source to resemble the solar visible spectral radiation. The performance of a Cr/TiO₂ photoreactor was measured in terms of its photocatalytic degradation of gaseous formaldehyde in a single pass of contaminated air flowing through the photoreactor. The experimental results demonstrated the promise of using light-transmitting glass substrate to allow transmission and distribution of light from an external source to achieve solar photocatalysis. In the design of a parallel-plate photoreactor, it is important to properly control the Cr ion loading so that each Cr/TiO₂-coated glass plate absorbs a portion of the incident light for its photocatalytic activation and allows light transmission available for the remaining coated plates.     

Determination of the energy storage efficiency of the photoisomerization of norbornadiene to quadricyclane as a potential means for the trapping of solar energy

The fraction of incident radiant energy stored in chemical form by the photosensitized isomerization of norbornadiene to quadricyclane has been estimated. This reaction has been widely viewed as a potential means for the conversion of solar energy to useful heat. Using a xenon lamp as a radiant energy source in lieu of natural sunlight, 1-M acetophenone-norbornadiene and benzophenone-norbornadiene solutions were irradiated for 2-h periods with 1–5-mW radiant power levels delivered in 4-nm bandwidths centered on 300 nm, 350 nm, 400 nm and 450 nm. In each case the reaction was carried out batchwise at four temperatures in the liquid-phase range of the solutions, namely, 278 K, 298 K, 318 K and 333 K, controlled to within ±2 K. The results indicate that about 8.8% of the total available incident light contained in the region from 300 nm (lower end of the solar spectrum that impinges upon the earth's surface) to 425 nm (approximate absorption threshold of the sensitizers acetophenone and benzophenone; maximum wavelength capable of promoting the reaction under the conditions of this study) can be stored in the chemical bonds of quadricyclane in the acetophenone-sensitized isomerization and about 14.5% when benzophenone is utilized. Based on the entire solar spectrum, these energy storage efficiencies translate into 0.9% and 1.5%, respectively. Efficiency values were estimated by determining the quantum yield of the reaction as a function of wavelength and temperature. While the data show no definitive temperature dependence, it has been established that the functional relationship with wavelength follows the same pattern as the absorption spectra of the sensitizers themselves.     

Analysis of multilayer convective flow of a hybrid nanofluid in porous medium sandwiched between the layers of nanofluid

AgBr acts as a good sensitizer for titanium oxide, hence TiO₂–AgBr nanoparticles exhibit high photocatalytic activity which helps decompose methyl orange under visible light irradiation. Methyl orange is a chemical compound that is hard to degrade and has high stability. It is photoreactive and can capture photons from the sun and is highly used as a light harvester in solar cells, hence, it is used in solar applications. In view of this, the present article deals with the analysis of heat transfer in a multilayer flow of two immiscible nanofluids in a vertical channel that finds application in the fields of solar reactors, electronic cooling, and so on. The mathematical model involving the effect of thermal radiation and the presence of heat source is in the form of a system of ordinary differential equations. This system of equations is simplified using the differential transform method-Padé approximant and the resulting equations are solved algebraically. It is observed that the temperature of the coolant does not reach its saturation point faster due to the presence of different base fluids that differ in their thermal conductivity. This helps in maintaining the optimum temperature of the system.     

Novel improvements in the sensitizers of dye‐sensitized solar cells for enhancement in efficiency—a review

In the present review article, we have focused our study on the novel improvements that have been brought about in the molecular design of various sensitizers for application in dye‐sensitized solar cells (DSSCs). The sensitizers based on noble metals such as ruthenium, osmium, and rhenium showed high efficiency, but their cost and complicated synthesis restrict their wide applications. Further, to reduce the cost of fabrication of DSSCs, researchers are focusing their interest in organic sensitizers. In this context, organic dyes have offered several possibilities, as by improving their molecular structure brings about improvement in the light harvesting ability of dyes, and with the help of such dyes, optimal DSSCs have been fabricated. Further, to reduce the cost of DSSCs, researchers are also focusing on natural sensitizers such as betacyanin and anthocyanin or chlorophyll, as natural sensitizers are easy to prepare, cost effective, and environmentally friendly. With the help of these natural sensitizers, eco‐friendly and more cost‐effective DSSCs can be fabricated. Thus, we found from our study that beside metal‐based sensitizers, organic and natural sensitizers also offer a vast potential for the optimization of efficiency in DSSCs. Copyright © 2015 John Wiley & Sons, Ltd.