Solar photocatalytic degradation of methylene blue in carbon-doped TiO2 nanoparticles suspension

Carbon-doped TiO₂ nanoparticles were prepared by sol–gel auto-combustion method and characterized by X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), Brunauer–Emmett–Teller method (BET), UV–vis diffuses reflectance spectroscopy (DRS). UV–vis diffuse reflectance spectra showed that carbon-doped TiO₂ exhibited obvious absorption in the visible light range. The visible light photocatalytic activity of carbon-doped TiO₂ was ascribed to the presence of oxygen vacancy state between the valence and the conduction bands because of the formation of Ti³⁺ species in the as-synthesized carbon-doped TiO₂. The sample calcined at 873 K showed the highest photocatalytic activity under solar irradiation. The effects of photocatalyst concentration, initial concentration of methylene blue, and pH value in aqueous solution were also presented.     

Visible-light-driven TiO2 catalysts doped with low-concentration nitrogen species

Visible-light-driven nitrogen-doped TiO₂ was synthesized using a novel nitrogen-ion donor of hydrazine hydrate. Low-concentration (0.2 at%) nitrogen species and Ti³⁺ were detected in the TiO₂-based photocatalyst by X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance (EPR) spectroscopy. The trace amount of Ti–N would contribute to the minor band-gap narrowing of about 0.02 eV. Those nitrogen-containing species, especially the NO₂²⁻ species, form surface states, which make the catalysts possible to degrade 4-chlorophenol (4-CP) under visible irradiation (λ>400 nm). Moreover, Ti³⁺ species induce oxygen vacancy states between the valence and the conduction bands, which would also contribute to the visible response. The photocatalytic activity of the nitrogen-doped TiO₂ catalyst was thought to be the synergistic effect of nitrogen and Ti³⁺ species. The catalysts showed higher photocatalytic activity for degradation of 4-CP than pure TiO₂ under not only visible but also UV irradiation. The visible response and the higher UV activity of the nitrogen-doped TiO₂ make it possible to utilize solar energy efficiently to execute photocatalysis processes.     

Solar photocatalytic treatment of simulated dyestuff effluents

The photocatalytic organic content reduction of two selected synthetic wastewater from the textile dyeing industry, by the use of heterogeneous and homogeneous photocatalytic methods under solar irradiation, has been studied at a pilot plant scale at the Plataforma Solar de Almeria. The effect of two different TiO₂ modifications with oxidants such as H₂O₂ and Na₂S₂O₈ on the decolourisation and the dissolved organic content reduction (DOC) of the wastewater was examined. The TiO₂/H₂O₂ system seems to be more efficient in comparison to the synergetic action that appears when using persulphate and TiO₂ in this specific wastewater. By an accumulated energy of 50 kJ l⁻¹ the synergetic effect of TiO₂ P-25 with H₂O₂ and Na₂S₂O₈ leads to a 70% and 57% DOC reduction, respectively, in the case of cotton synthetic wastewater, while the decolourisation was almost complete. The photocatalytic decolourisation, as well as the DOC reduction in the case of nylon simulated wastewater is a slower process and an accumulated energy of 50 kJ l⁻¹ leads to almost 54% mineralisation in both cases. The photo-Fenton process in both types of wastewater was more efficient in comparison to the TiO₂/oxidant system. An accumulated energy of 50 kJ l⁻¹ leads to 90% reduction of the organic content.     

Sol–gel preparation and characterization of anti-reflective and self-cleaning SiO2–TiO2 double-layer nanometric films

Glass substrates were first coated with SiO₂ and then TiO₂ by dipping into sols which were prepared by two different methods involving complex formation and hydrolysis, using ethanol (EtOH) or butyl glycol (BG). Concentration of TiO₂ in the sols was kept at 0.1 and 0.5 wt%. Prepared coatings were investigated by field-emission scanning electron microscope (FESEM), atomic force microscope (AFM), hazemeter, UV–visible spectrophotometer and goniometer. Rhodamine B (RhB) photodegradation tests were performed in order to evaluate photocatalytic activity. Application of SiO₂ as the bottom layer increased the transmittance by 6% points, thereby compensated for the loss of transmittance caused by the TiO₂ self-cleaning top layer. Pencil-hardness values of the obtained coatings were in 5B–3H range. TiO₂ coatings obtained from sols containing 0.5% TiO₂ and BG solvent represented the highest photocatalytic activity, with a rate constant of 0.44 ppm⁻¹ h⁻¹ and a half period of 5.5 h. Self-cleaning surfaces were obtained while maintaining the anti-reflectance.     

