Pilot-plant treatment of olive mill wastewater (OMW) by solar TiO2 photocatalysis and solar photo-Fenton

Olive mill wastewater (OMW), a highly polluted wastewater from the olive oil industry, was treated by solar photocatalysis and solar photo-Fenton. Among the tested systems the application of titanium dioxide alone was not successful. The addition of peroxydisulphate as an electron acceptor had only limited effect on degradation performance and led to high salt concentrations (30 g/l sulphate generated) and a pH value near zero. The photo-Fenton method successfully removed up to 85% COD and up to 100% of phenol index of OMW with different initial concentrations and from different sources. Two solar photocatalytic pilot-plant reactors were used; one of conventional CPC type and an open non-concentrating Falling Film Reactor. The latter, newly designed reactor worked properly and yielded comparable results to the CPC in terms of degradation rate referred to incident UV radiation energy per solution volume. The suspended solids in the OMW hinder light from entering the reactor. Therefore, flocculation induced by a commercial flocculation agent was successfully applied to remove suspended solids. Application of this pre-treatment led to considerable increase of degradation rates and decrease of hydrogen peroxide consumption.     

Removal of non-CO2 greenhouse gases by large-scale atmospheric solar photocatalysis

Large-scale atmospheric removal of greenhouse gases (GHGs) including methane, nitrous oxide and ozone-depleting halocarbons could reduce global warming more quickly than atmospheric removal of CO₂. Photocatalysis of methane oxidizes it to CO₂, effectively reducing its global warming potential (GWP) by at least 90%. Nitrous oxide can be reduced to nitrogen and oxygen by photocatalysis; meanwhile halocarbons can be mineralized by red-ox photocatalytic reactions to acid halides and CO₂. Photocatalysis avoids the need for capture and sequestration of these atmospheric components. Here review an unusual hybrid device combining photocatalysis with carbon-free electricity with no-intermittency based on the solar updraft chimney. Then we review experimental evidence regarding photocatalytic transformations of non-CO₂ GHGs. We propose to combine TiO₂-photocatalysis with solar chimney power plants (SCPPs) to cleanse the atmosphere of non-CO₂ GHGs. Worldwide installation of 50,000 SCPPs, each of capacity 200 MW, would generate a cumulative 34 PWh of renewable electricity by 2050, taking into account construction time. These SCPPs equipped with photocatalyst would process 1 atmospheric volume each 14–16 years, reducing or stopping the atmospheric growth rate of the non-CO₂ GHGs and progressively reducing their atmospheric concentrations. Removal of methane, as compared to other GHGs, has enhanced efficacy in reducing radiative forcing because it liberates more _°OH radicals to accelerate the cleaning of the troposphere. The overall reduction in non-CO₂ GHG concentration would help to limit global temperature rise. By physically linking greenhouse gas removal to renewable electricity generation, the hybrid concept would avoid the moral hazard associated with most other climate engineering proposals.     

Degradation of lincomycin in aqueous medium: Coupling of solar photocatalysis and membrane separation

The photocatalytic oxidation of a common antibiotic, the lincomycin was carried out in aqueous suspensions of polycrystalline TiO₂ Degussa P25 irradiated by sunlight. In order to improve the performance of the lincomycin degradation a hybrid system consisting of a solar photoreactor with the photocatalyst in suspension coupled with a membrane module, used to confine both photocatalyst and pollutants in the reaction environment, was tested.A preliminary study was carried out in order to determine some kinetics parameters of the drug photodegradation. The influence of initial substrate concentration on the lincomycin photooxidation rate was investigated. The photooxidation rate follows a pseudo-first order kinetics with respect to the lincomycin concentration under the used experimental conditions. The presence of the membrane reactor allows the catalyst separation and to operate in continuous mode as the membranes rejection for lincomycin and its oxidation products was quite high.     

Tunable band gap of NV co-doped Ca:TiO2B (CaTi5O11) for visible-light photocatalysis

Visible-light photocatalysis in layered perovskite CaTi₅O₁₁ is measured by band-gap tuning with co-doped anionic nitrogen (N) and cationic vanadium (V). The screening of hybrid density functional (HSE06) calculation reveals that strong Coulomb interaction between the dopants and other atoms makes NV co-doping energetically favorable and effective for narrowing the band gap. More importantly, co-doping can eliminate the impurity states to reduce the electron-hole combination and improve the efficiencies of photocatalysis, since the mono-doped N or V ion produces impurity near the Fermi level. The impurity state captures the photoexcitation-generated carriers and accelerates the recombination process of the electron-hole pairs, thus suppressing their photocatalytic performance. The alignment of the band edge position with respect to the water oxidation/reduction potential indicates that NV co-doped CaTi₅O₁₁ can act as a potential photocatalytic water catalyst. Co-doping is expected to be an effective way of improving the visible-light photocatalytic activity in layered perovskite due to the elimination of recombining the electron-hole pairs.     

