Dye-sensitized solar cells using TiO<Subscript>2</Subscript> nanoparticles transformed from nanotube arrays

Dye-sensitized solar cells (DSSCs) were fabricated using TiO₂ mesoporous layers obtained by very simple method—transformation of TiO₂ nanotube (NT) films grown by electrochemical oxidation to nanoparticle (NP) films. This transformation is based on thermal annealing of TiO₂ NT arrays formed by anodization of titanium foil in fluorine ambient. Performance of DSSCs fabricated with different size NPs was studied in the range from 35 to 350 nm. Highest nominal efficiency (9.05%) was achieved for DSSC with NP size 65 nm while the lowest nominal efficiency (1.48%) was observed for DSSCs with NP size 350 nm. The dependence of the solar cell parameters with NP size is discussed.     

Influence of the size-controlled TiO<Subscript>2</Subscript> nanotubes fabricated by low-temperature chemical synthesis on the dye-sensitized solar cell properties

Titanium dioxide nanotubes (TiO₂ NTs) with various sizes have been prepared by low-temperature chemical synthesis using commercial anatase TiO₂ particles with different crystallite size in NaOH solution and used as a photoelectrode in a dye-sensitized solar cell (DSSC). The relationship between the physicochemical properties of electrode materials and photovoltaic performance was investigated. The electrodes made from modified TiO₂ NTs showed a strong dependency on their specific surface area and resultant amount of dye adsorption; the surface area decreased with increase in the diameter of the NT from 9.8 to 23.6 nm. The conversion efficiency of the cell made from TiO₂ NT, 12.9 nm in diameter, was enhanced by 12% compared to that of the smallest NT. These results suggested that the photovoltaic performance improved by the suppression of photogenerated charge recombination in spite of a 25.3% reduction in the specific surface area. In addition, larger TiO₂ NTs could be utilized as a scattering layer on the top of the TiO₂ nanoparticulate working electrode. It was observed that this controlled TiO₂ photoelectrode architecture exhibited enhanced conversion efficiency without TiCl₄ treatment.     

Enhanced efficiency and improved photocatalytic activity of 1 : 1 composite mixture of TiO<Subscript>2</Subscript> nanoparticles and nanotubes in dye-sensitized solar cell

TiO₂-based nanotubes (NTs), nanoparticles (NPs) and composite structural film (50% NP + 50% NT film) were synthesized by sol-gel hydrothermal process. Synthetic indigo dye was used as a sensitizer with the unique combination of electrolyte (EMII + BMII + PMII) and with cobalt sulphide as counter electrode. The structure and morphology of the three films, namely, NP, NT and NPNT is studied through X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The absorption spectra and incident photon-to-current conversion efficiency (IPCE) of the three films were compared and found to be higher for NPNT film. The efficiency and photocatalytic activity of three films were evaluated. The composite structure showed improved efficiency (1.72%) than NP (1%) and NT films (0.78%). The photocatalytic activity of the three films were measured using organic dye, methylene blue under UV light radiation. The composite structure showed higher dye absorption and higher rate of reaction with time. This paper certainly proves that there are many rooms to focus on the photoanode configuration, which plays a key role to improve the efficiency of dye-sensitized solar cell (DSSC).     

Enhanced photovoltaic performance of solar cell based on front-side illuminated CdSe/CdS double-sensitized TiO2 nanotube arrays electrode

