Novel construction of CdS-encapsulated TiO2 nano test tubes corked with ZnO nanorods

A novel TiO₂ nanotube array/CdS nanoparticle/ZnO nanorod (TiO₂ NT/CdS/ZnO NR) photocatalyst was constructed by chemical assembling CdS into the TiO₂ NTs, and then laying ZnO NRs on the surface. The SEM results showed that the TiO₂ NTs looked like many “nano test tubes” and the ZnO NRs served as the corks to seal the nozzle. This photocatalyst exhibited a wide absorption range (200-535 nm) in both ultraviolet and visible regions (UV-vis region), and maintained very high photoelectrocatalytic (PEC) activities. The maximum photoelectric conversion efficiencies (η) of TiO₂ NT/CdS/ZnO NRs are 31.8 and 5.98% under UV light (365 nm) and visible light (420-800 nm), respectively.     

Formation of aligned ZnO nanotube arrays by chemical etching and coupling with CdSe for photovoltaic application

High density aligned ZnO nanotube (NT) arrays were synthesized using a facile chemical etching of electrochemically deposited ZnO nanorods (NRs). The influence of etching time and solution concentration on the ZnO NT formation was investigated. Moreover, cadmium selenide (CdSe) nanoparticles as sensitizers were assembled onto the ZnO NT and NR arrays for solar cell application. A conversion efficiency (η) of 0.44% was achieved for CdSe/ZnO NT-based solar cell under the white light illumination intensity of 85 mW/cm². An 8% enhancement in η was observed between the CdSe/ZnO NT-based and NR-based solar cell due to the enhancement of the photocurrent density. ZnO NT arrays have been proved to have a superior ability as compared with ZnO NR arrays when employed as a semiconductor film.     

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.     

Fabrication of periodically aligned vertical single-crystalline anatase TiO2 nanotubes with perfect hexagonal open-ends using chemical capping materials

A vertically aligned anatase TiO2 （A-TiO2） nanotube array has been fabricated by coating a ZnO nanorod （NR） template with a TiO2 precursor solution. After coating, the ZnO NR cores were selectively etched in an acidic environment to form TiO2 nanotubes （NTs）. More specifically, after growing the ZnO NRs via a hydrothermal method, one drop of the TiO2 precursor solution was cast to coat the ZnO NRs, the tops of which were previously covered with chemical capping materials by electrostatic interaction, and then the sample was sintered. Finally, the sample was immersed in an acidic solution resulting in selective etching of the ZnO NR cores. Thus, only TiO2 NTs remained on the substrate. The capping material is effectively used to create a perfect, hexagonal open-ended TiO2 NT array, which interestingly extends onset absorption towards the visible region.     

The influence of ultrasound on the formation of TiO2 nanotube arrays

Electrochemically anodized TiO₂ nanotube (NT) arrays on Ti foils have been in great interests recently attributing to the wide applications. The net growth rate of TiO₂ NT array is determined by electrochemical etching of Ti foil and chemical dissolution of the formed TiO₂ NTs. While the TiO₂ NT growth becomes a diffusion controlled process after certain growth time. Here, we report the influence of ultrasound on the growth process of TiO₂ NT arrays. The results indicate that ultrasound can significantly alter the relative rates of these reactions and adjust the geometries of the TiO₂ NT arrays.     

Influence of growth time and annealing on rutile TiO2 single-crystal nanorod arrays synthesized by hydrothermal method in dye-sensitized solar cells

Vertically aligned TiO₂ nanorod (NR) arrays have been grown on the fluorine doped tin oxide (FTO) substrates by hydrothermal methods and the structures were employed to fabricate the dye-sensitized solar cells (DSSCs). The charge transport properties were investigated by the current to voltage curves and the electrochemical impedance spectroscopy. It was found that DSSCs containing the as-prepared and 500 °C annealed TiO₂ NRs exhibit different trends with the growth time (t). The DSSCs assembled with the un-sintered NR arrays showed the highest power conversion efficiency (η) of 1.87%. When DSSCs were assembled with the sintered NR arrays, nearly 400% enhanced efficiency were obtained, and the values (η) showed a positive correlation with t. This behavior may be attributed to the improved adhesion and electric contact between TiO₂ and FTO, as well as the reduced number of recombination sites.     

