Characterization of N, N′-bis-2-(1-hydoxy-4-methylpentyl)-3, 4, 9, 10-perylene bis (dicarboximide) sensitized nanocrystalline TiO2 solar cells with polythiophene hole conductors

We have fabricated solid-state, dye-sensitized nanocrystalline TiO₂ solar cells (DSSC) based on perylene derivative dye, N,N′-bis-2-(1-hydoxy-4-methylpentyl)-3,4,9,10-perylene bis (dicarboximide) (HMPER) with two different polythiophenes as hole conductors; i.e. poly (3-octyl thiophene) (P3OT) and poly (3-hexyl thiophene) (P3HT), respectively. HMPER adsorbs strongly to the surface of nanocrystalline TiO₂ and inject electrons into TiO₂ conduction band upon absorption of light. Polythiophene derivatives are well-known materials as hole conductors in solid-state dye-sensitized solar cells. We obtained quite similar results with P3OT and P3HT yielding a short-circuit current density of around 80 μA/cm² and open-circuit voltage of around 0.7 V at 80 mW/cm² AM 1.5 light intensity. The results are compared with Ru-535 TBA-sensitized nc-TiO₂ cells prepared by using the same polythiophene derivatives.     

Oxide/polymer interfaces for hybrid and organic solar cells: Anatase vs. Rutile TiO2

In this work, we study the effect of the transparent conducting oxide (TCO) and the polymer applied (MEH-PPV or P3HT) on the photovoltaic properties of TCO/TiO₂/polymer/Ag bi-layer solar cells. The solar cells were analyzed under inert atmosphere conditions resembling an encapsulated or sealed device. We demonstrate that the substrate applied, ITO or FTO, modifies the crystalline structure of the TiO₂: on an ITO substrate, TiO₂ is present in its anatase phase, on an FTO, the rutile phase predominates. Devices fabricated on an FTO, where the rutile phase is present, show better stability under inert atmospheres than devices fabricated on an ITO, anatase phase. With respect to the polymer, devices based on MEH-PPV show higher Voc (as high as 1 V), while the application of P3HT results in lower Voc, but higher J_sc and longer device stability. These observations have been associated to (a), the crystalline structure of TiO₂ and (b) to the form the polymer is bonded to the TiO₂ surface. In-situ IPCE analyses of P3HT-based solar cells show a red shift on the peak corresponding to TiO₂, which is not present on the MEH-PPV-based solar cells. The latter suggest that P3HT can be linked to the TiO₂ though the S-end atom, which results in devices with lower Voc. All these observations are also valid for devices, where the bare TiO₂ is replaced by an Nb-TiO₂. The application of an Nb-TiO₂ with rutile structure in these polymer/oxide solar cells is the reason for their higher stability under inert atmospheres. We conclude that the application of TiO₂ in its rutile phase is beneficial for long-term stability devices. Moreover there is an interplay between low Voc and J_sc in devices applying P3HT, since power conversion efficiency can be partially canceled by their lower Voc in comparison with MEH-PPV. These findings are important for polymer/oxide solar cells, but also for organic solar cells, where a layer of semiconductor oxides are in direct contact with a polymer, like in an inverted or tandem organic solar cells.     

The influence of surface morphology of TiO2 coating on the performance of dye-sensitized solar cells

The optimization of solar energy conversion efficiency of dye-sensitized solar cells (DSSCs) was investigated by the tuning of TiO₂ photoelectrode's surface morphology. Double-layered TiO₂ photoelectrodes with four different structures were designed by the coating of TiO₂ suspension, incorporated with low and high molecular weight poly(ethylene glycol) as a binder. Among these four systems, P2P1, where P1 and P2 correspond to the molecular weight of 20,000 and 200,000, respectively, showed the highest efficiency under the conditions of identical film thickness and constant irradiation. This can be explained by the larger pore size and higher surface area of P2P1 TiO₂ electrode than the other materials as revealed by scanning electron microscopic (SEM) and Brunauer–Emmett–Teller (BET) analyses. Electrochemical Impedance Spectroscopy (EIS) analysis shows that P2P1 formulation displayed a smaller resistance than the others at the TiO₂/electrolyte interface. The best efficiency (η) of 9.04% with the short-circuit photocurrent density (J_sc) and open-circuit voltage (V_oc) of 18.9 mA/cm² and 0.74 V, respectively, was obtained for a solar cell by introducing the light-scattering particles to the TiO₂ nanoparticles matrix coated on FTO electrode having the sheet resistivity of 8 Ω/sq.     

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.     