The design of a hierarchical photocatalyst inspired by natural forest and its usage on hydrogen generation

A novel photocatalyst was designed from the inspiration of natural forest's high efficient on light harvesting and energy conversion. This novel “forest-like” photocatalyst was successfully synthesized by a facile continuously-conducted three steps methods: electrospinning TiO₂ nanofiber acts as the trunks, hydrothermal growth ZnO nanorods on the surface of TiO₂ nanofiber acts as the branches, while photodeposition of Cu nanoparticles on the surface of TiO₂ nanofiber and ZnO nanorods act as the leaves. This novel photocatalyst demonstrated higher photocatalytic hydrogen generation rate than most of semiconductor catalysts and many newly developed catalysts such as Pt/TiO₂ catalyst and artificial leaves Pt/N–TiO₂ catalyst in a water/methanol sacrificial reagent system under the light irradiation as a result of its enhanced light absorption ability, enlarged specific surface area promoting mass transfer and providing more reaction sites and its potential on anti-recombination of electrons and holes. Meanwhile, it is interesting to note that the photocatalytic hydrogen generation activity has a liner relationship with the hierarchy of materials, which means higher hierarchy materials display higher photocatalytic hydrogen generation activity. It is reasonable to believe that this natural mimic photocatalyst without noble metals will benefit the energy generation and novel materials development.     

Photodegradation of chlorofluorocarbon alternatives on metal oxide

HCFC and HFC were photodegraded on metal oxides. Degradation rate on several metal oxides was in the order: TiO₂ZnO > Fe₂O₃kaolin SiO₂ Al₂O₃. Principal degradation products were CO₂, Cl⁻ and F⁻. Minor products which oxidize I⁻ were included for some HCFC's. A photocatalytic process is suggested for the degradation reaction. An overall reaction equation is proposed on the basis of mass balance.     

Solar-based photoreduction of methyl orange using zeolite supported photocatalytic materials

A new class of novel photocatalysts has been prepared by supporting TiO₂ on the zeolite matrix by various routes of synthesis. Different transition metals like cobalt, nickel, and ruthenium have been incorporated in these photocatalysts, alongwith molybdenum based heteropolyacid (HPA) to improve the photocatalytic activity of these materials. Photoreduction of methyl orange under solar radiation was compared with photoreduction in presence of artificial visible light illumination to evaluate their photocatalytic activity. The quantity of methyl orange photoreduced by the cobalt containing photocatalyst was about 2.40 mg/g of TiO₂ under the influence of sunlight as compared to 4.111 mg/g of TiO₂ under artificial visible light illumination. However, the efficiency of the photocatalyst is high as compared to P25 TiO₂ under solar light (0.508 mg/g of TiO₂). The high photocatalytic activity of these materials is due to the synergistic effect of incorporation of transition metals in combination with TiO₂ and HPA supported by the zeolite matrix. These materials are being evaluated for photocatalytic water splitting.     

Enhanced hydrogen generation using ZrO2-modified coupled ZnO/TiO2 nanocomposites in the absence of noble metal co-catalyst

Mesoporous ZrO₂-modified coupled ZnO/TiO₂ nanocomposites were prepared by a surfactant assisted sol–gel method. The photocatalytic performance of these materials was investigated for H₂ evolution without noble metal co-catalyst using aqueous methanol media under AM1.5 simulated light. The H₂ evolution was compared with coupled ZnO/TiO₂, TiO₂, ZnO and Degussa P25. The ZrO₂-modified nanocomposites exhibited higher H₂ generation, specifically 0.5 wt.% ZrO₂ loading produced 30.78 mmol H₂ g⁻¹ compared to 3.55 mmol H₂ g⁻¹ obtained with coupled ZnO/TiO₂. A multiple absorbance thresholds at 435 nm and 417 nm were observed with 0.5 wt.% ZrO₂ loading, corresponding to 2.85 eV and 2.97 eV band gap energies. The high surface area, large pore volume, uniform crystallite sizes and enhanced light harvesting observed in ZrO₂-modified nanocomposites were contributing factors for effective charge separation and higher H₂ production. The possible mechanism of H₂ generation from aqueous methanol solution over ZrO₂-modified nanocomposite is presented.     