Cosensitized Quantum Dot Solar Cells with Conversion Efficiency over 12%

The improvement of sunlight utilization is a fundamental approach for the construction of high‐efficiency quantum‐dot‐based solar cells (QDSCs). To boost light harvesting, cosensitized photoanodes are fabricated in this work by a sequential deposition of presynthesized Zn–Cu–In–Se (ZCISe) and CdSe quantum dots (QDs) on mesoporous TiO₂ films via the control of the interactions between QDs and TiO₂ films using 3‐mercaptopropionic acid bifunctional linkers. By the synergistic effect of ZCISe‐alloyed QDs with a wide light absorption range and CdSe QDs with a high extinction coefficient, the incident photon‐to‐electron conversion efficiency is significantly improved over single QD‐based QDSCs. It is found that the performance of cosensitized photoanodes can be optimized by adjusting the size of CdSe QDs introduced. In combination with titanium mesh supported mesoporous carbon as a counterelectrode and a modified polysulfide solution as an electrolyte, a champion power conversion efficiency up to 12.75% (V_oc = 0.752 V, J_sc = 27.39 mA cm^?2, FF = 0.619) is achieved, which is, as far as it is known, the highest efficiency for liquid‐junction QD‐based solar cells reported.     

Growth of single-crystalline rutile TiO2 nanorods on fluorine-doped tin oxide glass for organic–inorganic hybrid solar cells

Single-crystalline TiO₂ nanorods (TiO₂ NRs) are grown directly on FTO substrates by hydrothermal methods. The diameters and lengths of TiO₂ NRs are easily controlled by growth conditions. When used in hybrid solar cells, TiO₂ NRs function as the continuous pathway for fast electron transport to charge collecting electrode, demonstrating a high power conversion efficiency (PCE) of 3.21% with 140?nm long TiO₂ NRs. The bilayer polymer coating are introduced into 500?nm long TiO₂ NRs to reduce the surface roughness, resulting in the improved contact between the polymer blend and silver electrode and an enhanced PCE from 2.70 to 3.07%.     

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.     

Chemical synthesis of CdS onto TiO2 nanorods for quantum dot sensitized solar cells

A quantum dot sensitized solar cell (QDSSC) is fabricated using hydrothermally grown TiO₂ nanorods and successive ionic layer adsorption and reaction (SILAR) deposited CdS. Surface morphology of the TiO₂ films coated with different SILAR cycles of CdS is examined by Scanning Electron Microscopy which revealed aggregated CdS QDs coverage grow on increasing onto the TiO₂ nanorods with respect to cycle number. Under AM 1.5G illumination, we found the TiO₂/CdS QDSSC photoelectrode shows a power conversion efficiency of 1.75%, in an aqueous polysulfide electrolyte with short-circuit photocurrent density of 4.04 mA/cm² which is higher than that of a bare TiO₂ nanorods array.     

Different TiO2 nanotubes for back illuminated dye sensitized solar cell: fabrication,characterization and electrochemical impedance properties of DSSCs

In the present work we reported the fabrication of different TiO₂ nanotube arrays (TiO₂ NTs) by anodization method. When used in dye-sensitized solar cells, the TiO₂ NTs prepared in the two-step anodization process (2-step TiO₂ NTs) showed better efficiency than those of TiO₂ NTs prepared in one step anodization process (1-step TiO₂ NTs). The 2-step TiO₂ NTs show a remarkable efficiency of 1.56 %. This is higher than those of TiO₂ NTs prepared in one step anodization process. Electrochemical impedance spectroscopy has been performed for qualitative analysis of charge transport process in dye-sensitized solar cells.     

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.     

Fabrication of quantum dot-sensitized solar cells with multilayer TiO2/PbS(X)/CdS/CdSe/ZnS/SiO2 photoanode and optimization of the PbS nanocrystalline layer

In this work, two multilayer photoanode structures of TiO₂/PbS(X)/CdS/ZnS/SiO₂ and TiO₂/PbS(X)/CdS/CdSe/ZnS/SiO₂ were fabricated and applied in quantum dot-sensitized solar cells (QDSCs). Then, the effect of PbS QDs layer on the photovoltaic performance of corresponding cells was investigated. The sensitization was carried out by PbS and CdS QDs layers deposited on TiO₂ scaffold through successive ionic layer adsorption and reaction (SILAR) method. The CdSe QDs film was also formed by a fast, modified chemical bath deposition (CBD) approach. Two passivating ZnS and SiO₂ layers were finally deposited by SILAR and CBD methods, respectively. It was shown that the reference cell with TiO₂/CdS/ZnS/SiO₂ photoanode demonstrated a power conversion efficiency (PCE) of 3.0%. This efficiency was increased to 4.0% for the QDSC with TiO₂/PbS(2)/CdS/ZnS/SiO₂ photoelectrode. This was due to the co-absorption of incident light by low-bandgap PbS nanocrystalline film and also the CdS QDs layer and well transport of the charge carriers. For the CdSe included QDSCs, the PbS-free reference cell represented a PCE of 4.1%. This efficiency was improved to 5.1% for the optimized cell with TiO₂/PbS(2)/CdS/CdSe/ZnS/SiO₂ photoelectrode. The maximized efficiency was enhanced about 25% and 70% compared to the PbS-free reference cells with and without the CdSe QDs layer.

