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Enhancement of the photoelectrochemical properties of TiO2 Nanofibers supported on Ti sheets by polyol-made CdSe quantum-dots impregnation
Abstract
Hetero-nanostructures with improved photoelectrochemical properties were assembled by impregnating well-crystallized CdSe quantum-dots onto TiO2 nanofibers on a Ti substrate. A significant enhancement of the photocurrent density was observed for the resulting CdSe-TiO2/Ti photoanode in a photoelectrochemical cell under solar simulated illumination: 0.138mA.cm⁻² (versus 0.008mA.cm⁻² on TiO2/Ti photoanode), at room temperature and zero-bias in a Na2S-Na2SO3 (0.25M, pH=7.0) electrolyte solution. Our results highlight the improvement in visible-light utilization by a facile quantum-dot sensitization of TiO2.
TiO2 nanofibers (NFs), grown on Ti sheets by hydrothermal treatment, were coated with Cu2O nanoparticles (NPs) using sputtering or chemical bath deposition (CBD) in order to improve their photoelectrochemical (PEC) water splitting capabilities as photoanodes. Scanning electron microscopy (SEM) and X-ray photoelectron (XPS) spectroscopy confirmed the production of the desired Cu2O-TiO2/Ti hetero-nanostructures, while PEC measurements evidenced a net improvement of the produced photocurrent under standard xenon lamp illumination, compared to the pristine TiO2/Ti structure. Moreover, it appears that the sputtered composite photoanodes provide a higher photocurrent compared to the CBD made ones, meaning that their oxide/oxide interfaces are of better crystalline quality, supplying a larger number of electrons to the Pt cell cathode for H⁺ reduction to H2, through the external PEC cell circuit.
In order to solve the rapid recombination of electron–hole pairs, limited optical response range, and single electron and hole transmission path, polydopamine (PDA)/WO3 and TiO2 composite (PDA-11.5%/WO3/TiO2/Ti) photoanode was prepared. Its maximum photocurrent density is 2.03 mA·cm−2 at 1.1 V (vs. SCE), which is 1.47 times higher than that of WO3/TiO2/Ti. The maximum power density, photocurrent density, and Rhodamine B degradation rate of the photocatalytic fuel cell (PFC) with PDA-11.5%/WO3/TiO2/Ti photoanode were 18.68 μW·cm−2, 0.18 mA·cm−2, and 84.6%, respectively. Its excellent performance is mainly due to the good carrier mobility of PDA and the stepped heterojunction structure formed by PDA-11.5%/WO3/TiO2/Ti so that photogenerated electrons and holes gather in the conduction bands of WO3 and HOMO of PDA, respectively. This not only expands the light absorption range and improves the light absorption intensity but also promotes the separation of electron–hole pairs and finally significantly enhances the Rhodamine B degradation and power generation performance of PFC.
In this study, polyol-made CdS and CdSe crystalline nanoparticles (NPs) are loaded by impregnation on TiO2 nanotube arrays (TNTAs) for solar-simulated light-driven photoelectrochemical (PEC) water vapor splitting. For the first time, we introduce a safe way to utilize toxic, yet efficient photocatalysts by integration in solid-state PEC (SSPEC) cells. The enabling features of SSPEC cells are the surface protonic conduction mechanism on TiO2 and the use of polymeric electrolytes, such as Nafion instead of liquid ones, for operation with gaseous reactants, like water vapor from ambient humidity. Herein, we studied the effects of both the operating conditions in gaseous ambient atmospheres and the surface modifications of TNTAs-based photoanodes with well-crystallized CdS and CdSe NPs. We showed 3.6 and 2.5 times increase in the photocurrent density of defective TNTAs modified with CdS and CdSe, respectively, compared to the pristine TNTAs. Electrochemical impedance spectroscopy and structural characterizations attributed the improved performance to the higher conductivity induced by intrinsic defects as well as to the enhanced electron/hole separation at the TiO2/CdS heterojunction under gaseous operating conditions. The SSPEC cells were evaluated by cycling between high relative humidity (RH) (80%) and low RH levels (40%), providing direct evidence of the effect of RH and, in turn, adsorbed water, on the cell performance. Online mass spectrometry indicated the corresponding difference in the H2 production rate. In addition, a complete restoration of the SSPEC cell performance from low to high RH levels was also achieved. The presented system can be employed in off-grid, water depleted, and air-polluted areas for the production of hydrogen from renewable energy and provides a solution for the safe use of toxic, yet efficient photocatalysts.