Free-standing TiO₂ nanotube (NT) arrays have been prepared by a two-step anodization method. These translucent TiO₂ NT arrays can be transferred to the fluorine-doped tin oxide glass substrates to form front-side illuminated TiO₂ NT electrodes. The TiO₂ NT electrodes were double-sensitized by CdSe/CdS quantum dots (QDs) through successive ionic layer adsorption and reaction (SILAR) process. The absorption range of the TiO₂ NT electrode was extended from ~380 to 700 nm after sensitization with CdSe/CdS QDs. The SILAR cycles were investigated to find out the best combination of CdS and CdSe QDs for photovoltaic performance. The power conversion efficiency of 2.42 % was achieved by the CdSe(10)/CdS(8)/TiO₂ NT solar cell. A further improved efficiency of 2.57 % was obtained with two cycles of ZnS overlayer on the CdSe(10)/CdS(8)/TiO₂ NT electrode, which is 45.19 % higher than that of back-side illuminated solar cell. Furthermore, the ZnS(2)/CdSe(10)/CdS(8)/TiO₂ NT solar cell possesses a higher stability than CdSe(10)/CdS(8)/TiO₂ NT solar cell during the same period. The better photovoltaic performance of the ZnS(2)/CdSe(10)/CdS(8)/TiO₂ NT solar cell has demonstrated the promising value to design quantum dots-sensitized solar cells with double-sensitized front-side illuminated TiO₂ NT arrays strategy.     

Photocatalytic reduction of CO<Subscript>2</Subscript> with H<Subscript>2</Subscript>O over modified TiO<Subscript>2</Subscript> nanofibers: Understanding the reduction pathway

Nanosized metal (Pt or Pd)-decorated TiO₂ nanofibers (NFs) were synthesized by a wet impregnation method. CdSe quantum dots (QDs) were then anchored onto the metal-decorated TiO₂ NFs. The photocatalytic performance of these catalysts was tested for activation and reduction of CO₂ under UV-B light. Gas chromatographic analysis indicated the formation of methanol, formic acid, and methyl formate as the primary products. In the absence of CdSe QDs, Pd-decorated TiO₂ NFs were found to exhibit enhanced performance compared to Pt-decorated TiO₂ NFs for methanol production. However, in the presence of CdSe, Pt-decorated TiO₂ NFs exhibited higher selectivity for methanol, typically producing ～90 ppmg^?1·h^?1 methanol. The CO₂ photoreduction mechanism is proposed to take place via a hydrogenation pathway from first principles calculations, which complement the experimental observations.

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Preparation and characterization of photocatalytic TiO<Subscript>2</Subscript> films on functionalized stainless steel

In this study, TiO₂ was synthesized using controlled hydrolysis of TiCl₄, followed by dialysis. We produced a transparent emulsion that was later dried into TiO₂ powder. TiO₂ photocatalyst films were deposited applying the technique of pulsed electrophoretic deposition, which decreased bubble formation caused by direct current. The substrates were bare stainless steel (SS) and stainless steel pre-functionalized in a conversion bath. Film surface morphology, crystallinity, elemental composition, and wettability were determined using XRD, SEM–EDAX, and contact angle measurements. The mechanical properties were determined by nano-indentation test. The adhesion was investigated using scratch test. The obtained results showed that the TiO₂ film over a conversion layer had better adhesion and mechanical properties than TiO₂ over bare SS. The optical characteristics of TiO₂ films were tested using PL measurement. The photocatalytic decolourization of the amido black-10B dye was studied over TiO₂ coating under UV and visible light irradiation. TiO₂ film over a conversion layer without heat treatment exhibited the best photocatalytic activity as a result of its crystalline size and three-phase structure as well as the synergetic effect of TiO₂ and Fe₂O₃.     

Tuning photocurrent response through size control of CdSe quantum dots sensitized solar cells