     

Effect of photoelectrode morphology of single-crystalline anatase nanorods on the performance of dye-sensitized solar cells

TiO₂ terpineol-based pastes with nanorods (NRs) of over 25 μm thickness have been prepared for the photoactive electrodes of the dye-sensitized solar cells (DSSCs). The NRs, with a length of approximately 80 nm and an aspect ratio of about 3, are made by a two-step hydrothermal process. They have the single crystalline anatase structure and can be dispersed well in water and ethanol. With a high thermal stability and larger surface area (47.2 m² g^− 1) than commercial TiO₂ particles (P25, 39.1 m² g^− 1), the well-dispersed anatase NR films with aggregate-free morphology are transparent. For the photocurrent-voltage measurements, the NR cell exhibits high short-circuit photocurrent (J_SC) under 1 Sun AM 1.5 simulated sunlight due to the higher surface area and transmittance. The open-circuit voltage (V_OC) of NR films is not obviously reduced with incremental thickness, which results from the one-dimensional single crystalline structure of NR due to less surface defects. As compared with the P25 cell, DSSCs made with NRs have a higher fill factor (FF) because of the uniform void spaces. An enhancement of conversion efficiency from 4.88% for P25 to 5.67% for NR is achieved. The P25 particles are incorporated in NR films as light-scattering centers, while the R1P1 containing 50 wt.% of P25 has a high V_OC and FF as compared with P25, but the J_SC is still lower than that of the NR.     

Construction of TiO<Subscript>2</Subscript> NP@TiO<Subscript>2</Subscript> NT double-layered structural photoanode to enhance photovoltaic performance of CdSe/CdS quantum dots sensitized solar cells

TiO₂ nanoparticles (NP) at top of TiO₂ nanotube (TiO₂ NP@TiO₂NT) double-layered architecture is constructed to combine the advantages of TiO₂ NP and TiO₂ NT together. This composite TiO₂ NP@TiO₂NT architecture as photoanode possesses a larger surface area for more QDs loading, and highly tubular structure for electron swift transport. Based on this architecture, CdSe/CdS quantum dots (QDs) have been successfully synthesized by successive ionic layer adsorption reaction (SILAR) method for quantum dots-sensitized solar cell application. The photovoltaic performance of QDSSCs based on TiO₂ NP@TiO₂ NT have been investigated in contrast with bare TiO₂ NP and bare TiO₂ NT architectures with almost the same thickness. The results show that the power conversion efficiency (PCE) of QDSSCs could be enhanced using TiO₂ NP@TiO₂ NT and improved to 3.26%, which is 80% and 38% higher than QDSSCs based on bare TiO₂ NT and bare TiO₂ NP, respectively. The BET surface area, UV–vis absorption spectra, and incident photon to current conversion efficiency (IPCE) measurements results show the evidence that the TiO₂ NP@TiO₂ NT can combine advantages of TiO₂ NP and TiO₂ NT structures together and lead to a higher light harvesting efficiency, electron collecting efficiency, and efficient electron transport.     

TiO<Subscript>2</Subscript> nanotube arrays: hydrothermal fabrication and photocatalytic activities

Titanium dioxide nanotube arrays (TiO₂ NTAs) with rutile phase have been fabricated successfully via a two-step hydrothermal method. TiO₂ nanorod arrays (TiO₂ NRAs) are first hydrothermally grown on FTO substrate. Then the TiO₂ NTAs can be obtained by controlling the HCl concentration of the hydrothermal etching process. The TiO₂ NTAs have been characterized by X-ray diffractometer, scanning electron microscope, transmission electron microscopy, X-ray photoelectron spectroscopy, and ultraviolet–visible spectroscope. Evolution of TiO₂ nanoarrays are accompanied by enhanced of the surface area and optical properties. Compared with TiO₂ NRAs, the prepared TiO₂ NTAs is more efficient in the photodegradation of methyl orange. These results reveal that the hydrothermal chemical etching provide a flexible and straightforward route for design and preparation of TiO₂ NTAs, promising for new opportunities in photocatalysts and other fields.     