Nitrogen (N)-doped anatase/rutile TiO2 nanoparticulate (NP) films were successfully prepared by a sparking process under external electric (E) and magnetic (B) fields without heat treatment. As-deposited NP films annealed at 250 °C, 350 °C, and 450 °C prepared by the sparking process without E and B fields were compared with as-deposited NP films prepared with E+B fields. The results clearly show that the quantity of the NP films increases after sparking under E and B fields. Interestingly, sparking under E+B fields can enhance both the film crystallinity and photocatalytic efficiency. Furthermore, N-doped TiO2 was observed only in NP films prepared by sparking under E+B fields. Doping with N not only plays an important role in enhancing the photocatalytic activity but also manipulates the spin polarization of TiO2 under E+B fields to enhance the film crystallinity and photocatalytic efficiency without the need for an annealing system.
In order to improve the light utilization and electron transfer efficiency of the PFC system, an efficient BiVO4/UiO-66/TiO2/Ti photoanode with visible-light response was prepared by a simple sol–gel method and a hydrothermal synthesis method. The photocurrent density of the BiVO4/UiO-66/TiO2/Ti was 0.72 mA/cm² at 0.6 V (vs. SCE) under visible light, which was 2.4 times that of BiVO4/TiO2/Ti photoanode, indicating UiO-66 used as the electron transmission medium between TiO2 and BiVO4 could enhance the electron transmission efficiency. With BiVO4/UiO-66/TiO2/Ti photoanode and Cu2O/Cu cathode, the visible-light responsive photocatalytic fuel cell (PFC) exhibited high organics degradation and electricity conversion performance. The short circuit photocurrent density (Jsc), maximum power output and degradation efficiency of Rhodamine B were 0.23 mA·cm⁻², 29.26 μW·cm⁻² and 91.50% in 2 h, respectively.
In order to improve the utilization of visible light and enhance the electron transmission rate of photocatalytic fuel cell (PFC), core-shell BiVO4@PTh-wt% composite was successfully prepared by hydrothermal synthesis and chemical polymerization method, and used to modify TiO2/Ti to obtain BiVO4@PTh-wt%/TiO2/Ti photoanode with high efficient visible light-response. BiVO4@PTh-8%/TiO2/Ti obtained the maximum photocurrent density of 8.14 mA cm⁻² at 1.0 V (vs. SCE) under visible light illumination, which was 3.59 times higher than that of BiVO4/TiO2/Ti. This can be explained by that the existence of π conjugated system in PTh helps the delocalization of π electrons which leads to high conducting properties of photoanode. With BiVO4@PTh-8%/TiO2/Ti photoanode and Cu2O/Cu cathode, the visible-light responsive PFC exhibited high organics degradation and electricity conversion performance. The short circuit photocurrent density (Jsc), maximum power output and degradation efficiency of Rhodamine B were 0.48 mA cm⁻², 59.70 μW cm⁻² and 96.89%, respectively.
TiO2 nanotube arrays (TiO2 NTs) films were successfully derived from Ti substrate through a twice electrochemical anodization method. CdSe nanoparticles were then uniformly deposited on the TiO2 NTs (CdSe/TiO2 NTs) via in situ deposition method. CdSe/TiO2 NTs possess a better visible light harvesting ability compared to pure TiO2 NTs. Importantly, the generation, accumulation and transportation of free electrons of the CdSe/TiO2 hetero-junctions were systematically investigated by photoelectrochemical experiments. The open-circuit potential of 7-CdSe/TiO2 NTs (−0.31 V) is higher than that of TiO2 NTs (−0.18 V), suggesting increased electron generation and accumulation. Meanwhile, the flat band potential of 7-CdSe/TiO2 NTs (1.88 V) shows a positive shift compared with that of TiO2 NTs (−0.05 V), implying a significant chemical potential gradient from the electrolyte to the surface of CdSe/TiO2 NTs. Additionally, the 7-CdSe/TiO2 NTs exhibit the highest photoelectrocatalytic activity for methylene blue degradation. Those enhanced photoactivities are assigned to the synergistic effect of the one-dimensional nanotubular structure and the efficient separation of charge carriers.