The photovoltaic characterization of CdSe quantum dots sensitized solar cells (QDSSCs) by tuning band gap of CdSe quantum dots (QDs) through size control has been investigated. Fluorine doped tin oxide (FTO) substrates were coated with 20 nm in diameter TiO₂ nanoparticles (NPs). Pre-synthesized colloidal CdSe quantum dots of different sizes (from 4.0 to 5.4 nm) were deposited on the TiO₂-coated substrates using direct adsorption (DA) method. The FTO counter electrodes were coated with platinum, while the electrolyte containing I^?/I ₃ ^? redox species was sandwiched between the two electrodes. The current density-voltage (J-V) characteristic curves of the assembled QDSSCs were measured for different dipping times, and AM 1.5 simulated sunlight. The maximum values of short circuit current density (J_sc) and conversion efficiency (η) are 1.62 mA/cm² and 0.29 % respectively, corresponding to CdSe QDs of size 4.52 nm (542 nm absorption edge) and of 6 h dipping time. The variation of the CdSe QDs size mainly tunes the alignment of the conduction band minimum of CdSe with respect to that of TiO₂ surface. Furthermore, the J_sc increases linearly with increasing intensity of the sun light, which indicates the sensitivity of the assembled cells.     

SiO<Subscript>2</Subscript>/TiO<Subscript>2</Subscript> fibers from titanium-modified polycarbosilane

We developed a process for preparing SiO₂/TiO₂ fibers by means of precursor transformation method. After mixing PCS and titanium alkoxide, continuous SiO₂/TiO₂ fibers were fabricated by the thermal decomposition of titanium-modified PCS (PTC) precursor. The tensile strength and diameter of SiO₂/TiO₂ fibers are 2.0 GPa, 13 μm, respectively. Based on X-ray diffraction (XRD), scanning electron microscopy (SEM), and high resolution transmission electron microscopy (HRTEM) measurements, the microstructure of the SiO₂/TiO₂ fibers is described as anatase–TiO₂ nanocrystallites with the mean size of ~10 nm embedded in an amorphous silica continuous phase.     

Dye sensitized solar cell efficiency improvement using TiO<Subscript>2</Subscript>/nanodiamond nano composite

This paper aims to demonstrate the efficiency and recombination improvement of Dye-sensitized solar cells (DSSCs) by introducing of a Nanodiamond (NDs)-TiO₂ nano composite. The main challenge in the proposed application is to find the optimal wt.% of ND in TiO₂. The experimental tests were conducted to compare the developed NDs/TiO₂ cell with one of pure TiO₂ nanoparticles prepared in the same conditions. It was observed that short circuit current density, power conversion efficiency, fill factor and electron life time enhanced with increasing ND content. The best performance was obtained with 1 wt.% ND content; with a current density of 12.11 mA/cm² and light-to-electricity conversion efficiency of 4.95%. The improvement in efficiency of 18.7% was obtained as the standard DSSC was compared with that of pure TiO₂.     

One-dimension carbon self-doping g-C<Subscript>3</Subscript>N<Subscript>4</Subscript> nanotubes: Synthesis and application in dye-sensitized solar cells

One-dimension carbon self-doping g-C₃N₄ nanotubes (CNT) with abundant communicating pores were synthesized via thermal polymerization of saturated or supersaturated urea inside the framework of a melamine sponge for the first time. A ～16% improvement in photoelectric conversion efficiency (η) is observed for the devices fabricated with a binary hybrid composite of the obtained CNT and TiO₂ compared to pure TiO₂ device. The result of EIS analysis reveals that the interfacial resistance of the TiO₂-dye|I₃^?/I^? electrolyte interface of TiO₂-CNT composite cell is much lower than that of pure TiO₂ cell. In addition, the TiO₂-CNT composite cell exhibits longer electron recombination time, shorter electron transport time, and higher charge collection efficiency than those of pure TiO₂ cell. Systematic investigations reveal that the CNT boosts the light harvesting ability of the photovoltaic devices by enhancing not only the visible light absorption but also the charge separation and transfer.

