Single-crystalline TiO2 Nanowires Research Articles

Construction of a TiO2 heterostructure nanowire with a sulfurized shell via a simple sulfurization process for enhanced photoelectrochemical water oxidation

Promoted charge transport and separation as well as excellent light harvesting ability play a key role in enhancing photoelectrochemical (PEC) performance. Herein, a simple sulfurization approach was developed for forming polycrystalline S-doped TiO2 shell on the single crystalline TiO2 nanowire surface to form the TiO2/S-doped TiO2 core-shell homostructure. With the sulfurization time, the light harvesting ability, separation efficiency and carrier density increased for such homostructures. As a result, the highest photocurrent densities of 1.24 mA/cm2 at 1.23 V versus RHE was achieved over the TiO2/S-doped TiO2 core-shell homostructure sulfurized for 15 h, corresponding to 67.6% improvement in photocurrent density compared with the pristine TiO2. This work reveals a relatively simple strategy to simultaneously enhance light absorption and electron-hole pair separation.

Type-II band alignment in single crystalline TiO2 nanowires under twisting

Electronic structures with the type-II band alignment usually exist only in heterostructures. Using the generalized Bloch theorem, we reveal that an effective type-II band alignment can be induced in the single crystalline TiO2 nanowires (NWs) by an axial twisting deformation. With this, we further reveal distinct responses of the the valence band (VB) states and the conduction band (CB) states in the radial dimension of the nanowire. Specifically, in the twisted NW, the VB states migrate from the NW core toward the NW shell while the CB states migrate from the NW shell toward the NW core. Our results pave a new way to realize the type-II band alignment in single crystalline NWs, which is critical for actual applications such as NW-based photovoltaic effect and photocatalytic effect. Further, the significant variation of the quantum states also illustrates the possibility to tune the electronic properties of TiO2 NWs by strain engineering.

Hierarchical Titanium Dioxide Nanowire/Metal–Organic Framework/Carbon Nanofiber Membranes for Highly Efficient Photocatalytic Degradation of Hydrogen Sulfide

Photocatalysis is an efficient approach to degrade hydrogen sulfide (H2 S) and titanium dioxide (TiO2 ) is commonly used as a catalyst for H2 S degradation. However, the low separation rate of photoinduced carriers and low gas adsorption ability of TiO2 limit its H2 S photocatalytic decomposition rate. In this paper, single-crystalline TiO2 nanowires are assembled on one-dimensional carbon nanofibers (CNFs) and a tunable metal-organic framework (MOF) coating is fabricated on the surface of the TiO2 nanowires using a versatile step-by-step self-assembly strategy. The excellent photocatalytic properties of the resulting membrane originate from the ability of the CNFs to rapidly transport charge carriers and the high and regenerable H2 S adsorption ability of the MOF. The photocatalytic mechanism of the as-prepared material was also discussed. Therefore, this work provides a promising method to improve the photocatalytic performance of H2 S degradation.

High‐Performance Triphase Bio‐Photoelectrochemical Assay System Based on Superhydrophobic Substrate‐Supported TiO2 Nanowire Arrays

AbstractThe design and fabrication of bio‐photoelectrodes that simultaneously possess rapid charge and gas‐phase mass transport abilities are highly desirable for the development of high‐performance bio‐photoelectrochemical assay systems. Here, a solid–liquid–air triphase bio‐photoelectrode is demonstrated by immobilizing glucose oxidase, a model redox active enzyme, onto 1D single crystalline TiO2 nanowire arrays grown upon a superhydrophobic carbon textile substrate. Based on this triphase bio‐photoelectrode system, oxygen can be directly and constantly supplied from the air phase to sustain enzymatic reactions, leading to much enhanced oxidase kinetics. Further, the photogenerated electrons can transport rapidly and be efficiently collected along the nanowire arrays. The bio‐photoelectrochemical assay system demonstrates a significant wide detection range, high sensitivity, and selectivity as well as a low detection limit. The design principle is generally applicable to the fabrication of other triphase bio‐photoelectrodes, which offers an excellent opportunity for significant advances in environmental analysis and clinical diagnosis.

Physical Mechanism Behind Enhanced Photoelectrochemical and Photocatalytic Properties of Superhydrophilic Assemblies of 3D-TiO2 Microspheres with Arrays of Oriented, Single-Crystalline TiO2 Nanowires as Building Blocks Deposited on Fluorine-Doped Tin Oxide.

In comparison to the one-dimensional (1D) semiconductor nanostructures, the hierarchical, three-dimensional (3D) microstructures, composed of the arrays of 1D nanostructures as building blocks, show quite unique physicochemical properties due to efficient photon capture and enhanced surface to volume ratio, which aid in advancing the performance of various optoelectronic devices. In this contribution, we report the fabrication of surfactant-free, radially assembled, 3D titania (rutile-phase) microsphere arrays (3D-TMSAs) composed of bundles of single-crystalline titania nanowires (NWs) directly on fluorine-doped conducting oxide (FTO) substrates with tunable architecture. The effects of growth parameters on the morphology of the 3D-TMSAs have been studied thoroughly. The 3D-TMSAs grown on the FTO-substrate showed superior photon-harvesting owing to the increase in light-scattering. The photocatalytic and photon to electron conversion efficiency of dye-sensitized solar cells (DSSC), where the optimized 3D-TMSAs were used as an anode, showed around 44% increase in the photoconversion efficiency compared to that of Degussa P-25 as a result of the synergistic effect of higher surface area and enhanced photon scattering probability. The TMSA film showed superhydrophilicity without any prior UV irradiation. In addition, the presence of bundles of almost parallel NWs led to the formation of arrays of microcapacitors, which showed stable dielectric performance. The fabrication of single-crystalline, oriented, self-assembled TMSAs with bundles of titania nanowires as their building blocks deposited on transparent conducting oxide (TCO) substrates has vast potential in the area of photoelectrochemical research.

