TiO2 Nanotube Photonic Crystals Research Articles

Exploiting the Bragg Mirror Effect of TiO2 Nanotube Photonic Crystals for Promoting Photoelectrochemical Water Splitting.

Exploiting the Bragg mirror effect of photonic crystal photoelectrode is desperately desired for photoelectrochemical water splitting. Herein, a novel TiO2 nanotube photonic crystal bi-layer structure consisting of a top nanotube layer and a bottom nanotube photonic crystal layer is presented. In this architecture, the photonic bandgap of bottom TiO2 nanotube photonic crystals can be precisely adjusted by modulating the anodization parameters. When the photonic bandgap of bottom TiO2 nanotube photonic crystals overlaps with the electronic bandgap of TiO2, the bottom TiO2 nanotube photonic crystal layer will act as a Bragg mirror, leading to the boosted ultraviolet light absorption of the top TiO2 nanotube layer. Benefiting from the promoted UV light absorption, the TiO2 NT-115-NTPC yields a photocurrent density of 1.4 mA/cm2 at 0.22 V vs. Ag/AgCl with a Faradic efficiency of 100%, nearly two times higher than that of conventional TiO2 nanotube arrays. Furthermore, incident photon-to-current conversion efficiency is also promoted within ultraviolet light region. This research offers an effective strategy for improving the performance of photoelectrochemical water splitting through intensifying the light-matter interaction.

High-Throughput Multitarget Molecular Detection in an Automatic Light-Addressable Photoelectrochemical Sensing Platform.

Successively emerged high-throughput multitarget molecular detection methods bring significant development tides in chemical, biological, and environmental fields. However, several persistent challenges of intricate sample preparation, expensive instruments, and tedious and skilled operations still need to be further addressed. Here, we propose an automatic light-addressable photoelectrochemical (ALA-PEC) sensing platform for sensitive and selective detection of multitarget molecules. With Au nanoparticle-decorated TiO2 nanotube photonic crystals (Au-TiO2 NTPCs) as a photoelectrode and 8 kinds of antibiotics as target molecules, the ALA-PEC sensing system implements automatic detection of multimolecules in a short time with high sensitivity and good selectivity. Random samples with different amounts of antibiotics have been well-distinguished in the ALA-PEC system, and both the chemical components and concentrations have been well-illustrated in a pattern recognition model. It is worth noting that 8 samples are not the limit of the ALA-PEC sensing platform, which can be easily expanded to more complex detection arrays based on practical needs. The emerging ALA-PEC sensing platform provides a new solution for rapid screening and detection of multitarget and high-throughput substances and potentially brings the automatic, portable, sensitive, high-throughput, and cost-effective detection technique to an entire new realm.

Synergistic Effect of Photonic Crystals and Oxygen Vacancies on Photoelectrochemical Water Splitting of TiO2 Nanotube

TiO2 nanotube photonic crystals with a mass of oxygen vacancies (VO-TiO2 NTPCs) are prepared through a simple electrochemical reduction. The novel VO-TiO2 NTPCs produce a photocurrent density of 1.4 mA/cm2 at 0.22 V versus Ag/AgCl with Faradic efficiency of 100%, which is 1.75 times that of pristine TiO2NTPCs. The improved performance is contributed to the fact that the oxygen vacancies created by electrochemical reduction, which not only promote efficient utilization of the visible light absorption, but also increase electrical conductivity. This work will contribute to the design and fabrication of the TiO2nanotube-based photoelectric devices.

Bandgap engineering of TiO2nanotube photonic crystals for enhancement of photocatalytic capability

TiO2nanotube photonic crystals with a tunable photonic band gap were constructed by the anodization technique and used as efficient photocatalytic devices.