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Synthesis and characterization of polythiophene-modified TiO<Subscript>2</Subscript> nanotube arrays

The highly ordered and uniform TiO₂ nanotube arrays were fabricated by anodic oxidation method and PTh(polythiophene)/TiO₂ nanotube arrays electrode were obtained by electrochemical polymerization. X-ray powder diffraction (XRD) analysis confirmed the formation of TiO₂ phase. The morphologies and optical characteristics of the TiO₂ nanotube arrays were studied by scanning electron microscope (SEM), UV-Vis absorption spectra and Raman spectra. The results demonstrate that the PTh/TiO₂ electrode could enlarge the visible light absorption region and increase the photocurrent in visible region. The modified TiO₂ electrode with light-to-electric energy conversion efficiency of 1·46%, the short-circuit current density of 4·52 mAcm^− 2, open-circuit voltage of 0·74 V and fill factor of 0·44, were obtained.     

Fabrication of dye-sensitized solar cells with multilayer photoanodes of hydrothermally grown TiO<Subscript>2</Subscript> nanocrystals and P25 TiO<Subscript>2</Subscript> nanoparticles

TiO₂ nanocrystals (NCs) with sizes around 20 nm were synthesized by hydrothermal method in acidic autoclaving pH. The hydrothermally grown TiO₂ NCs and P25 TiO₂ nanoparticles (NPs) were used in the preparation of two different pastes using different procedures. These pastes with different characteristics were separately deposited on FTO glass plates to form multilayer photoanodes of the dye-sensitized solar cells. The aim of this study was to search how a thin sub-layer of the hydrothermally grown TiO₂ NCs in the photoanodes could improve the efficiency of TiO₂ P25-based solar cells. The highest efficiency of 6.5% was achieved for a cell with a photoanode composed of one transparent sub-layer of hydrothermally grown TiO₂ NCs and two over-layers of P25 NPs. Higher energy conversion efficiencies were also attainable using two transparent sub-layers of hydrothermally grown TiO₂ NCs. In this case, an efficiency of 7.2% was achieved for a cell with a photoelectrode made of one over-layer of P25 TiO₂ NPs. This could show an increase of about 30% in the efficiency compared to the similar cell with a photoanode made of two layers of hydrothermally grown TiO₂ NCs.     

Enhanced visible-light photoelectrochemical activity of TiO<Subscript>2</Subscript> nanorod arrays decorated by Sb<Subscript>2</Subscript>S<Subscript>3</Subscript> particles

Single-crystal TiO₂ nanorod arrays (NRAs) were synthesized successfully onto transparent conducting substrates through a facile hydrothermal route for photo-electrochemical (PEC) water-splitting. Further, the ordered TiO₂ NRAs were treated by SbCl₃ solution for preparing a layer of Sb₂S₃ and enhancing their PEC activity. A series of methods were employed to compare and analyze the differences between the TiO₂ NRA samples with/without Sb₂S₃ decoration. It was demonstrated that the PEC performance were enhanced significantly after being treated with SbCl₃ aqueous solution (precursor) under a hydrothermal environment. Compared to pure TiO₂ NRAs, the sample that decorated with Sb₂S₃ showed a more positive flat-band level as well as a higher donor density (i.e. electron density). Thus, it suggested that the enhanced PEC performance after Sb₂S₃ modification might be attributed to the widening of spectral response as well as the improvement of charge transfer / transport occurring at the solid/liquid interfaces.     

Photocatalytic CO<Subscript>2</Subscript> conversion over Au/TiO<Subscript>2</Subscript> nanostructures for dynamic production of clean fuels in a monolith photoreactor