In this work, we developed a new type of nanostructured photoanodes for photoelectrochemical water splitting. They are based on CdS–TiO2 nanocomposite films, supported on conductive Ti sheets, prepared by an easy-to-achieve three-step method. It involves the production of TiO2 nanofibers (NFs) using a controlled corrosion route of polished Ti sheets, the preparation of size-controlled CdS quantum dots (QDs) by the polyol process and the direct impregnation of TiO2/Ti sheets by QDs in suspension. The photoelectrochemical (PEC) properties of the resulting nanostructures were measured, using a homemade electrochemical cell illuminated with a standard Xenon lamp, and compared to those of bare TiO2 NFs. A net enhancement of the photocurrent was observed after CdS impregnation, suggesting a low carrier recombination rate and a higher efficiency of the PEC device for solar water splitting, as the induced photocurrent is related to the electrons needed to reduce H+ ions into H2 at the cathode electrode (Pt wire).
Abstract We developed a novel one-pot polyol approach for the synthesis of biocompatible CdSe quantum dots (QDs) using poly(acrylic acid) (PAA) as a capping ligand at 240°C. The morphological and structural characterization confirmed the formation of biocompatible and monodisperse CdSe QDs with several nanometers in size. The encapsulation of CdS thin layers on the surface of CdSe QDs (CdSe/CdS core–shell QDs) was used for passivating the defect emission (650 nm) and enhancing the fluorescent quantum yields up to 30% of band-to-band emission (530–600 nm). Moreover, the PL emission peak of CdSe/CdS core–shell QDs could be tuned from 530 to 600 nm by the size of CdSe core. The as-prepared CdSe/CdS core–shell QDs with small size, well water solubility, good monodispersity, and bright PL emission showed high performance as fluorescent cell labels in vitro. The viability of QDs-labeled 293T cells was evaluated using a 3-(4,5-dimethylthiazol)-2-diphenyltertrazolium bromide (MTT) assay. The results showed the satisfactory (>80%) biocompatibility of as-synthesized PAA-capped QDs at the Cd concentration of 15 μg/ml.
Quantum dot-metal oxide junctions are an integral part of next-generation solar cells, light emitting diodes, and nanostructured electronic arrays. Here we present a comprehensive examination of electron transfer at these junctions, using a series of CdSe quantum dot donors (sizes 2.8, 3.3, 4.0, and 4.2 nm in diameter) and metal oxide nanoparticle acceptors (SnO(2), TiO(2), and ZnO). Apparent electron transfer rate constants showed strong dependence on change in system free energy, exhibiting a sharp rise at small driving forces followed by a modest rise further away from the characteristic reorganization energy. The observed trend mimics the predicted behavior of electron transfer from a single quantum state to a continuum of electron accepting states, such as those present in the conduction band of a metal oxide nanoparticle. In contrast with dye-sensitized metal oxide electron transfer studies, our systems did not exhibit unthermalized hot-electron injection due to relatively large ratios of electron cooling rate to electron transfer rate. To investigate the implications of these findings in photovoltaic cells, quantum dot-metal oxide working electrodes were constructed in an identical fashion to the films used for the electron transfer portion of the study. Interestingly, the films which exhibited the fastest electron transfer rates (SnO(2)) were not the same as those which showed the highest photocurrent (TiO(2)). These findings suggest that, in addition to electron transfer at the quantum dot-metal oxide interface, other electron transfer reactions play key roles in the determination of overall device efficiency.