Preparation, optical properties and solar cell applications of CdS quantum dots synthesized by chemical bath deposition

We reported on temperature-dependent photoluminescence (PL) studies of CdS quantum dots (QDs, ~5 nm in diameter) grown onto TiO₂ nanorod arrays by chemical bath deposition. By constructing a liquid-junction solar cell, a power conversion efficiency of 0.88 % was demonstrated. In addition, we observed anomalous emission behavior, specifically a ‘Λ’-shaped (blueshift–redshift) temperature dependence of the peak energy for CdS related PL with increasing temperature. From a study of the integrated PL intensity as a function of temperature, it was revealed that thermionic dissociation of excitons (carriers) out of local potential minima into higher energy states was the dominant mechanism leading to the quenching behavior of the QDs related PL. At 110 K, the localized excitons were totally dissociated and converted to the free excitons. Our results shed light on the exciton-dissociation process in CdS QDs and can be used for the proposed solar cell application.     

Dye sensitized solar cell of TiO2 nanoparticle/nanorod composites prepared via low-temperature synthesis in oleic acid

Titania (TiO₂) nanorods (NRs) and nanoparticles (NPs) were synthesized using oleic acid as a surfactant and employed as photoanodes for dye sensitized solar cell (DSSC) fabrication. The synthesized NRs and NPs were characterized using transmission electron microscopy and X-ray diffraction. The photovoltaic performances were compared between NRs, NPs, and their composites. The results showed that the power conversion efficiencies (η) of the composites depend on the relative compositions of NRs and NPs in photoanodes, reaching the greatest at 10% NR content. η of the pure NRs DSSC was lower than that of the NPs DSSC. Electrochemical impedance spectroscopy revealed that the highest η at 10% NRs is mainly due to reduced charge transport resistance at the TiO₂/dye/electrolyte interface and electrolyte diffusion resistance, overcoming the reduction of the number of adsorbed dye molecules.     

Heterostructured TiO2 Nanorod@Nanobowl Arrays for Efficient Photoelectrochemical Water Splitting

Heterostructured TiO₂ nanorod@nanobowl (NR@NB) arrays consisting of rutile TiO₂ nanorods grown on the inner surface of arrayed anatase TiO₂ nanobowls are designed and fabricated as a new type of photoanodes for photoelectrochemical (PEC) water splitting. The unique heterostructures with a hierarchical architecture are readily fabricated by interfacial nanosphere lithography followed by hydrothermal growth. Owing to the two‐dimensionally arrayed structure of anatase nanobowls and the nearly radial alignment of rutile nanorods, the TiO₂ NR@NB arrays provide multiple scattering centers and hence exhibit an enhanced light harvesting ability. Meanwhile, the large surface area of the NR@NB arrays enhances the contact with the electrolyte while the nanorods offer direct pathways for fast electron transfer. Moreover, the rutile/anatase phase junction in the NR@NB heterostructure improves charge separation because of the facilitated electron transfer. Accordingly, the PEC measurements of the TiO₂ NR@NB arrays on the fluoride‐doped tin oxide (FTO) substrate show significantly enhanced photocatalytic properties for water splitting. Under AM1.5G solar light irradiation, the unmodified TiO₂ NR@NB array photoelectrode yields a photocurrent density of 1.24 mA cm^–2 at 1.23 V with respect to the reversible hydrogen electrode, which is almost two times higher than that of the TiO₂ nanorods grown directly on the FTO substrate.     

CeO2 quantum dot functionalized ZnO nanorods photoanode for DSSC applications

Cerium oxide quantum dots (CeO₂ QDs) decorated zinc oxide nanorods (ZnO NRs) heterostructures were grown by a combination of solvothermal and chemical bath deposition methods and used for dye sensitized solar cell (DSSC) applications. Transmission electron microscope images showed the formation of CeO₂/ZnO NRs, where ~5 nm CeO₂ QDs were decorated on ZnO NRs having 1–2.5 μm length and 100–150 nm width. Photoluminescence spectra showed the significant increase in UV emission after decoration of ZnO NRs with CeO₂ QDs. DSSC results revealed that the ZnO NRs with CeO₂ QDs leads to an increase in the open circuit voltage and fill factor and exhibited a maximum efficiency of 2.65 %, which was 2.01 times higher than that of unmodified ZnO NRs. The decoration of CeO₂ QDs on the ZnO NRs surface may lead to the formation of barrier layer and hindered the back electron transfer and thereby high light harvesting efficiency.