Global economic development intensifies the consumption of fossil fuels which results in increase of carbon dioxide (CO₂) concentration in the atmosphere. The technologies for carbon capture and utilization to produce cleaner fuels are of great significance. However, phototechnology provides one perspective for economical CO₂ conversion to cleaner fuels. In this study, CO₂ conversion with H₂ to selective fuels over Au/TiO₂ nanostructures using environment friendly continuous monolith photoreactor has been investigated. Crystalline nanoparticles of anatase TiO₂ were obtained in the Au-doped TiO₂ samples. The Au deposited over TiO₂ in metal state produced plasmonic resonance. CO₂ was efficiently converted to CO as the main product over Au/TiO₂ with a maximum yield rate of 4144 µmol g-catal.^?1 h^?1, 345 fold-higher than using un-doped TiO₂ catalyst. The significantly enhanced photoactivity of Au/TiO₂ catalyst was due to hindered charges recombination rate and Au metallic-interband transition. The photon energy in the UV range was high enough to excite the d-band electronic transition in the Au to produce CO, CH₄, and C₂H₆. The quantum efficiency over Au/TiO₂ catalyst for CO was considerably improved in the continuous monolith photoreactor. At higher space velocity, the yield rates of CO gradually reduced, but the initial rates of hydrocarbon yields increased. The stability of the recycled Au/TiO₂ catalyst was sustained in cyclic runs. Thus, Au-doped TiO₂ supported over monolith channels is promising for enhanced CO₂ photoreduction to high energy products. This provides pathway that phototechnology to be explored further for cleaner and economical fuels production.     

Enhanced electrochemical performance of Na<Subscript>3</Subscript>V<Subscript>2</Subscript>(PO<Subscript>4</Subscript>)<Subscript>2</Subscript>F<Subscript>3</Subscript> for Na-ion batteries with nanostructure and carbon coating

Carbon-coating Na₃V₂(PO₄)₂F₃ nanoparticles (NVPF@C NP) were prepared by a hydrothermal assisted sol–gel method and applied as cathode materials for Na-ion batteries. The as-prepared nanocomposites were composed of Na₃V₂(PO₄)₂F₃ nanoparticles with a typical size of ~?100 nm and an amorphous carbon layer with the thickness of ~?5 nm. Cyclic voltammetry, rate and cycling, and electrochemical impedance spectroscopy tests were used to discuss the effect of carbon coating and nanostructure. Results display that the as-prepared NVPF@C NP demonstrates a higher rate capability and better long cycling performance compared with bare Na₃V₂(PO₄)₂F₃ bulk (72 mA h g^?1 at 10 C vs 39 mA h g^?1 at 10 and 1 C capacity retention of 95% vs 88% after 50 cycles). The remarking electrode performance was attributed to the combination of nanostructure and carbon coating, which can provide short Na-ion diffusion distance and rapid electron migration.     

Ag<Subscript>2</Subscript>S/Bi<Subscript>2</Subscript>S<Subscript>3</Subscript> co-sensitized TiO<Subscript>2</Subscript> nanorod arrays prepared on conductive glass as a photoanode for solar cells

TiO₂ nanorod arrays (TiO₂ NRAs) were synthesized through a hydrothermal method. Ag₂S and Bi₂S₃ were then grown on the surface of TiO₂ NRAs with successive ionic layer adsorption and reaction method. The pristine rutile TiO₂ NRAs, Ag₂S/TiO₂, Bi₂S₃/TiO₂, and Bi₂S₃/Ag₂S/TiO₂ electrodes were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, ultraviolet–visible absorption spectroscopy, and electrochemical analysis. According to photoelectrochemical (PEC) measurement, an enhanced short circuit current density was obtained for the co-sensitized TiO₂ NRAs under simulated sunlight illumination, which was 10.7 times higher than that of the TiO₂ NRAs. Appropriate potential positions of conduction band and valence band of Bi₂S₃ that match well those of rutile TiO₂ NARs and Ag₂S lead to the improved PEC performance. In addition, the PEC property of the co-sensitized TiO₂ NRAs under visible light irradiation was also investigated and showed a dramatically enhanced photocurrent response.     