Ag loaded TiO2 nanotube photonic crystals self-doped Ti3+ periodically by anodization process and their performance of surface enhanced Raman scattering

Here, the preparation of TiO2 nanotube photonic crystals (PCs) and their periodical Ti3+ self-doping were synchronously accomplished for forming Ti3+-TiO2 PCs by introducing an appropriate reduction pulse voltage (−4 V) into the periodical anodic oxidation voltage waveform. Subsequently, some Ag nanoparticles were deposited on the surface of the Ti3+-TiO2 PCs by vacuum evaporation for getting Ag/Ti3+-TiO2 PCs nanostructures, which were ideally suited for the surface-enhanced Raman scatting (SERS) substrates. Compared to the Ag loaded TiO2 nanotubes, the SERS signal of the Ag/Ti3+-TiO2 PCs for RhB can be improved 8.4 times. The desirable SERS property would be attributed to the synergistic effects of the localized electromagnetic (EM) field enhancement and charge-transfer (CT) chemical enhancement mechanisms, which was also discussed in detail.

Bulk/Surface Defects Engineered TiO2 Nanotube Photonic Crystals Coupled with Plasmonic Gold Nanoparticles for Effective in Vivo Near-Infrared Light Photoelectrochemical Detection.

Photoelectrochemical (PEC) techniques are of fundamental and practical importance, and they have been widely used for solar energy conversion and experimental protection. Besides these important applications, an emerging and fast developing PEC application of PEC bioanalysis is receiving more attention from both academic and clinic communities. However, the typical PEC biosensing is still limited under illumination of ultraviolet and visible (UV/vis) light, which hampers its in vivo detection in deep tissues. Expanding the optical absorption wavelength of photoelectrodes from the UV/vis light region into the near-infrared (NIR) light region is highly desirable due to its deep tissue penetrability and minimal invasiveness for organisms, but the exploration of a facile strategy to implement efficient NIR absorption with biocompatible materials is still a big challenge. Herein, under the guidance of theorical calculations, we propose a strategy through modulation of bulk/surface defects and decoration of Au nanoparticles on TiO2 nanotube photonic crystals to implement efficient NIR response and thus successfully realize sensitive and selective PEC detection of antibiotics in real bio- and experimental-samples under NIR illumination. In addition, we first implement the in vivo PEC detection under illumination of NIR light. We have faith that this new NIR photoelectric responsive strategy will provide a broad platform for detection of life-related biomolecules in deep tissues or even in vivo for real-time measurement and shed light on the intrinsic connections between biomarkers and clinical diseases.

Partially disordered TiO2 nanotube photonic crystal: randomness characterization and tuning of reflected scattering light

Introducing disorder into a periodic nanostructure can lead to specific optical behaviors. We present a method of anodic oxidation by adjusting the applied voltage and process time to introduce disorder to TiO2 nanotubes. The surface morphology of TiO2 was numerically investigated according to the morphologies measured with a scanning electron microscope by imaging processing and a statistical method. TiO2 nanotubes obtained under different fabrication conditions have various tube radii ranging from 20–40 nm and wall thicknesses ranging from 20–70 nm. We also evaluated the degree of disorder of the tube radius of the TiO2 nanotubes. The reflected scattering light distributions of laser sources were optically measured at different observing distances, which indicate that the presence of nanotubes enhances the scattering effect, reducing the scattered light intensity by more than 75%, and provide the relationship between the scattering effect and surface morphology of nanotubes. This discovery offers TiO2 nanotubes important application prospects in optical limiting and light confinement, such as stealth coating.

Hydrogenated TiO2 nanotube photonic crystals for enhanced photoelectrochemical water splitting

We report the design, fabrication and characterization of novel TiO2 nanotube photonic crystals with a crystalline core/disordered shell structure as well as substantial oxygen vacancies for photoelectrochemical (PEC) water splitting. The novel TiO2 nanotube photonic crystals are fabricated by annealing of anodized TiO2 nanotube photonic crystals in hydrogen atmosphere at various temperatures. The optimized novel TiO2 nanotube photonic crystals produce a maximal photocurrent density of 2.2 mA cm–2 at 0.22 V versus Ag/AgCl, which is two times higher that of the TiO2 nanotube photonic crystals annealed in air. Such significant PEC performance improvement can be ascribed to synergistic effects of the disordered surface layer and oxygen vacancies. The reduced band gap owing to the disordered surface layer and localized states induced by oxygen vacancies can enhance the efficient utilization of visible light. In addition, the disordered surface layer and substantial oxygen vacancies can promote the efficiency for separation and transport of the photogenerated carriers. This work may open up new opportunities for the design and construction of the high efficient and low-cost PEC water splitting system.

