Quantum Dot-sensitized TiO2 Nanotube Research Articles

Constructing quantum dots sensitized TiO2 nanotube p-n heterojunction for photoelectrochemical hydrogen generation

Photoelectrochemical (PEC) water splitting is a promising approach to convert solar radiation into hydrogen (H2) as a clean fuel. The PEC device performance depends on the light harvesting efficiency of the photoanode and the carrier dynamics (i.e. separation/transport rate) at the photoanode/electrolyte interface. Herein, we report a photoanode architecture consisting of self-organized TiO2 nanotubes (NTs) sensitized by CdS/CdSe quantum dots (QDs) and treated with a Cu-based solution to create a p-n heterojunction. Our results demonstrate that the TiO2 NTs/QDs PEC device yields a photocurrent density of 4.18 mA.cm−2 at 0.5 V vs RHE, which is 51 times higher than the device based on TiO2 NTs only (i.e., 0.08 mA.cm−2) and 7 times compared to TiO2/QDs nanoparticles (NPs) (i.e. 0.45 mA.cm−2) under one sun illumination. The p-type CuSe coating over the TiO2/QDs NTs photoanodes forms a p-n heterojunction that improves the carrier dynamics. The resulting PEC device shows a 13% improvement in the photocurrent density. In addition, employing longer TiO2 NTs improves the device's stability. Our results offer a simple and scalable method for the design and optimization of the photoanodes to enhance the performance of PEC and other optoelectronic devices.

Enhanced photocathodic protection performance of graphene quantum dots sensitized TiO2 nanotube arrays for 304 stainless steel

Graphene quantum dots sensitized TiO2 nanotube arrays (GQDs/TNAs) are fabricated via a modified electrophoretic deposition (MEPD) method, in which TNAs were pretreated by silane coupling agent. The influences of GQDs on the morphology, optical property and photocathodic protection performance of the hybrid GQDs/TNAs were investigated. By varying the time of MEPD, appropriate quantity of GQDs can be well attached to the inner walls and tube entrance of TNAs. Compared with the bare TNAs and GQDs/TNAs prepared by impregnation-precipitation and conventional EPD method, the GQDs/TNAs acquired at 15 min exhibits excellent visible-light response and enhanced photocathodic protection of the coupled 304 stainless steel. A higher photocurrent density of 800 μA cm−2 is achieved and the open-circuit potential shifts to a more negative value of 740 mV under white-light irradiation. The improved performance is attributed to GQDs, which is not only a green photosensitizer, but also a superb mediator for fast separation and transfer of the photogenerated charge.

Label-free Photoelectrochemical Aptasensor for the Determination of Carcinoembryonic Antigen Using a Cadmum Sulfide Quantum Dot Sensitized Titanium (IV) Oxide Nanotube Electrode

ABSTRACTA label-free photoelectrochemical aptasensor for the sensitive and selective determination of carcinoembryonic antigen was constructed based on a CdS quantum dot sensitized TiO2 nanotube electrode. TiO2 nanotubes with highly ordered structure and more active sites than bulk TiO2 were prepared with an electrochemical anodic oxidation process. The CdS quantum dots were immobilized on the TiO2 nanotubes using poly(diallyldimethylammonium chloride) as a bridge. Due to the energy level match between TiO2 and CdS, the CdS quantum dots/TiO2 nanotubes electrode exhibits excellent photoelectrochemical performance. The large surface area of the electrode also allows for capturing large numbers of aptamers. The fine photoelectrochemical performance and the large surface area of the electrode greatly enhanced the detection sensitivity. Under the optimal conditions, the prepared photoelectrochemical aptasensor presents desirable analytical properties for the determination of carcinoembryonic antigen in the range of 0.05 to 10 ng mL−1 with a detection limit of 0.014 ng mL−1. The application of the designed protocol was investigated by analyzing carcinoembryonic antigen in human serum samples with recoveries from 80.0 to 115.0%. This simple and sensitive method provides an alternative tool to standard biochemical assays.

The influence of linker molecule on photovoltaic performance of CdS quantum dots sensitized translucent TiO2 nanotube solar cells

Based on translucent TiO2 nanotube (NT) film, CdS quantum dots (QDs) were anchored onto TiO2 NT film using different linker molecules including thioglycolic acid (TGA), mercaptopropionic acid (MPA) and cysteine via hydrothermal method. The CdS QDs sensitized TiO2 NT (CdS/TiO2 NT) electrodes were constructed to quantum dots-sensitized solar cells (QDSSCs). The photovoltaic performance analysis results of the QDSSCs show that the linker molecule plays a significant role in the solar cell performance. The QDSSC using cysteine as linker molecule exhibits best power conversion efficiency than the other QDSSCs using TGA or MPA as linker molecules. The incident photon-to-current conversion efficiency (IPCE) and the electrochemical impedance spectroscopy (EIS) analysis results indicate that amino group of cysteine may has two aspects of the role to improve the power conversion efficiency of the QDSSC, one is that the good charge transfer properties of the cysteine might depend on the good donor–acceptor abilities of the free electron pair of the amino group, another is that distance between the TiO2 NT and the CdS QDs could be slightly decreased due to the cysteine could anchor the CdS QDs via both amino and thiol group.