Photocatalytic oxidation of nitrite by sunlight using TiO2 supported on hollow glass microbeads

The feasibility of photocatalytic oxidation of nitrite using TiO₂ supported on hollow glass microbeads as a photocatalyst by sunlight was studied. The results showed that 1.1×10⁻⁴ mol/dm³ of nitrite can be completely photocatalytically oxidized after 120-min illumination by sunlight. The conversion of nitrite was increased rapidly by adding a small amount of H₂O₂, Fe³⁺ and Pd²⁺. The effects of parameters such as the amount of TiO₂/beads, air flow, initial pH, dichromate, SO₄²⁻, NO₃⁻, Cl⁻ and organic compounds on the photocatalytic oxidation of nitrite were also studied. The possible mechanisms of photocatalytic oxidation of nitrite were investigated. After 150-h illumination by sunlight, there was no significant loss of the photocatalytic activity of TiO₂/beads.     

Photocatalytic H2 production from water splitting under visible light irradiation using Eosin Y-sensitized mesoporous-assembled Pt/TiO2 nanocrystal photocatalyst

Sensitized photocatalytic production of hydrogen from water splitting is investigated under visible light irradiation over mesoporous-assembled titanium dioxide (TiO₂) nanocrystal photocatalysts, without and with Pt loading. The photocatalysts are synthesized by a sol–gel process with the aid of a structure-directing surfactant and are characterized by N₂ adsorption–desorption analysis, X-ray diffraction, UV–vis spectroscopy, scanning electron microscopy, transmission electron microscopy and energy-dispersive X-ray analysis. The dependence of hydrogen production on the type of TiO₂ photocatalyst (synthesized mesoporous-assembled and commercial non-mesoporous-assembled TiO₂ without and with Pt loading), the calcination temperature of the synthesized photocatalyst, the sensitizer (Eosin Y) concentration, the electron donor (diethanolamine) concentration, the photocatalyst dosage and the initial solution pH is systematically studied. The results show that in the presence of the Eosin Y sensitizer, the Pt-loaded mesoporous-assembled TiO₂ synthesized by a single-step sol–gel process and calcined at 500 °C exhibits the highest photocatalytic activity for hydrogen production from a 30 vol.% diethanolamine aqueous solution with dissolved 2 mM Eosin Y. Moreover, the optimum photocatalyst dosage and initial solution pH for the maximum photocatalytic activity for hydrogen production are 3.33 g dm⁻³ and 11.5, respectively.     

Facile synthesis of noble-metal free polygonal Zn2TiO4 nanostructures for highly efficient photocatalytic hydrogen evolution under solar light irradiation

Designing of noble-metal free and morphologically controlled advanced photocatalysts for photocatalytic water splitting using solar light is of huge interest today. In the present work, novel polygonal Zn₂TiO₄ (ZTO) nanostructures have been synthesized by citricacid assisted solid state method for the first time and synthesized nanostructures were characterized by using various techniques like PXRD, UV-Vis-DRS, PL, FT-IR, BET, FE-SEM and TEM for their structural, optical, chemical, surface and morphological properties. The PXRD and UV-Vis-DRS analysis show the existence of cubic and tetragonal phases. FE-SEM and TEM results confirm the formation of polygonal ZTO nanostructures. Synthesised ZTO nanostructures have been potentially applied for solar light-driven photocatalytic hydrogen evaluation from water splitting and compare the photocatalytic activity with synthesized conventional Zn₂TiO₄ and commercially available TiO₂, ZnO photocatalysts. A high rate of 529 μmolh^?1g^?1 solar light-driven photocatalytic H₂ evolution has been achieved by using a small amount (5 mg) of polygonal Zn₂TiO₄ nanostructures from glycerol-water solution. The enhanced photocatalytic performance of the polygonal Zn₂TiO₄ nanostructures compare to conventional Zn₂TiO₄ under solar light irradiation is due to the large surface area and low recombination rate. However having the same bandgap, the polygonal Zn₂TiO₄ nanostructures have shown enhanced photocatalytic performance than that of commercially available TiO₂, ZnO photocatalysts.     