Coupling plasmonic nanoparticles with TiO2 nanotube photonic crystals for enhanced dye-sensitized solar cells performance

In order to break through the limit of low power conversion efficiency (PCE) of dye-sensitized solar cell (DSSC) with single photon management architecture in photoanode, herein, we propose a novel architecture, composed of a TiO2 nanotube photonic crystal (TiO2 NTPC) layer and in-suit deposited Au plasmonic nanoparticles (Au NPs), as photoanode for DSSC applications. The designed photoanode is expected to significantly increase the light harvesting in DSSCs, due to the synergistic effect of the photonic crystal (PC) effect originates from TiO2 NTPC and the surface plasmon resonance (SPR) effect from Au NPs. This synergistic effect of the newly designed photoanode and its functionality in DSSCs are discussed by both experimental and simulated results. Moreover, when the SPR band of Au NPs has been tailored to best match with the bandgap of TiO2 NTPC, the maximum enhancement in power conversion efficiency (PCE, 44.7%), which exceeds the sum enhancement producing by coupling individual TiO2 NTPC (22.1%) or Au NPs (18.0%), has been achieved by introducing the designed Au NPs/TiO2 NTPC photoanode, yielding a PCE of 5.63% in the NT-based DSSCs. The work presented here provides new insights into the design and application of SPR/PC coupling architectures for high efficient light management and synergistically enhanced light harvesting in photovoltaic devices.

TiO2 nanotube photonic crystal fabricated by two-step anodization method for enhanced photoelectrochemical water splitting

TiO2 nanotube photonic crystal (TiO2 TSANTPC) is successfully fabricated via a facile two-step anodization method. The TiO2 TSANTPC exhibits a strong light absorption over a broad visible range. When it was used as a photoanode in photoelectrochemical (PEC) water splitting, the photocurrent density of the TiO2 TASNTPC was identified to be 1.1mA/cm2 at 0.22V vs. Ag/AgCl electrode with Faradic efficiency of 100%, which is approximately 4.8-fold enhanced over pristine TiO2 nanotube. These findings contribute to further enhancement of the PEC performance of TiO2-based photoelectrode.

Broadband and omnidirectional light harvesting enhancement in photovoltaic devices with aperiodic TiO2 nanotube photonic crystal

Design of more effective broadband light-trapping elements to improve the light harvesting efficiency under both normal and tilted light for solar cells and other photonic devices is highly desirable. Herein we present a theoretical analysis on the optical properties of a novel TiO2 nanotube aperiodic photonic crystal (NT APC) following an aperiodic sequences and its photocurrent enhancement effect for dye-sensitized solar cells (DSSCs) under various incidence angles. It is found that, compared to regular PC, the designed TiO2 NT APC owns broader reflection region and a desired omnidirectional reflection (ODR) bandgaps, leading to considerable and stable photocurrent enhancement under both normal and oblique light. The effects of the structural parameters of the TiO2 NT APC, including the average lattice constant and the common sequence difference, on the optical properties, ODR bandgaps and absorption magnification of the integrated DSSCs are investigated in detail. Moreover, the angular dependence of photocurrent enhancement and angular compensation effect of such TiO2 NT APCs are also provided to offer a guidance on the optimum structural parameters design under different engineering application conditions.

Defect-engineered TiO2 nanotube photonic crystals for the fabrication of near-infrared photoelectrochemical sensor.