Efficient hybrid carboxylated polythiophene/nanocrystalline TiO2 heterojunction solar cells

Efficient hybrid solar cells fabricated from TiO₂, novel carboxylated polythiophene poly (3-thiophenemalonic acid) P3TMA as sensitizer as well as hole conductor and poly (3-hexylthiophene) (P3HT) as hole transporter was described. UV-Vis absorption and morphology of the active layer were investigated. Device J/V characterizations with different P3HT layer thickness were measured and discussed. Efficiency improvements were observed in thinner P3HT layer thickness and with poly[3,4-(ethylenedioxy)-thiophene]:poly(styrene sulfonate) (PEDOT:PSS) as charge collection layer, and such device showed a short-circuit current density of 1.32 mA/cm², an open-circuit voltage of 0.44 V, a fill factor of 0.43, and a energy conversion efficiency of 0.25% at A.M. 1.5 solar illumination (100 mW/cm²).     

Improvement in light harvesting and performance of P3HT:PCBM solar cell by using 9,10-diphenylanthracene

We have studied the effect of 9,10-diphenylanthracene (DPA) as a conjugated dye with different concentrations on light harvesting and performance of solar cell composed from poly (3-hexylthiophene) (P3HT):[6,6]-phenyl-C₆₁butyric acid methyl ester (PCBM) blend films. The dye concentration effect was investigated with optical absorption spectroscopy, photocurrent spectroscopy, and current density-voltage characteristic measurements on devices under AM1.5 white light illumination with intensity of 100 mW/cm². The incorporation of the conjugated DPA inside P3HT:PCBM blend improved the light harvesting, slightly, and conjugation length indicated from the optical absorption and external quantum efficiency spectra. By adding specific amounts of the DPA into P3HT:PCBM blend, the external quantum efficiency and solar cell performance parameters, i.e., short circuit current density, fill factor, and power conversion efficiency improved as a result of improvement in the light harvesting and charge carrier transfer taking place between P3HT and PCBM through the conjugated DPA molecules.     

P3HT/ZnO bulk-heterojunction solar cell sensitized by a perylene derivative

In this work, a soluble perylene-derivative dye, N, N′-didodecyl-3,4,9,10-perylene tetracarboxylic diimide (PDI), was used to improve the photovoltaic performance of poly(3-hexylthiophene) (P3HT)/ZnO bulk heterojunction cells through blending with the composite. Results show that by incorporation of PDI in the P3HT/ZnO composite, the light absorption and exciton separation can be significantly improved. The photocurrent under white-light irradiation can be increased from 6.35 to 9.55 mA/cm². Solar decay experiment shows that V_OC of the ITO/PEDOT:PSS/P3HT:ZnO:PDI/Al device decreases rapidly to almost zero in 1 h under persistent white-light illumination. After placing a 420 nm cutoff filter between the cell and the xenon lamp, the stability of the cell can be significantly improved. The device performance can maintain about 80% of the original value within 30 h and I_SC degraded to zero after 142 h. The addition of PDI into the P3HT/ZnO device up to 5 wt% does not show observable effect on the solar cell decay behavior.     

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.     

Preparation of semiconducting copolymer and its application in nanocrystalline TiO2 solar cells

Abstract

The past decade has witnessed increasing attention in the nanocrystalline TiO₂ solar cells (TSSCs). In this work, we have studied a novel TiO₂/PCBM/PPy solar cell based on blends of the semiconducting copolymer polypyrrole (PPy) and [6,6]-phenyl C₆₁ butyric acid methyl (PCBM) coated titanium dioxide (TiO₂) nanocrystal film to substitute the I₃^?/I^? redox electrolyte and the dye using in DSSCs. The research by incident photon to current efficiency spectra shows that the TiO₂ films had a stronger absorption in 300–500 nm light range. The performance of the resulting photovoltaic devices was investigated, and the effects of the PCBM/PPy ratio by photocurrent–voltage characteristics were researched. By the optimised PCBM/PPy ratio was 3∶1, The TSSC exhibited a short circuit current of 1·28 mA cm^?2, an open circuit voltage of 0·788 V, a fill factor of 0·654 and a light to electric energy conversion efficiency of 0·622% under a simulated solar light irradiation of 100 mW cm^?2.     