Improvement of photocatalytic hydrogen generation from CdSe/CdS/TiO2 nanotube-array coaxial heterogeneous structure

The fabrication and characterization of CdSe/CdS/TiO₂ nanotube-array coaxial heterogeneous structure that has potential applications in photocatalytic water splitting and toxic pollutants degradation are investigated. CdSe(top)/CdS(under) double-layer is conformally deposited onto TiO₂ nanotubes by successive ionic layer adsorption and reaction (SILAR) and electrochemical atomic layer deposition (ECALD), respectively, for the CdS under layer and the CdSe top layer. Such double sensitized TiO₂ nanotubular photoelectrode exhibits significant enhancements in photoconversion efficiency, visible light response, and efficient hydrogen generation. The detailed synthesis process and the surface morphology, phase structure, elemental analysis, and photoelectrochemical properties of the resulting films with the CdSe/CdS/TiO₂ nanotube-array coaxial heterogeneous structure are discussed. The photoconversion efficiency of 9.47% and hydrogen generation rate of 10.24 ml h⁻¹ cm⁻² were observed. Both values are a 7-fold enhancement compared with that of the pure TiO₂ nanotube. The as-prepared photoelectrode presents potential application for industrialized photocatalytic hydrogen generation in the future.     

Preparation and photocatalytic properties of HLaNb2O7/(Pt,TiO2) perovskite intercalated nanomaterial

A novel perovskite intercalated nanomaterial HLaNb₂O₇/(Pt, TiO₂) is fabricated by successive intercalated reaction of HLaNb₂O₇ with [Pt(NH₃)₄]Cl₂ aqueous solution, n-C₆H₁₃NH₂/C₂H₅OH organic solution and acidic TiO₂ colloid solution, followed by ultraviolet light irradiation. The gallery height and the band gap energy of HLaNb₂O₇/(Pt, TiO₂) is less than 0.6 nm and 3.14 eV, respectively. The photocatalytic activity of HLaNb₂O₇/TiO₂ is superior to that of unsupported TiO₂ and is enhanced by the co-incorporation of Pt. The photocatalytic hydrogen evolution based on HLaNb₂O₇/(Pt, TiO₂) is 240 cm³ h⁻¹ g⁻¹ using methanol as a sacrificial agent under irradiation with wavelength more than 290 nm from a 100-W mercury lamp. High photocatalytic activity of HLaNb₂O₇/(Pt, TiO₂) may be due to the host with rare earth La element and perovskite structure, the quantum size effect of intercalated semiconductor and the coupling effect between host and guest.     

Assessment and influence of operational parameters on the TiO2 photocatalytic degradation of sodium benzene sulfonate under highly concentrated solar light illumination

Sodium benzene sulfonate (BS) was decomposed in aqueous TiO₂ dispersions under highly concentrated solar light illumination to examine the photocatalytic characteristics of a parabolic round concentrator (PRC) reactor to degrade the pollutant without visible light absorption. The effects of such operational parameters as initial concentration, volume of the aqueous BS solution, oxygen purging, and TiO₂ loading on the kinetics of decomposition of BS were investigated. An effective photodegradation necessitates a suitable combination of initial volume and concentration of BS solution. Relative to atmospheric air, oxygen purging significantly accelerates the degradation process at high initial concentrations of BS (0.40 mM or 1.0 mM). Optimal TiO₂ loading was 9 g l⁻¹, greater than previously reported. Elimination of TOC (total organic carbon) followed pseudo first-order kinetics in the initial stages of the photodegradation process. The relative photonic efficiency for the photodegradation of BS is ζ_rel=1.0.     

Significant improvement of photocatalytic hydrogen generation rate over TiO2 with deposited CuO

TiO₂ photocatalyst with deposited CuO (CuO-TiO₂) was synthesized by the impregnation method using P25 (Degussa) as support, and exhibited high photocatalytic hydrogen generation activity from methanol/water solution. A substantial hydrogen evolution rate of 10.2 ml min⁻¹ (18,500 μmol h⁻¹ g⁻¹_catalyst) was observed over this efficient CuO-TiO₂ with optimal Cu content of 9.1 mol% from an aqueous solution containing 10 vol% methanol; this improved hydrogen generation rate is significantly higher than the reported Cu-containing TiO₂, including some Pt and Pd loaded TiO₂. Optimal Cu content of 9.1 mol% provided maximum active sites and allowed good light penetration in TiO₂. Over this efficient CuO-TiO₂, the hydrogen generation rate was accelerated by increasing the methanol concentration according to Freundlich adsorption isotherm. However, the photocatalytic hydrogen generation rate was suppressed under long time irradiation mainly due to accumulation of by-products, reduction of CuO and copper leaching, which requires further investigation.     