The use of photoelectrochemical (PEC) sensors, with their outstanding advantages of remarkable sensitivity, inherent miniaturization, portability and easy integration, is becoming a promising analytical detection technique. The rational design of PEC materials and convenient establishment of PEC analysis platforms have given this technique tremendous popularity in both analytical and medical communities for biomolecule detection. However, most of the current efforts in the development of PEC analysis have been made with ultraviolet and visible light (UV-vis) as light source, which are detrimental to biomolecules because of their high energy. On the contrary, near-infrared (NIR) light is biocompatible and is available for in vivo detection. Herein, a prototype of NIR light-responsive PEC analysis platform is first proposed with defect-engineered TiO2 nanotube photonic crystals as photoelectrode and dopamine as target molecule. The coupled strategies of defect engineering for electronic structure modification and morphology design for photon manipulation open up a distinctive avenue to not only implement sensitive NIR PEC detection of dopamine but also have potential multi-target detection ability by integrating bio-recognition units, thus promoting NIR PEC analysis as a versatile analysis method.

Phosphorus Cation Doping: A New Strategy for Boosting Photoelectrochemical Performance on TiO2 Nanotube Photonic Crystals.

Photoelectrochemical (PEC) water splitting is a promising technique for sustainable hydrogen generation. However, PEC performance on current semiconductors needs further improvement. Herein, a phosphorus cation doping strategy is proposed to fundamentally boost PEC performance on TiO2 nanotube photonic crystal (TiO2 NTPC) photoelectrodes in both the visible-light region and full solar-light illumination. The self-supported P-TiO2 NTPC photoelectrodes are fabricated by a facile two-step electrochemical anodization method and subsequent phosphidation treatment. The Ti4+ is partially replaced by P cations (P5+) from the crystal lattice, which narrows the band gap of TiO2 and induces charge imbalance by the formation of Ti-O-P bonds. We believe the combination of unique photonic nanostructures of TiO2 NTPCs and P cation doping strategy will open up a new opportunity for enhancing PEC performance of TiO2-based photoelectrodes.

Recognition unit-free and self-cleaning photoelectrochemical sensing platform on TiO2 nanotube photonic crystals for sensitive and selective detection of dopamine release from mouse brain

For implementing sensitive and selective detection of biological molecules, the biosensors are been designed more and more complicated. The exploration of detection platform in a simple way without loss their sensitivity and selectivity is always a big challenge. Herein, a prototype of recognition biomolecule unit-free photoelectrochemical (PEC) sensing platform with self-cleaning activity is proposed with TiO2 nanotube photonic crystal (TiO2 NTPCs) materials as photoelectrode, and dopamine (DA) molecule as both sensitizer and target analyte. The unique adsorption between DA and TiO2 NTPCs induces the formation of charge transfer complex, which not only expends the optical absorption of TiO2 into visible light region, thus significantly boosts the PEC performance under illumination of visible light, but also implements the selective detection of DA on TiO2 photoelectrode. This simple but efficient PEC analysis platform presents a low detection limit of 0.15nm for detection of DA, which allows to realize the sensitive and selective determination of DA release from the mouse brain for its practical application after coupled with a microdialysis probe. The DA functionalized TiO2 NTPCs PEC sensing platform opens up a new PEC detection model, without using extra-biomolecule auxiliary, just with target molecule naturally adsorbed on the electrode for sensitive and selective detection, and paves a new avenue for biosensors design with minimalism idea.

Enhanced efficiencies in thin and semi-transparent dye-sensitized solar cells under low photon flux conditions using TiO2 nanotube photonic crystal

The photovoltaic output of dye-sensitized solar cells (DSSCs) are greatly dependent on the amount of absorbed photons, which is limited by the thickness of active layer of DSSCs and the illumination conditions. To improve the cell performance under low irradiance condition, a photoanode was designed by attaching a TiO2 nanotube photonic crystal (NTPC) onto the thin TiO2 nanoparticle (NP) layer for applications in thin and semi-transparent DSSCs. It is found that the introduction of the TiO2 NTPC significantly increases the light harvesting and hence the power conversion efficiency (PCE) of the respective DSSCs. The TiO2 NTPC provides multi-functionalities, such as Bragg reflection, light scatting and additional light harvesting from its nanotube structure, leading to more significant light harvesting enhancement in these thin and semi-transparent DSSCs. Compared with the single-layer TiO2 NP based reference DSSCs, the above-mentioned synergic effects in a cell incoporated with a ∼2.3-μm-thick TiO2 NTPC yield PCE enhancements up to 99.1% and 130%, under 1 and 0.5 Sun conditions, respectively. Meanwhile, an obvious compensation effect of TiO2 NTPC to reduce the output power drop of these cells under tilted incient light is also demonstrated. The work will boost the practical applications of PC in irradiance sensitive devices.