In2O3:Sn/TiO2/CdS heterojunction nanowire array photoanode in photoelectrochemical cells

The design of photoanode with highly efficient light harvesting and charge collection properties is important in photoelectrochemical (PEC) cell performance for hydrogen production. Here, we report the hierarchical In₂O₃:Sn/TiO₂/CdS heterojunction nanowire array photoanode (ITO/TiO₂/CdS-nanowire array photoanode) as it provides a short travel distance for charge carrier and long light absorption pathway by scattering effect. In addition, optical properties and device performance of the ITO/TiO₂/CdS-nanowire array photoanode were compared with the TiO₂ nanoparticle/CdS photoanode. The photocatalytic properties for water splitting were measured in the presence of sacrificial agent such as SO₃²⁻ and S²⁻ ions. Under illumination (AM 1.5G, 100 mW/cm²), ITO/TiO₂/CdS-nanowire array photoanode exhibits a photocurrent density of 8.36 mA/cm² at 0 V versus Ag/AgCl, which is four times higher than the TiO₂ nanoparticle/CdS photoanode. The maximum applied bias photon-to-current efficiency for the ITO/TiO₂/CdS-nanowire array and the TiO₂ nanoparticle/CdS photoanode were 3.33% and 2.09%, respectively. The improved light harvesting and the charge collection properties due to the increased light absorption pathway and reduced electron travel distance by ITO nanowire lead to enhancement of PEC performance.     

Field solar E. coli inactivation in the absence and presence of TiO2: is UV solar dose an appropriate parameter for standardization of water solar disinfection?

Evaluation of solar treatment in the absence and presence of TiO₂ has been made to assess its effectiveness in reducing bacterial load with respect to drinking water standards.Field experiments under direct solar radiation were carried out using a compound parabolic collector (CPC) placed at the Swiss Federal Institute of Technology (EPFL), Lausanne, Switzerland. Water contaminated with E. coli K12 was exposed to sunlight in different seasons. The obtained results indicate that the presence of TiO₂ accelerates the detrimental action of light. Total photocatalytic disinfection was obtained in both periods of year and no bacterial recovery was observed during 24 h after stopping sunlight exposure. In the absence of TiO₂, total disinfection was not always reached; and bacterial recovery was observed, especially when inactivation was not complete. Bacterial decay was mainly dependent on light intensity. It was also demonstrated that solar UV dose is not a pertinent parameter to standardize solar disinfection. The influence of the following topics on solar water disinfection is also studied in this paper: (a) UV and total solar spectra characteristics (b) volume of phototreated water (c) post-irradiation events.     

TiO2 thin films as protective material for transparent-conducting oxides used in Si thin film solar cells

Nb-doped TiO₂ films have been fabricated by RF magnetron sputtering as protective material for transparent-conducting oxide (TCO) films used in Si thin film solar cells. It is found that TiO₂ has higher resistance against hydrogen radical exposure, utilizing the hot-wire CVD (catalytic CVD) apparatus, compared with SnO₂ and ZnO. Further, the minimum thickness of TiO₂ film as protective material for TCO was experimentally investigated. Electrical conductivity of TiO₂ in the as-deposited film is found to be 10⁻⁶ S/cm due to the Nb doping. Higher conductivity of 10⁻² S/cm is achieved in thermally annealed films. Nitrogen treatments of Nb-doped TiO₂ film have been also performed for improvements of optical and electric properties of the film. The electrical conductivity becomes 4.5×10⁻² S/cm by N₂ annealing of TiO₂ films at 500 °C for 30 min. It is found that the refractive index n of Nb-doped TiO₂ films can be controlled by nitrogen doping (from n=2.2 to 2.5 at λ = 550 nm) using N₂ as a reactive gas. The controllability of n implies a better optical matching at the TCO/p-layer interface in Si thin film solar cells.     

The effect of temperature on the charge transport and transient absorption properties of K27 sensitized DSSC

The charge transport and transient absorption properties of K27 dye-sensitized solar cell have been investigated. The current–voltage (I–V) characteristics of the solar cell were analyzed by the thermionic emission theory. The ideality factor, barrier height and series resistance values of the solar cell were determined. The ideality factor higher than unity indicated the presence of non-ideal behavior in current–voltage characteristics at lower voltages. At the higher voltages, the charge transport mechanism for the solar cell is controlled by a space-charge limited current (SCLC) with an exponential distribution of traps. The built potential values are determined from capacitance–voltage plot and were found to be 0.14 and 0.58 V, respectively. The transient absorption data of K27 DSSC device suggest that the fast and slow phases are taking place. While the fast phase corresponds to regeneration of the dye cation by the iodide redox couple, the slow phase corresponds to the decay of long-lived I₂⁻/ TiO₂ electron absorption. The best conversion efficiency for K27 DSSC was found to be 0.317% under 100 mW/cm² (FF=0.584, V_oc=480 mV, I_sc=1.131 mA). The photocurrent results indicate that the photogeneration of charge carriers is a monophotonic process.     