Enhanced photoelectrochemical performance of quantum dot-sensitized TiO2 nanotube arrays with Al2O3 overcoating by atomic layer deposition.

While TiO2 nanotube arrays cosensitized with CdS and PbS quantum dots can achieve water splitting under visible light excitation, the use of quantum dots is limited by the relatively slow interfacial hole transfer rate and low internal quantum efficiencies in the visible region. Al2O3 overcoating by atomic layer deposition (ALD) can drastically enhance the photoelectrochemical performance of the quantum dot-sensitized TiO2 nanotube arrays. 30 ALD cycles of the Al2O3 overlayer can achieve a good balance between surface coverage and charge transfer resistance. The resulting maximum photocurrent density of 5.19 mA cm(-2) under simulated solar illumination shows a 52 times improvement over the pure TiO2 nanotube arrays, and more significantly, a 60% enhancement over bare quantum dot-sensitized TiO2 nanotube arrays. The incident photon-to-current conversion efficiency can reach the record value of 83% at 350 nm and remain above 30% up to 450 nm. A systematic examination of the role of the ALD Al2O3 overlayer indicates that surface recombination passivation, catalytic improvement in interfacial charge transfer kinetics, and chemical stabilization might synergistically enhance the photoelectrochemical performance in the visible region. These results provide a physical insight into the facile surface treatment, which could be applied to develop and optimize high-performance photoelectrodes for artificial photosynthesis.

Recent progress in all-solid-state quantum dot-sensitized TiO2 nanotube array solar cells

All-solid-state quantum dot-sensitized TiO2 nanotube array solar cells have been drawing great attention to solar energy conversion, which break through restrictions in traditional solar cells, such as the high recombination at interfaces of porous TiO2 films/sensitizers/hole conductors/counter electrodes, instability of dyes, and leakage of solution electrolyte, and so the novel solar cells exhibit promising applications in the future. In this Minireview article, the assembling of solar cells including the preparation of TiO2 nanotube array photoanodes, quantum dot preparation and sensitization on photoanodes, filling of hole conductors in TiO2 nanotubes, and selection of counter electrodes are overviewed, and the development course of all-solid-state quantum dot-sensitized TiO2 nanotube array solar cells in recent years are summarized in detail. Moreover, the influences of TiO2 nanotube array photoanodes, quantum dots, solid electrolyte, and counter electrodes on photon-to-current efficiencies of solar cells are summarized. In addition, current problems of solid-state quantum dot-sensitized TiO2 nanotube array solar cells are analyzed, and the corresponding improvements, such as multisensitizers and passivation layers, are proposed to improve the photoelectric conversion efficiency. Finally, this Minireview provides a perspective for the future development of this novel solar cell.

Effects of Hydroxylation on PbS Quantum Dot Sensitized TiO2 Nanotube Array Photoelectrodes

ABSTRACT The contact state at the heterojunction interfaces greatly influences the interfacial kinetics of the photoinduced charge carriers. In this study, we used a facile NaOH pretreatment to replenish the hydroxyl groups lost during the heat treatment for crystallization of TiO 2 nanotube arrays (TNAs) prepared via anodic oxidization. By reacting the carboxylic acid groups of thioglycolic acid (TGA) with the TiO 2 surface hydroxyl groups, TGA molecules were covalently linked to the TiO 2 surface and then PbS quantum dots (QDs) were anchored onto the TNAs via the successive ionic layer adsorption and reaction (SILAR) method. The sample microstructure and photoelectrochemical properties were analyzed with X-ray diffraction (XRD), transmission electron microscopy (TEM), field-emission scanning electron microscopy (FE-SEM),current–voltage characteristics ( J – V ), electrochemical impedance spectroscopy (EIS), transient photovoltage plots and Mott-Schottky curves. The contact state and electrostatic potential distribution between TiO 2 {1 0 1} and PbS {1 1 1} planes were estimated by using first principle simulation. It was found that the NaOH pretreatment could enhance the crystallization degree of PbS QDs, decrease the crystal face mismatch, dangling bond density and the interfacial resistance between PbS QDs and TiO 2 , and accelerate the interfacial separation and transfer of photoinduced charge carriers. The first principle calculations demonstrated that the PbS QDs and TiO 2 interfacial contact was strengthened, and the built-in electric field was induced from TiO 2 {1 0 1} towards PbS {1 1 1}. These combined effects apparently improved the device photoelectrochemical performance. Compared to the sample without pretreatment, the specimen pretreated with NaOH demonstrated 19.96% and 29.93% increases in peak photoconversion efficiency after five and ten cycles of SILAR deposition, respectively.