Hydrothermal Growth of TiO2 Nanorod Arrays and In Situ Conversion to Nanotube Arrays for Highly Efficient Quantum Dot‐Sensitized Solar Cells

TiO₂ nanorod (NR) and nanotube (NT) arrays grown on transparent conductive substrates are attractive electrode for solar cells. In this paper, TiO₂ NR arrays are hydrothermally grown on FTO substrate, and are in situ converted into NT arrays by hydrothermally etching. The TiO₂ NR arrays are reported as single crystalline, but the TiO₂ NR arrays are demonstrated to be polycrystalline with a bundle of 2–5 nm single crystalline nanocolumns grown along [001] throughout the whole NR from bottom to top. TiO₂ NRs can be converted to NTs by hydrothermal selective etching of the (001) core and remaining the inert sidewall of (110) face. A growth mechanism of the NR and NT arrays is proposed. Quantum dot‐sensitized solar cells (QDSCs) are fabricated by coating CdSe QDs on to the TiO₂ arrays. After conversion from NRs to NTs, more QDs can be filled in the NTs and the energy conversion efficiency of the QDSCs almost double.     

Preparation and photoelectrochemical characterization of WO<Subscript>3</Subscript>/TiO<Subscript>2</Subscript> nanotube array electrode

WO₃/TiO₂ nanotube array electrode was fabricated by incorporating WO₃ with TiO₂ nanotube array via a wet impregnation method using ammonium tungstate as the precursor. TiO₂ and WO₃/TiO₂ nanotube arrays were characterized by field emission scanning electron microscopy, X-ray diffraction, and energy dispersive X-ray analysis. In order to characterize the photoelectrochemical properties of WO₃/TiO₂ electrode, electrochemical impedance spectroscopy, and steady-state photocurrent (i _ss) measurement at a controlled potential were performed in the supporting electrolyte containing different concentrations of glucose. The photoelectrochemical characterization results reveal that WO₃/TiO₂ nanotube array electrode possesses a much higher separation efficiency of the photogenerated electron–hole pairs and could generate more photoholes on the electrode surface compared with the pure TiO₂ nanotube array electrode. The i _ss for glucose oxidation at WO₃/TiO₂ nanotube array electrode is much higher than that at the pure TiO₂ nanotube array electrode.     

General strategy for embedding high quality Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> quantum dots and ZnS:Mn<Superscript>2+</Superscript> quantum dots in a silica matrix

In this paper, ZnS:Mn²⁺ quantum dots (QDs) Fe₃O₄ quantum dots (QDs)/SiO₂ nanocomposites were successfully synthesized by reverse microemulsion method. The average diameter of ZnS:Mn²⁺ QDs, Fe₃O₄ QDs and ZnS:Mn²⁺ QDs Fe₃O₄ QDs/SiO₂ nanocomposites was about 5.8, 9 and 29 nm, respectively. As the mass ratio of ZnS:Mn²⁺ to Fe₃O₄ QDs increased from 2.5:4 to 7.5:4, the intensity of the yellow–orange emission coming from Mn²⁺ ions was increased. The superparamagnetic property of ZnS:Mn²⁺ QDs Fe₃O₄ QDs/SiO₂ nanocomposites was observed at room temperature, and the saturation magnetization was decreased as the amount of ZnS:Mn²⁺ QDs increased.     

Rutile TiO<Subscript>2</Subscript> films as electron transport layer in inverted organic solar cell

Titanium dioxide (TiO₂) thin films were prepared by sol–gel spin coating method and deposited on ITO-coated glass substrates. The effects of different heat treatment annealing temperatures on the phase composition of TiO₂ films and its effect on the optical band gap, morphological, structural as well as using these layers in P3HT:PCBM-based organic solar cell were examined. The results show the presence of rutile phases in the TiO₂ films which were heat-treated for 2 h at different temperatures (200, 300, 400, 500 and 600 °C). The optical properties of the TiO₂ films have altered by temperature with a slight decrease in the transmittance intensity in the visible region with increasing the temperature. The optical band gap values were found to be in the range of 3.28–3.59 eV for the forbidden direct electronic transition and 3.40–3.79 eV for the allowed direct transition. TiO₂ layers were used as electron transport layer in inverted organic solar cells and resulted in a power conversion efficiency of 1.59% with short circuit current density of 6.64 mA cm^?2 for TiO₂ layer heat-treated at 600 °C.