Cu2O/Cu/TiO2 nanotube Ohmic heterojunction arrays with enhanced photocatalytic hydrogen production activity

Cu₂O/Cu/TiO₂ nanotube heterojunction arrays were prepared by assembling Cu@Cu₂O core-shell nanoparticles on TiO₂ nanotube arrays (NTAs) using a facile impregnation-reduction method. SEM and TEM results show that Cu@Cu₂O plate-like nanoparticles with tens of nanometers in size are confined inside TiO₂ NTAs. Only the outmost several nanometers of the nanoparticles are Cu₂O and the predominant inner of the nanoparticles are Cu metals. Cu L₃VV Auger spectra of Cu₂O/Cu/TiO₂ NTAs suggest that Cu metals are enveloped by at least several nanometers Cu₂O on the surface, which further confirms the Cu@Cu₂O core shell structure of Cu nanoparticles. The ability of light absorption of Cu₂O/Cu/TiO₂ NTAs is enhanced. The range of absorption wavelengths changes from 400 to 700 nm due to the surface plasmon response of Cu metals core and Cu₂O nanoparticles shell. The photocatalytic hydrogen production rate of Cu₂O/Cu/TiO₂ heterojunction arrays is enhanced when compared with those of Cu₂O/TiO₂ NTAs and TiO₂ NTAs under UV light. Moreover, a stable H₂ generation property was obtained under visible light (λ gt; 400 nm). The Cu metal core is believed to play a key role in the enhancement of photocatalytic properties of Cu₂O/Cu/TiO₂ nanotube heterojunction arrays.     

Poly(3-hexylthiophene)/C60 heterojunction solar cells: Implication of morphology on performance and ambipolar charge collection

The performance of heterojunction organic solar cells is critically dependent on the morphology of the donor and acceptor components in the active film. We report results of photovoltaic devices consisting of bilayers and bulk heterojunctions using poly(3-hexylthiophene) (P3HT) and Buckminsterfullerene C₆₀. White light power efficiencies of η2.2% (bulk heterojunction) and 2.6% (bilayer) were measured after a thermal annealing step on completed devices. Optical and structural investigations on non-annealed bilayer thin films indicated a distinct porosity of the spin-coated polymer, which allows C₆₀ to penetrate the P3HT layer and to touch the anode. This resulted for these bilayer solar cells in the experimental observation that electrons were collected predominantly at the cathode after photo-excitation of P3HT, but predominantly at the anode after C₆₀ excitation. A morphological model to explain the ambipolar charge collection phenomenon is proposed.     

A technique to compare polythiophene solid-state dye sensitized TiO2 solar cells to liquid junction devices

In this communication, we report on a technique to fabricate solid-state polythiophene-based dye sensitized solar cells (DSSCs) that can be directly compared to analogous liquid junction devices. The device configuration is based on non-porous TiO₂ thin films and one of the three undoped polythiophene hole conductors: poly[3-(11 diethylphosphorylundecyl) thiophene], P3PUT, poly(4-undecyl-2,2′-bithiophene), P4UBT, or poly(3-undecyl-2,2′-bithiophene), P3UBT. These polymers were spin coated and cast from organic solutions onto the TiO₂ films. The dense TiO₂ thin films (ca. 30 nm) were deposited on conductive glass via facile spray pyrolysis and sol–gel techniques. After that, cis-(SCN)₂ Bis(2,2′ bipyridyl-4,4′-dicarboxylate) ruthenium(II) (a.k.a. Ru N3 dye) was adsorbed on the TiO₂ surface, and the polythiophenes were utilized as hole conductors in a simplified solar cell geometry. The results were compared to the control DSSC device made with dense TiO₂ and a liquid electrolyte, or 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (a.k.a. Spiro-MeOTAD). The polythiophenes exhibited bandgaps in the range 1.9–2.0 eV, and HOMO energy levels of approximately 5 eV (vs. vacuum). The P3PUT DSSC device exhibited an AM1.5 V_OC=0.8 V, a J_SC=0.1 mA/cm², as well as an IPCE=0.5–1%. The AM1.5 short-circuit photocurrents and quantum efficiencies for DSSCs made with the polythiophenes, the Spiro-MeOTAD and the standard liquid electrolyte (I⁻/I₃⁻) were found to be identical within the limits of experimental uncertainty and reproducibility. Our results indicate that a solid-state replacement to the liquid junction is not necessarily limited by the fundamental aspect of hole transfer, one of the three fundamental aspects that must be met for an efficient DSSC. Rather than suggest that P3UBT or P4UBT could be used to create efficient “organic solar cells” with the exclusion of the Ru dye, we suggest that transparent thiophene compounds could be attractive candidates for high-surface area solid-state DSSCs, and that the technique presented can be applied to other hole conductors. It can allow a verification of one of the things necessary for the DSSC, so that parallel studies using high-surface area materials can proceed with confidence.     