A strategy to reduce the angular dependence of a dye-sensitized solar cell by coupling to a TiO2nanotube photonic crystal

Almost all types of solar cells suffer from a decreased power output when the incident light is tilted away from normal since the incident intensity generally follows a cosine law of the incident angle. Making use of the blue shift nature of the Bragg position of a TiO2 nanotube photonic crystal (NT PC) under oblique incidence, we demonstrate experimentally that the use of the NT PC can partially compensate the cosine power loss of a dye-sensitized solar cell (DSSC). The strategy used here is to purposely choose the Bragg position of the NT PC to be at the longer wavelength side of the dye absorption peak. When the incident light is tilted, the blue shift of the Bragg position results in more overlap with the dye absorption peak, generating a higher efficiency that partially compensates the reduced photon flux due to light inclination. Moreover, the unique structure of the vertically aligned TiO2 nanotubes contributes an additional scattering effect when the incident light is tilted. As a result, the power output of a DSSC coupled with the NT PC layer shows a much flatter angular dependence than a DSSC without the NT PC. At all the incident angles, the DSSC coupled with the NT PC layer also shows a higher power conversion efficiency than the one without. The concept of using NT PC to mitigate the angular dependence of DSSCs can be easily extended to many other optoelectronic devices that are irradiance sensitive.

Enhanced Light Harvesting in Dye-Sensitized Solar Cells Coupled with Titania Nanotube Photonic Crystals: A Theoretical Study

Herein we present a theoretical analysis on the optical properties and the photocurrent enhancement of nanotube-based dye-sensitized solar cells (DSSCs) coupled with a TiO2 nanotube (NT) photonic crystal (PC). It is found that the introduction of a TiO2 nanotube PC produces both Bragg mirror effect and Fabry-Perot cavity behavior, leading to a significant enhancement of light harvesting for photons in the photonic bandgap and at the two band edges. In addition, an increased amount of surface-anchored dye due to the larger surface area in the NT PC layer also causes absorption enhancement in the whole visible spectrum. The effects of structural parameters of the PC, such as the thickness of the PC layer, the axial lattice constant, the diameter of the nanotube, and light incident angle, on the optical properties and photocurrent magnification are thoroughly studied. The optimum structural parameters are proposed, which not only provide guidance but also offer further opportunities in the design and applications of TiO2 nanotube photonic crystals.

Design and coupling of multifunctional TiO2 nanotube photonic crystal to nanocrystalline titania layer as semi-transparent photoanode for dye-sensitized solar cell

In order to conquer the challenges in the current technology to couple a dye-sensitized solar cell (DSSC) with a photonic crystal (PC), we propose a novel design of PC-based photoanode, composed of a thick TiO2 nanoparticle absorption layer and a thin TiO2 nanotube photonic crystal (TiO2 NT PC) membrane. In this architecture, the bandgap of TiO2 NT PC can be precisely tailored by modulating the anodization parameters using the current-pulse anodization process. Owing to the selective reflectivity of TiO2 NT PC, the power conversion efficiency (PCE) of the electrodes reveals a strong wavelength dependence. Strategies to enhance the efficiency of the newly designed DSSC and its relation to the selective reflection of the photoanode are discussed and evaluated by experimental and simulated results. Meanwhile, the functionality of TiO2 NT PC, offering both PC and light-scattering effects, has also been clarified. The combined effects of PC and light-scattering yield the maximum enhancement in PCE (39.5%) when the tailored TiO2 NT PC, with the best matching of its reflectance maximum to the dye absorption maximum, is integrated into a DSSC. The work presented here provides new insights into the design and tailoring of a photonic crystal to enhance the PCE of DSSCs for practical applications.