Inactivation of Enterococcus sp. by photolysis and TiO2 photocatalysis with H2O2 in natural water

This paper presents research carried out into the disinfectant power of a series of treatments based on the individual application and possible combinations of TiO₂ (1 g/L), H₂O₂ (0.04 mM) and irradiation (290–800 nm and 320–800 nm) on Enterococcus sp., a faecal bacterial indicator used in water analysis. The main aims are the determination of the influence of the UVB range on the inactivation of Enterococcus sp. in natural water, the mode of application of irradiation (intermittent or continuous) and the capacity of bacterial recovery after the application of the treatments in darkness conditions. The results show that when the irradiation includes the UVB range, a very high degree of inactivation is obtained in an Enterococcus sp. solution in natural water (N₀ = 10⁸ CFU 100 mL^?1) by irradiation alone. Neither the addition of TiO₂ nor of H₂O₂ are very relevant in these conditions. However, if the irradiation does not include the UVB range, the bactericidal action of photolysis is practically nonexistent, highlighting the positive effect of TiO₂ and H₂O₂ on the irradiation, the photocatalysis and photocatalysis/H₂O₂ treatments obtaining a higher degree of disinfection. Continuous and intermittent illumination give rise to similar inactivation levels in all the treatments studied, except for photolysis in which a significant increase in inactivation is observed when the irradiation is continuous. During 3 h of darkness following application of the treatments, there is no change or else a slight recovery of the bacterial population.     

Performance of TiO2 nanoparticles synthesized by microwave and solvothermal methods as photoanode in dye-sensitized solar cells (DSSC)

In this work, a direct comparison of the properties of the TiO₂ nanoparticles prepared by microwave and solvothermal methods were carried out and its performance as photoanode in dye-sensitized solar cells (DSSC) was analyzed. Though previously some works exist on the preparation of TiO₂ nanoparticles by solvothermal or microwave methods, they could not be compared directly as the experiment conditions such as choice of solvent, precursors and reaction temperatures were not virtually same. Herein, TiO₂ nanoparticles were synthesized by microwave and solvothermal methods using the same initial precursors and properties of the prepared nanoparticles were compared. From the X-ray diffraction pattern and Raman analysis, the prepared nanoparticles in both the cases were found to be of anatase phase. Optical properties and its carrier lifetime were studied using UV–Vis absorption, photoluminescence (PL) analysis and PL lifetime studies, respectively. Further, its morphology analyzed using scanning electron microscope (SEM) and transmission electron microscope (TEM) images, and SAED (selected area electron diffraction) patterns reveals the polycrystalline nature of the prepared nanoparticles. The surface area and the pore size distribution were studied using BET (Brunauer–Emmett–Teller) and BJH (Barrett–Joyner–Halenda) analysis, which revealed its mesoporous nature and uniform pore distribution. The chemical states of the prepared nanoparticles were further characterized using X-ray photoelectron spectroscopy. The DSSC was fabricated using the prepared TiO₂ nanoparticles as photoanodes. Further, the power conversion efficiency and the electron transport properties were analyzed.     

Polymer solar cell based on poly(2,6-bis(3-alkylthiophen-2-yl)dithieno-[3,2-b;2′,3′-d]thiophene)

Polymer photovoltaic solar cells using poly (2,6-bis(3-alkylthiophen-2-yl)dithieno-[3,2-b;2′,3′-d]thiophene) (PBTDT) as the donor is demonstrated. The UV-vis spectra show that PBTDT has strong absorption in the visible spectrum. By adjusting the ratio of PBTDT to [6,6]-phenyl C61-butyric acid methyl ester (PCBM) and optimizing the annealing temperature, the PBTDT-based polymer solar cells show a power conversion efficiency of 0.42% under 100 mW/cm² AM1.5G simulated sunlight.     