Influence of electrolyte on the photovoltaic performance of a dye-sensitized TiO2 solar cell based on a Ru(II) terpyridyl complex photosensitizer

We have investigated the influence of electrolyte composition on the photovoltaic performance of a dye-sensitized nanocrystalline TiO₂ solar cell (DSSC) based on a Ru(II) terpyridyl complex photosensitizer (the black dye). We have also spectroscopically investigated the interaction between the electrolyte components and the adsorbed dye. The absorption peaks attributed to the metal-to-ligand charge transfer transitions of the black dye in solution and adsorbed on a TiO₂ film, were red-shifted in the presence of Li cations, which led to an expansion of the spectral response of the solar cell toward the near-IR region. The photovoltaic performance of the DSSC based on the black dye depended remarkably on the electrolyte composition. We developed a novel efficient organic liquid electrolyte containing an imidazolium iodide such as 1,2-dimethyl-3-n-propylimidazolium iodide or 1-ethyl-3-methylimidazolium iodide (EMImI) for a DSSC based on the black dye. A high solar energy-to-electricity conversion efficiency of 9.2% (J_sc=19.0 mA cm⁻², V_oc=0.67 V, and FF=0.72) was attained under AM 1.5 irradiation (100 mW cm⁻²) using a novel electrolyte consisting of 1.5 M EMImI, 0.05 M iodine, and acetonitrile as a solvent with an antireflection film.     

Photocatalytic splitting of seawater and degradation of methylene blue on CuO/nano TiO2

The main objective of this study was to prepare effective photocatalysts for splitting of seawater for solar fuel – H₂ and degradation of seawater organic pollutants such as dyes. To enhance photocatalytic activities, CuO is supported on nano TiO₂ (CuO/nano TiO₂). By X-ray absorption near edge structure (XANES) spectroscopy, CuO clusters are found on nano TiO₂. The 2.5% CuO/nano TiO₂ has greater activities in photocatalytic splitting of water and seawater than nano TiO₂ by 9.9 and 7.8 times, respectively. Interestingly, the 2.5% CuO/nano TiO₂ is also very active for photocatalytic splitting of water and seawater contaminated with dyes such as methylene blue (MB) (10 ppm). Under a 5-h irradiation of the UV–Vis light, about 99% of MB is degraded while 3.1 μmol/h g cat of H₂ are generated from seawater in the photocatalysis process.     

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.     

Cobalt doped TiO2: A stable and efficient photocatalyst for continuous hydrogen production from glycerol: Water mixtures under solar light irradiation

Glycerol is the main by-product during the trans-esterification of vegetable oils to biodiesel. In this study, we investigate the process of photocatalytic hydrogen production from glycerol aqueous solution, with the use of cobalt doped TiO₂ photocatalyst under solar light irradiation. Cobalt doped TiO₂ photocatalysts are prepared by impregnation method and these catalysts are characterized by XRD, EDAX, DRS, TEM, EPR and XPS techniques. DRS studies clearly show the expanded photo response of TiO₂ into visible region on impregnation of Co²⁺ ions on surface of TiO₂. XPS studies also show change in the binding energy values of O1s, Ti 2p and Co 2p, indicating that Co²⁺ ions are in interaction with TiO₂. Maximum hydrogen production of 220 μ mol h⁻¹ g⁻¹ is observed on 2 wt% cobalt doped TiO₂ catalysts in pure water under solar irradiation. A significant improvement in hydrogen production is observed in glycerol: water mixtures; and maximum hydrogen production of 11,021 μ mol h⁻¹ g⁻¹ is obtained over 1 wt% cobalt doped TiO₂ in 5% glycerol aqueous solutions. Furthermore, to evaluate some reaction parameters such as cobalt wt% on TiO₂, glycerol concentration, substrate effect (alcohols) and pH of the solution on the hydrogen production activity are systematically investigated. When the catalysts are examined under UV irradiation, a 3–4 fold increase in activity is observed where this activity seems to decrease with time; however, a continuous activity is observed under solar irradiation on these catalysts. The decreased activity could be ascribed the loss of cobalt ions under UV irradiation, as evidenced by EDAX and TEM analysis. A possible explanation for the stable and continuous activity of cobalt doped TiO₂ photocatalysts under solar irradiation is proposed.