Effect of loaded silver nanoparticles on TiO2 for photocatalytic degradation of Acid Red 88

Nanometer sized Ag–TiO₂ nanoparticles were prepared (by photoreduction of Ag⁺ ions) in order to assess its photocatalytic degradation ability of target pollutant (textile dye; Acid red 88) upon visible light irradiation. Furthermore, oxidative reagents such as peroxomonosulfate (PMS), peroxodisulfate (PDS) and hydrogen peroxide (H₂O₂) were added to the photocatalytic system, which may act as an alternative electron acceptor and result in a notably enhanced degradation rate (seven-fold increase with PMS as oxidant) of pollutant destruction. Mineralization of target pollutant was also performed by total organic carbon (TOC) analysis and from the results, it was confirmed that 65% mineralization was achieved in 7 h using PMS as electron acceptor. Overall, this system is relatively inexpensive, reproducible, extremely stable and efficient in complete degradation of dye in aqueous solution. In order to obtain maximum information about the performance of Ag–TiO₂ photocatalyst, we did experiments under different operating conditions, i.e., variation of amount of catalyst, concentration of dye and electron acceptors. In addition to the above, a comparative study on the photocatalytic activities of TiO₂ was also made.     

Synthesis, characterization and fabrication of solar cells making use of [Ru(dcbpy)(tptz)X]X (where X = Cl, SCN, CN) complexes

Dye-sensitized TiO₂ solar cells were fabricated using tridentate ligand ruthenium(II) complexes, [Ru(dcbpy)(tptz)X]X (where dcbpy = 4,4′-dicarboxy-2,2′-bipyridine, tptz = 2,4,6-Tris(2-pyridyl)-s-triazine and X = Cl⁻, SCN⁻, CN⁻) attached to sol–gel processed TiO₂ electrodes. The ligand tptz functions as spectator ligand and dcbpy functions as the anchoring ligand with sufficient visible light absorption. The synthesized complexes were characterized before using them in solar cells. The functioning of the solar cells fabricated using different conducting glasses was monitored and the current–voltage characteristics were measured. The efficiencies of different cells were calculated and compared.     

Charge transport properties in the nanostructured ZnO thin film electrode – electrolyte system studied with time resolved photocurrents

The charge transport in nanoporous ZnO was studied by laser flash induced photocurrent transients. The results are discussed using a diffusion model and compared with previous results on TiO₂. The charge transport was highly dependent on the potential giving apparent diffusion coefficients for the electron ranging from 1×10⁻⁴ to 1×10⁻⁶ cm²/s with an applied bias of +100 and +300 mV vs. Ag/AgCl in ethanol, respectively. The electrolyte was 0.5 M LiClO₄ in ethanol. The potential dependence was much more pronounced for ZnO than for TiO₂. The charge transport was also dependent on the electrolyte giving a linear dependence between the conductivity of the electrolyte and the apparent electronic diffusion coefficient. The dependence of the light intensity was also studied. Intensity-dependent losses were observed.     

Morphological, optical and photocatalytic properties of TiO2–Fe2O3 multilayers

Morphological, optical and photocatalytic properties of TiO₂, Fe₂O₃ and TiO₂–Fe₂O₃ samples (formed by 1, 3 and 5 coatings) were studied. The layers were deposited on glass substrate by the sol–gel method. The catalytic activity of the samples was studied by the photodecomposition of methylene blue (MB) under visible light illumination. The FTIR results indicate that all samples present surface OH radicals that are bound either to the Ti or Fe atoms. This effect is better visualized at larger number of coatings in the TiO₂–Fe₂O₃/glass systems. Also, two mechanisms are observed during the photodecomposition of the MB.     

Photoelectric behavior of nanocrystalline TiO2 electrode with a novel terpyridyl ruthenium complex

The photoelectric behavior of a black dye, tris (isothiocyanato)-[N-(2,2′:6′,2″-terpyridine-4′-(4-carboxylic acid) phenyl)] ruthenium (II) complex, was examined under different conditions. The dye was adsorbed on nanocrystalline TiO₂ surface strongly and generated incident monochromatic photon-to-current conversion efficiency (IPCE) of about 90% at maximum absorption wavelength and greater than 20% in the near-IR region. A sandwich-type solar cell fabricated by this dye-sensitized nanocrystalline TiO₂ film generated 6.1 mAcm⁻² of short-circuit photocurrent, 0.58 V of open-circuit photovoltage and 2.9% of overall yield under irradiation of white light (78.0 mWcm⁻²) from a Xe lamp. Since the title dye shows better photoresponse than the N3 dye in the near-IR region, it would be a promising panchromatic sensitizer after optimization.