Transformation of TiO2 nanoparticles to nanotubes by simple solvothermal route and its performance as dye-sensitized solar cell (DSSC) photoanode

Titania nanotubes were synthesized by simple solvothermal method using quasi crystalline TiO₂ nanoparticles as the starting material without using autoclave. In the presence of concentrated NaOH (sodium hydroxide), TiO₂ nanoparticles were transformed into nanotubes. The complete transformation of nanoparticles to nanotubes was witnessed using Field Emission Scanning Electron Microscopy (FESEM) and High-resolution transmission electron microscopy (HRTEM) images and further, the materials were found to be polycrystalline in nature. From the X-ray diffraction pattern and Raman analysis, the TiO₂ nanoparticles were found to exhibit the anatase phase and the nanotubes were found to exhibit the titanate phase. The surface area and pore size distribution were analysed using BET (Brunauer–Emmett–Teller) and BJH (Barrett–Joyner–Halenda) analysis. The surface area of the nanotubes was found to be higher compared to the initial nanoparticles and bimodal type pore distributions were observed from the BJH study. The bandgap and defect emissions of the nanotubes and nanoparticles were analysed using UV–Vis absorption and photoluminescence (PL) analysis. The chemical states of the prepared nanoparticles were further characterized using X-ray photoelectron spectroscopy. The Dye-sensitized solar cells (DSSC) were fabricated using the prepared TiO₂ nanostructures as photoanodes and their power conversion efficiencies were analysed.     

Rational design and fabrication of TiO2 nano heterostructure with multi-junctions for efficient photocatalysis

In addition to the extended light absorption, the effective spatial charge separation is a crucial factor for highly efficient metal-oxide semiconductor-based photocatalysts. Herein, a rational design of metal-semiconductor-metal nano heterostructure for enhancing photocatalytic performance is proposed. The semiconductor nanoparticles are integrated with two metals in one single nano heterostructure. The disordered layers are induced on the surface of TiO₂ to promote the light absorption capacity. More importantly, the n-n⁺ junction is fabricated at the contact region between crystalline TiO₂ (n-TiO₂) and disordered layers (n⁺-TiO₂). Besides, the Schottky diode and Ohmic contact are formed on n-TiO₂ and n⁺-TiO₂, respectively. As a result, the existence of multi-junctions leads to the formation of multiple continuous built-in electric fields, thus remarkably accelerating the spatial separation of charge carriers. The resulting nano heterostructure with multi-junctions (Pt–TiO₂–H–Ag) exhibits remarkably promoted photocatalytic performance. The maximum hydrogen generation rate of Pt–TiO₂–H–Ag under solar illumination (18001.0 μmol/h/g) is 8.3, 9.3, and 1.5 times superior to that of Pt-loaded P25 (Pt–P25), Pt loaded TiO₂ (Pt–TiO₂), and hydrogenated Pt–TiO₂ (Pt–TiO₂–H), respectively. Moreover, the photocatalytic performance under visible illumination is significantly enhanced by Pt–TiO₂–H–Ag. Specifically, the H₂ generation rate of Pt–TiO₂–H–Ag (2382.7 μmol/h/g) is about 15.1, 17.2, and 1.4 times higher than that of Pt–P25, Pt–TiO₂, and Pt–TiO₂–H, respectively. The corresponding apparent quantum efficiency of Pt–TiO₂–H–Ag is 15.8% (420 nm). The nano heterostructure with multi-junctions also exhibits excellent stability after five cycles, remaining hydrogen evolution rates of 15581.5 and 2211.4 μmol/h/g under solar and visible illumination, respectively. This effective and controllable manufacturing strategy could provide new opportunities to simultaneously extend optical absorption and facilitate the spatial charge separation and transport of wide-bandgap metal-oxide semiconductors.     

Photoelectrochemical studies on [MnMoO2(NCS)(Ox)3(H2O)2] (Ox=8‐quinolinol): a novel system for solar energy conversion

Photoelectrochemical studies were carried out on [MnMoO₂(NCS)(Ox)₃(H₂O)₂] {Ox=8‐quinolinol} complex in aqueous dimethylformamide medium in a Honda cell. The system developed a maximum potential of 335 mV when exposed to visible light at 30°C and was found to be reversible. The photogalvanic behaviour has been further investigated by varying the pH, temperature and photosensitizers. When a temperature difference between the illuminated and dark half‐cells was maintained, the system generated 410 mV at 60°C. A solid‐state galvanic cell, developed using the complex mixed with tetraethylammonium perchlorate (TEAP), showed a maximum voltage of 25 mV. A sandwich galvanic cell, constructed from transparent tin oxide‐glass/complex/platinum, developed a maximum photovoltage of 88 mV when irradiated with a tungsten halogen lamp. Copyright © 2000 John Wiley & Sons, Ltd.     