In this work, we grow oriented photonic TiO2nanotubes (TNTs) with a well-established anodization method and their intrinsic catalytic and electronic properties are studied by means of (photo)electrochemical methods. The oriented photonic TNTs are assessed by the RctCdl(ΩF) product, which is a measure of the intrinsic photoelectrocatalytic activity (IPA) of the material. The RctCdlproduct decouples any surface area effects and it can be applied at any potential and accompanied reaction without knowing the actual electrochemically active surface area (EASA). Moreover, electrochemical impedance spectroscopy (EIS) is used in order to investigate the frequency dispersion in the single frequency Mott-Schottky (MS) measurement. The choice of frequency and the frequency dispersion in MS measurements in nanostructured and of high surface area semiconductors are rarely addressed and can result in erroneous values of two important parameters, i.e. flat band position and donor or acceptor densities. This work provides further insights into important electrochemical aspects and catalytic properties of high surface area nanomaterials for photoelectrochemical applications.
ZnS- and CdS-TiO2 hetero-nanostructures were synthesized by impregnation of polyol-made ZnS, CdS quantum dots (QDs) on the surface of TiO2 nanofibers that were grown on Ti sheets by controlled corrosion. Due to their finite size, the grafted QDs were partly oxidized, when air exposed, reducing their ability to sensitize TiO2 toward photoelectrochemical (PEC) water splitting application. We showed that a subsequent aqueous sulfidation allowed recovering the initial chemical composition of the produced catalysts improving their chemical stability and consequently their PEC properties.
Background
The conversion of sunlight to electrical power has been dominated by solidstate junction devices, often made of silicon. However, this dominance is now being challenged by the emergence of the new generation of water splitting cell (integration of photovoltaic system with an electrolyzer to generate clean and portable H2 energy carrier. This cell normally is based on nanocrystalline materials, which offers the prospect of cheap fabrication together with other attractive feature such as high chemical stability and flexibility in aqueous solution under evolving oxygen (O2) gases. However, nanocrystalline materials are facing few drawback such as recombination losses of charge carriers and less response under visible spectrum. Therefore, an effort to minimize the recombination losses of charge carriers and extended the spectral response of TiO2 NTs into visible spectrum by incorporating an optimum amount of lower band gap and suitable band edge position semiconductor (cadmium selenide [CdSe]) into the lattice of TiO2 NTs.
Methods
An efficient approach has been demonstrated in this research work to enhance the solardriven photoelectrochemical (PEC) water splitting performance by decorating CdSe species into highly ordered TiO2 nanotubes (NTs) film through a facile and cost-effective chemical bath deposition. Morphology, chemical properties, and electronic structures have been studied.
Results
A maximum photocurrent density of ~2.50 mA/cm2 at 0.6V versus Ag/AgCl electrode was exhibited by TiO2 NTs with the presence of approximately 1 at % of CdSe species. The presence of CdSe species offered an improvement of photocurrent density under solar irradiation due to the effective mediators to trap the photo-induced electrons and minimizes the recombination of charge carriers within the lattice of TiO2 NTs.
Conclusion
Hybrid CdSe-TiO2 NTs were successfully fabricated through chemical bath deposition method in order to study the synergistic coupling effect of CdSe with TiO2 NTs on the PEC performance. By bathing pure TiO2 NTs film in a 5 mM CdSe precursor solution extensively covered by approximately 1 at % CdSe exhibited the highest jp of 2.50 mA/cm2 among the samples. However, excessive deposition (≥5 mM) was neither negatively affected by the self-organized NTs nor decreased in jp. This condition inferred that higher ionic product (Cd and Se ions) leaded to rapid ion-by-ion condensation or adsorption of colloidal particles clogged the opening pore’s mouth of TiO2 NTs. Thus, an improvement in the photoresponse was observed when optimum amount (~ 1 at %) of the CdSe was deposited on TiO2 NTs film.
The photocatalytic decomposition of water is believed to be able to help mitigate the crisis of fossil fuel depletion. However, the photocatalytic hydrogen production remains challenge to obtain high and stable photoconversion efficiency. Here we report an epitaxial hetero-structure of CdSe/TiO2 nanotube arrays as efficient photo-anodes via simple room-temperature, low-cost electrochemical deposition. With the help of the similar d spacing with TiO2, CdSe sensitization layer is epitaxially grown on the tube wall of the TiO2 nanotubes, resulting in an ideal coherent grain boundary and single crystal growth. The resultant photo-anode produces 30% more photocurrent than those samples without coherent grain boundary. Notably, the especial epitaxial hetero-structure is beneficial to decrease the recombination site and accelerate the separation of photogenerated electron-hole pairs. Furthermore, an ultrathin PEDOT surface layer was developed on the epitaxial hetero-structure of CdSe/TiO2 nano-tube arrays in which it functions as both a physical passivation barrier and a hole transfer layer. As a result, significantly enhanced photocurrent density and substantially better stability have been achieved. This methodology may be providing a new pathway of epitaxial growth for preparing the heterogeneous junction materials which have similar d spacing.