Target-induced nano-enzyme reactor mediated hole-trapping for high-throughput immunoassay based on a split-type photoelectrochemical detection strategy.

Photoelectrochemical (PEC) detection is an emerging and promising analytical tool. However, its actual application still faces some challenges like potential damage of biomolecules (caused by itself system) and intrinsic low-throughput detection. To solve the problems, herein we design a novel split-type photoelectrochemical immunoassay (STPIA) for ultrasensitive detection of prostate specific antigen (PSA). Initially, the immunoreaction was performed on a microplate using a secondary antibody/primer-circular DNA-labeled gold nanoparticle as the detection tag. Then, numerously repeated oligonucleotide sequences with many biotin moieties were in situ synthesized on the nanogold tag via RCA reaction. The formed biotin concatamers acted as a powerful scaffold to bind with avidin-alkaline phosphatase (ALP) conjugates and construct a nanoenzyme reactor. By this means, enzymatic hydrolysate (ascorbic acid) was generated to capture the photogenerated holes in the CdS quantum dot-sensitized TiO2 nanotube arrays, resulting in amplification of the photocurrent signal. To elaborate, the microplate-based immunoassay and the high-throughput detection system, a semiautomatic detection cell (installed with a three-electrode system), was employed. Under optimal conditions, the photocurrent increased with the increasing PSA concentration in a dynamic working range from 0.001 to 3 ng mL(-1), with a low detection limit (LOD) of 0.32 pg mL(-1). Meanwhile, the developed split-type photoelectrochemical immunoassay exhibited high specificity and acceptable accuracy for analysis of human serum specimens in comparison with referenced electrochemiluminescence immunoassay method. Importantly, the system was not only suitable for the sandwich-type immunoassay mode, but also utilized for the detection of small molecules (e.g., aflatoxin B1) with a competitive-type assay format.

Fabrication of CdTe/ZnS core/shell quantum dots sensitized TiO2 nanotube films for photocathodic protection of stainless steel

A highly ordered TiO2 nanotube array film on a titanium substrate was fabricated by an anodic oxidation method. Then CdTe/ZnS core/shell quantum dots were successfully prepared on the film by pulsed potential electrodeposition and chemical bath deposition in succession. The results indicated that the CdTe/ZnS quantum dots sensitized TiO2 composite film displayed a significant increase in photocurrent compared with a pure TiO2 nanotube film, and its photoabsorption was extended to the visible region. Under white light illumination, the composite film as a sustainable photoanode had better photoelectric conversion performance and could provide striking photocathodic protection for 403 stainless steel.

CdS–CdSe (CdTe) core–shell quantum dots sensitized TiO2 nanotube array solar cells

CdS–CdSe and CdS–CdTe core–shell quantum dots sensitized TiO2 nanotube array solar cells have been controllably prepared based on the anion exchange method using CdS quantum dots on the TiO2 NTs as sacrificial templates. The formation of CdS–CdSe and CdS–CdTe core–shell quantum dots on the upper and inner surface of the TiO2 NTs was confirmed by scanning electron (SEM) and transmission electron microscopes (TEM). These two photoelectrodes exhibit significant improvement of optical and electrochemical properties. Although TiO2 NTs/CdS–CdTe with a more favorable visible light activity, TiO2 NTs/CdS–CdSe have higher photostability and photoelectrochemical performance.

Preparation of coaxial heterogeneous graphene quantum dot-sensitized TiO2 nanotube arrays via linker molecule binding and electrophoretic deposition

Coaxial heterogeneous graphene quantum dot-sensitized TiO2 nanotube arrays (GQDs/TNTs) are prepared by a coupling technique of linker molecule binding and electrophoretic deposition (EPD). The silane linker molecules act as a superb medium for integrating GQDs and TNTs by covalent amide linkage, thus preventing GQDs from clogging the tube entrances and forming a uniform GQD layer tightly attached to the inside tube walls during the following EPD process. By adjusting the time of EPD, appropriate thickness of the deposited GQDs in the internal tube walls of TNTs can be controlled. Compared to the pristine TNTs and GQDs/TNTs prepared by the conventional impregnation–precipitation method, the hybrids fabricated by EPD exhibit significantly enhanced photoelectrochemical water-splitting activity and photocatalytic organic dye decomposition performance for their broad photo-absorption range, fast separation of photogenerated charge, and stability.