Degradation of solar cell efficiency by sheet resistance

The efficiency degradation of solar cells due to sheet resistance-generated losses is computed as a function of light intensity and electrode spacing. The degradation derived by Heizer and Chu is shown to be 33 per cent too large at small electrode spacing. We give an approximate analytic relation for the efficiency degradation and derive from it the electrode spacing which minimizes the combined losses due to sheet resistance and electrode shading. An electrode configuration which prevents extreme power loss by local short circuits in the photojunction is described.     

Concentrated solar photocatalysis for hydrogen generation from water by titania-containing gold nanoparticles

Photocatalysis is an effective way to utilize solar energy to produce hydrogen from water. Au/TiO₂ nanoparticles (NPs) have a better performance in photocatalytic hydrogen generation because of the localized surface plasmon resonance (LSPR) effect of Au/TiO₂ NPs. In the photocatalytic hydrogen generation experiments, it was found that light intensity plays a key role in the photocatalytic reaction rate of Au/TiO₂ NPs. At a light intensity of 0–7 kW/m², the reaction rate has a super-linear law dependence on the light intensity (Rate ∝ Intensityⁿ, with n > 1). However, at a light intensity of 7–9 kW/m², the dependency becomes sub-linear (n < 1). This means that the increase rate of photocatalytic rate is smaller than that of light intensity when the light intensity exceeds 7 kW/m². In addition, the finite element method (FEM) was utilized to further elucidate the role of light intensity by calculating the absorption power and nearfield intensity mapping of a Au/TiO₂ nanoparticle. The variation trend of the calculated total absorption power agrees with the photocatalytic experimental results for different light intensities. These results shed light on the utilization of concentrated solar photocatalysis to increase the solar-to-hydrogen performance of Au/TiO₂ NPs.     

Photovoltage enhancement of dye sensitised solar cells by using ZnO modified TiO2 electrode

Abstract

A ZnO modified TiO₂ (ZnO/TiO₂) film was prepared by immersing TiO₂ electrodes in Zn(Ac)₂ aqueous solution. The open circuit voltage of a dye sensitised solar cell (DSSC) with the ZnO/TiO2 film electrode has a dramatic enhancement, compared to the DSSC with the TiO₂ film electrode. However, the short circuit current density of the DSSC with the ZnO/TiO₂ film electrode is lower than that with TiO₂ electrode. The film electrodes were characterised by SEM, EDX and UV-vis, and the photoelectric performance of DSSCs were measured. The photovoltage enhancement is attributed to the formation of a flat-band potential energy barrier by ZnO at TiO₂/electrolyte interface. The decline of the photocurrent with ZnO/TiO₂ film electrode is due to poor dye absorption on larger particles of ZnO.     

Let the games begin [solar powered homes]

The 2002 Solar Decathlon challenged students to create, and operate, energy-efficient solar-powered homes. The competition also required that the solar-powered homes be designed and constructed such that they could be transported to Washington, DC, set up in a solar village on the National Mall for competition, and then removed from the Mall. The participants competed in ten different contests. Seven of these contests were based strictly on energy. This article outlines the ten contests and discusses the design issues and transportation and setup issues.     

Surface engineering of Ba-doped TiO2 nanorods by Bi2O3 passivation and (NiFe)OOH Co-catalyst layers for efficient and stable solar water oxidation

The rational design of heterostructures as an ideal photoelectrode system for H₂ and O₂ conversion in photoelectrochemical (PEC) system has been regarded as an essential key to boost PEC performance. In this work, to demonstrate the energetic photoanode cell, deposition of a thin layer of Bi₂O₃ is utilized to hybridize with the 5 wt% Ba-doped TiO₂ nanorod heterostructure under the cascading band diagram, where Ba-doping can enhance the charge transport/separation rate in bulk phase, in terms of increasing the donor density, enhancing the bulk electronic conductivity, and increasing the band bending. Furthermore, with optimizing the thickness (～15 nm) of Bi₂O₃, the (NiFe)OOH as a cocatalyst was adapted to improve the interfacial charge transfer rate in the PEC cell, reaching the high photocurrent density (J) of ～4.1 mA/cm² at 1.23 V (vs. Reversible Hydrogen Electrode) and stability retention of 100%, even after 15 h at 1 M NaOH under 1 Sun illumination condition. The improvement mainly comes from the extended absorption of visible light from the thin Bi₂O₃ layer, effective transfer/separation of photogenerated charge carriers, and acceleration of water oxidizing reaction, caused by the narrowed band gap and the favorable charge transfer under the cascading band alignment built by the heterojunction, as well as electrocatalyst, offering the timely consumption of photogenerated holes accumulated at the electrode surface.     