Hydrogen production from water and sunlight through photocatalysis could become one of the channels, in the not-so-distant future, to meet a part of ever growing energy demands. However, accomplishing solar water splitting through semiconductor particulate photocatalysis seems to be the ‘Holy Grail’ problem of science. In the present mini-review, some of the critical strategies of semiconductor photocatalysis are focused with the aim of enumerating underlying critical factors such as visible light harvesting, charge carrier separation, conduction and their utilization that determine the quantum efficiency. We attempted to bring out the essential requirements expected in a material for facile water splitting by explaining important and new designs contributed in the last decade. The newly emerged designs in semiconductor architecture employing nanoscience towards meeting the critical factors of facile photocatalysis are elucidated. The importance of band gap engineering is emphasized to utilize potential wide band gap semiconductors. Assistance of metal nanostructures and quantum dots to semiconductors attains vital importance as they are exuberant visible light harvesters and charge carrier amplifiers. Benevolent use of quantum dots in solar water splitting and photoelectrochemical water splitting provides scope to revolutionize the quantum efficiency by its multiple exciton generation features. A list of drawbacks and issues that hamper the much needed breakthrough in photocatalysis of water splitting is provided to invite attention to address them and move towards sustainable water splitting.
Present mini-review focuses on some of the major aspects of solar water splitting to produce hydrogen. Electronic structure changes due to doping of conventional oxides are correlated with visible light absorption. Quantum dot assisted water splitting and persistent problems associated, in general, with solar water splitting are highlighted with a possible path forward.
An efficient photoelectrode is prepared by sequentially assembled CdS and CdSe quantum dots (QDs) onto a nanocrystalline TiO2 film. The CdS/CdSe co-sensitized photoelectrode was found to have a complementary effect in the light absorption. Furthermore, the cascade structure, TiO2/CdS/CdSe, exhibits a significant enhancement in the current−voltage response, both in dark conditions and under light illumination. On the contrary, the performance of the reverse structure, TiO2/CdSe/CdS, is much less than the electrode using a single sensitizer. The open circuit potentials measured in the dark for these electrodes indicates that a Fermi level alignment occurs between CdS and CdSe after their contact, causing downward and upward shifts of the band edges, respectively, for CdS and CdSe. A stepwise band edge structure is, therefore, constructed in the TiO2/CdS/CdSe electrode, which is responsible for the performance enhancement of this photoelectrode. The saturated photocurrent achieved by the TiO2/CdS/CdSe electrode under the illumination of UV cutoff AM1.5 (100 mW/cm2) is 14.9 mA/cm2, which is three times the value obtained by the TiO2/CdS and TiO2/CdSe electrode. When a ZnS layer is further deposited for passivating the QDs, the corresponding hydrogen evolution rate measured for the TiO2/CdS/CdSe/ZnS electrode is 220 μmol/(cm2 h) (5.4 mL/(cm2 h)). This performance is presently the highest reported for the QD-sensitized photoelectrochemical cells.
This review discusses salient features of interfacial electron transfer reactions in colloidal semiconductor solutions and thin films and their application for solar light energy conversion and photocatalytic water purification. This research is interdisciplinary and is situated at the limit between colloid science, and electrochemistry, and semiconductor physics. The section on colloidal semiconductors discusses material science aspects, optical properties, electronic properties, and charge carrier reactions in colloidal semiconductor solutions. The section on nanocrystalline semiconductor electrodes discusses some basic properties and preparation procedures of nanocrystalline TiO[sub 2] films, energetics and operations of the nanoporous solar cell, observations and importance of electronic states in the band gap region, photoinduced processes in nanocrystalline semiconductor electrodes, the electric field in a nanocrystalline electrode, and kinetic rate constants in the photosensitized solar cell. 156 refs.