Fabrication of ZnxIn1−xS quantum Dot-sensitized TiO2 nanotube arrays and their photoelectrochemical properties

Complete composition-tuned ZnxIn1−xS (0<x<1) quantum dots were deposited on TiO2 nanotube arrays by the successive ionic layer absorption and reaction (SILAR) method. The deposition of ZnxIn1−xS nanoparticles on the upper and inner surface of the TiO2 NTs was confirmed by field emission scanning electron (FE-SEM) and transmission electron microscopes (TEM). The ZnxIn1−xS/TiO2 nanotube arrays exhibited high absorption in the visible light region due to the narrow band gap of ZnxIn1−xS. Zn0.4In0.6S/TiO2 NTs electrodes showed the optimal photoelectrochemical properties under visible light illumination.

A visible-light-driven composite photocatalyst of TiO2 nanotube arrays and graphene quantum dots.

TiO2 nanotube arrays are well-known efficient UV-driven photocatalysts. The incorporation of graphene quantum dots could extend the photo-response of the nanotubes to the visible-light range. Graphene quantum dot-sensitized TiO2 nanotube arrays were synthesized by covalently coupling these two materials. The product was characterized by Fourier-transform infrared spectrometry (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), thermogravimetric analysis (TGA) and UV–vis absorption spectroscopy. The product exhibited high photocatalytic performance in the photodegradation of methylene blue and enhanced photocurrent under visible light irradiation.

A new method to disperse CdS quantum dot-sensitized TiO2 nanotube arrays into P3HT:PCBM layer for the improvement of efficiency of inverted polymer solar cells.

We report that the efficiency of ITO/nc-TiO2/P3HT:PCBM/MoO3/Ag inverted polymer solar cells (PSCs) can be improved by dispersing CdS quantum dot (QD)-sensitized TiO2 nanotube arrays (TNTs) in poly (3-hexylthiophene) and [6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) layer. The CdS QDs are deposited on the TNTs by a chemical bath deposition method. The experimental results show that the CdS QD-sensitized TNTs (CdS/TNTs) do not only increase the light absorption of the P3HT:PCBM layer but also reduce the charge recombination in the P3HT:PCBM layer. The dependence of device performances on cycles of CdS deposition on the TNTs was investigated. A high power conversion efficiency (PCE) of 3.52% was achieved for the inverted PSCs with 20 cyclic depositions of CdS on TNTs, which showed a 34% increase compared to the ITO/nc-TiO2/P3HT:PCBM/MoO3/Ag device without the CdS/TNTs. The improved efficiency is attributed to the improved light absorbance and the reduced charge recombination in the active layer.

Stability of Photoactive Oxide Semiconductors

ZnxCd1-xSe quantum dot-sensitized TiO2 nanotube (TiO2 NT) array photoelectrodes were synthesized employing successive ionic layer adsorption and reaction (SILAR) technique. The influence of the number of deposition cycles and post-deposition annealing temperature on photoelectrochemical properties and stability behavior of photoanodes was investigated. The photoanodes were characterized using scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) techniques. Photoelectrochemical measurements indicated that TiO2 NTs coupled with ZnCdSe quantum dots synthesized with 7 SILAR cycles, annealed at 400oC for 1hr under N2 atmosphere exhibited excellent photoelectrochemical properties. The high and stable photocurrent produced demonstrates the potential of ZnxCd1-xSe/TiO2 heterostructure as an efficient photoanode.

Photoelectrochemical performance of CdTe sensitized TiO2 nanotube array photoelectrodes

CdTe and ZnS quantum dots sensitized TiO2 nanotube array photoelectrodes were prepared by depositing CdTe and ZnS QDs on the tube walls of the self-organized TiO2 NTs using successive ionic layer adsorption and reaction (SILAR) technique. The deposition of CdTe and ZnS on the upper and inner surface of the TiO2 NTs was confirmed by field emission scanning electron (FE-SEM) and transmission electron microscopes (TEM). The obtained results indicated that TiO2 NTs photoelectrodes sensitized by CdTe quantum dots for 5 cycles exhibited excellent photoelectrochemical properties, with an open-circuit voltage of 0.95V and a short circuit current density of 11.15mAcm−2, and ZnS coating protected the photoelectrodes against photocorrosion.

Carbon quantum dot sensitized TiO2 nanotube arrays for photoelectrochemical hydrogen generation under visible light

TiO2 nanotube arrays loaded with carbon quantum dots were used as a photoanode for efficient hydrogen generation under visible light.