Efficiency enhancement of dye‐sensitized TiO2 solar cell based on ruthenium(II) terpyridyl complex photosensitizer

We theoretically investigated the influence of the ligands on the enhancement of the efficiency toward absorption in the solar spectrum of ruthenium(II) terpyridyl complex. In the present work, the NCS ligands of the parent black dye 4,4′,4″‐tricarboxy‐2,2′:6′,2″‐terpyridine)ruthenium(II) complex (BD0) were kept unchanged, while π‐conjugating spacers were introduced between the terpyridine ligands and the anchoring groups ―COOH. Molecular structures, electronic, and spectroscopic properties of four designed black dyes, in addition to the parent black dye (BD0), were examined. Compared with the parent black dye BD0, broad and intense absorption bands in the infrared and near‐infrared region (red‐shifted) were found, featuring the enhancement of the absorption efficiency resulting from the insertion of the proposed π‐conjugating spacers. Copyright © 2015 John Wiley & Sons, Ltd.     

Effect of TiO2 photo-electrode growth condition on dye-sensitised solar cells

Titanium dioxide- (TiO₂) based photo-electrode for dye-sensitised solar cells (DSSCs) use is fabricated with the electro-phoretic deposition (EPD) technique on indium-tin-oxide (ITO). TiO₂ films were synthesised at different EPD biases ranging from 70 to 110 V. We correlate the morphological and optical properties of formed films to the electrical characteristics of fabricated DSSCs. In addition, by neglecting the tunnelling and the tunnelling-assisted thermo-ionic currents with respect to the pure thermo-ionic current at the ITO/TiO₂ interface, we evaluate the high potential barrier, e?_B. Investigation of the electrical properties of the formed DSSCs shows a best result when the TiO₂ film is elaborated at 100 V. Furthermore, by taking into account the high band energy of 0.6 eV at an aluminium-based counter electrode/electrolyte interface, we deduce that aluminium reduces drastically the short-circuit current of the DSSC.     

Nanocrystalline TiO2 (anatase) for Li-ion batteries

Nanocrystalline TiO₂ (anatase) was synthesized successfully by the direct conversion of TiO₂-sol at 85 °C. The as-prepared TiO₂ at 85 °C were calcined at different temperatures and time in order to optimize the system with best electrochemical performance. The particle sizes of the synthesized materials were found to be in the range of 15–20 nm as revealed by the HR-TEM studies. Commercial TiO₂ anatase (micron size) was also studied for its Li-insertion and deinsertion properties in order to compare with the nanocrystalline TiO₂. The full cell studies were performed with LiCoO₂ cathode with the best performing nano-TiO₂ as anode. The specific capacity of the nanocrystalline TiO₂ synthesized at 500 °C/2 h in a half-cell configuration was 169 mAh g⁻¹ while for the cell with LiCoO₂ cathode, it was 95 mAh g⁻¹ in the 2 V region. The specific reversible capacity and the cycling performance of the synthesized nano-TiO₂ anode in full cell configuration across LiCoO₂ cathode are superior to that reported in the literature. Cyclic voltammetry measurements showed a larger peak separation for the micro-TiO₂ than the nano-TiO₂, clearly indicating the influence of nano-particle size on the electrochemical performance.     

Natural dye (cyanidin 3-O-glucoside) sensitized nanocrystalline TiO2 solar cell fabricated using liquid electrolyte/quasi-solid-state polymer electrolyte

TiO₂ dye-sensitized solar cells (DSSCs) have been fabricated using TiO₂ photoelectrodes sensitized using the extracts of Delonix regia (May flower, locally called Vakai) and Eugenia Jambolana (Indian blackberry, locally called Naval) as natural sensitizers and their characteristics have been studied. Among them Eugenia Jambolana gave the best photosensitization effect and presents the prospect to be used as an environment-friendly, low-cost alternative system. The extracts having anthocyanin pigment (cyanidin 3-O-glucoside), which have carboxylic groups in the molecule can attach effectively to the surface of TiO₂ film. The solar cell constructed using the Eugenia Jambolana sensitized TiO₂ photo-electrode exhibited a short-circuit photocurrent of 1.49 mA and a power conversion efficiency of 0.5%.