Using migration enhanced epitaxy the self-organized formation of CdSe islands on ZnSe in open and overgrown structures has been studied by transmission electron microscopy, high-resolution X-ray diffraction, atomic force microscopy, photoluminescence and excitation spectroscopy. The transition from homogeneous CdSe quantum wells with flat interfaces to fluctuating CdSe films could be observed when exceeding the critical thickness. These interrupted layers contain CdSe quantum dots embedded in Cd1—xZnxSe with a concentration gradient. Islands observed on open structures are unstable in time. Their chemical nature is still unclear due to the fact that similar features are obtained on pure ZnSe. First results on other highly lattice mismatched II–VI systems like CdTe/ZnSe and ZnTe/ZnSe will be presented.
We report the synthesis and photoelectrochemical (PEC) studies of TiO(2) nanoparticles and nanowires simultaneously doped with nitrogen and sensitized with CdSe quantum dots (QDs). These novel nanocomposite structures have been applied successfully as photoanodes for PEC hydrogen generation using Na(2)S and Na(2)SO(3) as sacrificial reagents. We observe significant enhanced photoresponse in these nanocomposites compared to N-doped TiO(2) or CdSe QD sensitized TiO(2). The enhancement is attributed to the synergistic effect of CdSe sensitization and N-doping that facilitate hole transfer/transport from CdSe to TiO(2) through oxygen vacancy states (V(o)) mediated by N-doping. The results demonstrate the importance of designing and manipulating the energy band alignment in composite nanomaterials for fundamentally improving charge separation and transport and thereby PEC properties.
Conduction band electrons produced by band gap excitation of TiO2-particles reduce efficiently thiosulfate to sulfide and sulfite.
This reaction is confirmed by electrochemical investigations with polycrystalline TiO2-electrodes. The valence band process in alkaline TiO2-dispersions involves oxidation of S2O to tetrathionate which quantitatively dismutates into sulfite and thiosulfate, the net reaction being:
This photodriven disproportionation of thiosulfate into sulfide and sulfite: should be of great interest for systems that photochemically split hydrogen sulfide into hydrogen and sulfur.
ALTHOUGH the possibility of water photolysis has been investigated by many workers, a useful method has only now been developed. Because water is transparent to visible light it cannot be decomposed directly, but only by radiation with wavelengths shorter than 190 nm (ref. 1).
By using bifunctional surface modifiers (SH-R-COOH), CdSe quantum dots (QDs) have been assembled onto mesoscopic TiO(2) films. Upon visible light excitation, CdSe QDs inject electrons into TiO(2) nanocrystallites. Femtosecond transient absorption as well as emission quenching experiments confirm the injection from the excited state of CdSe QDs into TiO(2) nanoparticles. Electron transfer from the thermally relaxed s-state occurs over a wide range of rate constant values between 7.3 x 10(9) and 1.95 x 10(11) s(-1). The injected charge carriers in a CdSe-modified TiO(2) film can be collected at a conducting electrode to generate a photocurrent. The TiO(2)-CdSe composite, when employed as a photoanode in a photoelectrochemical cell, exhibits a photon-to-charge carrier generation efficiency of 12%. Significant loss of electrons occurs due to scattering as well as charge recombination at TiO(2)/CdSe interfaces and internal TiO(2) grain boundaries.
White light-emitting diodes (WLEDs) were fabricated by combining blue InGaN chips with luminescent colloidal core-shell CdSe-ZnSe quantum dots (QDs). The core-shell CdSe-ZnSe QDs synthesized by thermal deposition approach exhibited high photoluminescence efficiency with a quantum yield more than ∼40%, and size-tunable emission wavelengths from 510 to 620 nm. Three-band red-green-blue WLED was successfully assembled by blue InGaN chips and green-emitting and red-emitting CdSe-ZnSe QDs. Based on QDs with flexibly selected color, the WLEDs exhibited white light with a CIE-1931 coordinate of (0.33, 0.33) and color rendering index Ra of 91.