A Heterojunction Design of Single Layer Hole Tunneling ZnO Passivation Wrapping around TiO2Nanowires for Superior Photocatalytic Performance.

Nanostructured hybrid heterojunctions have been studied widely for photocatalytic applications due to their superior optical and structural properties. In this work, the impact of angstrom thick atomic layer deposited (ALD) ZnO shell layer on photocatalytic activity (PCA) of hydrothermal grown single crystalline TiO2 nanowires (NWs) is systematically explored. We showed that a single cycle of ALD ZnO layer wrapped around TiO2 NWs, considerably boosts the PCA of the heterostructure. Subsequent cycles, however, gradually hinder the photocatalytic activity (PCA) of the TiO2 NWs. Various structural, optical, and transient characterizations are employed to scrutinize this unprecedented change. We show that a single atomic layer of ZnO shell not only increases light harvesting capability of the heterostructure via extension of the absorption toward visible wavelengths, but also mitigates recombination probability of carriers through reduction of surface defects density and introduction of proper charge separation along the core-shell interface. Furthermore, the ultrathin ZnO shell layer allows a strong contribution of the core (TiO2) valence band holes through tunneling across the ultrathin interface. All mechanisms responsible for this enhanced PCA of heterostructure are elucidated and corresponding models are proposed.

Surface roughening and top opening of single crystalline TiO2 nanowires for enhanced photocatalytic activity

One dimensional (1D) single crystalline nanostructures facilitate electron transport and reduce the charge recombination; whilst nanotubular structure and a roughened surface provide more active sites as well as more opportunities for target molecules to reach the photocatalyst. As a result, TiO2 thin films with such a structural design can be anticipated to possess excellent photocatalytic activity. In this study, top opened nanowires with nanoprotrusions on the surface, which are hierarchical yet single crystalline as a whole, were synthesized by a simple acid treatment at 80°C for 36h of hydrothermally synthesized titanate nanowires, followed by a final calcination at 450°C for 1h. The hierarchical TiO2 films exhibited significantly enhanced efficiency when utilized to assist the photodegradation of rhodamine B and phenol in water under UV light illumination.

Metal-Organic Frameworks as Promising Photosensitizers for Photoelectrochemical Water Splitting.

Ti-based metal-organic frameworks (MOFs) are demonstrated as promising photosensitizers for photoelectrochemical (PEC) water splitting. Photocurrents of TiO2 nano wire photoelectrodes can be improved under visible light through sensitization with aminated Ti-based MOFs. As a host, other sensitizers or catalysts such as Au nanoparticles can be incorporated into the MOF layer thus further improving the PEC water splitting efficiency.

Ferroelectric Polarization-Enhanced Photoelectrochemical Water Splitting in TiO2-BaTiO3 Core-Shell Nanowire Photoanodes.

The performances of heterojunction-based electronic devices are extremely sensitive to the interfacial electronic band structure. Here we report a largely enhanced performance of photoelectrochemical (PEC) photoanodes by ferroelectric polarization-endowed band engineering on the basis of TiO2/BaTiO3 core/shell nanowires (NWs). Through a one-step hydrothermal process, a uniform, epitaxial, and spontaneously poled barium titanate (BTO) layer was created on single crystalline TiO2 NWs. Compared to pristine TiO2 NWs, the 5 nm BTO-coated TiO2 NWs achieved 67% photocurrent density enhancement. By numerically calculating the potential distribution across the TiO2/BTO/electrolyte heterojunction and systematically investigating the light absorption, charge injection and separation properties of TiO2 and TiO2/BTO NWs, the PEC performance gain was proved to be a result of the increased charge separation efficiency induced by the ferroelectric polarization of the BTO shell. The ferroelectric polarization could be switched by external electric field poling and yielded PEC performance gain or loss based on the direction of the polarization. This study evidence that the piezotronic effect (ferroelectric or piezoelectric potential-induced band structure engineering) holds great promises in improving the performance of PEC photoelectrodes in addition to chemistry and structure optimization.

Self-induced preparation of TiO2 nanowires by chemical vapor deposition

High-density single-crystalline TiO2 nanowires are successfully fabricated on a TiSi2 layer using a new self-induced catalyst-free method by APCVD. The results show that the high aspect ratio nanowires with diameters of 20–50nm and lengths of about 3μm are obtained on a TiSi2 layer at 720°C. The length of TiO2 nanowires increases with the preparation time until Ti is exhausted. The nanowires shape changes with the concentration of O2 and temperature. When the temperature is above 720°C or the flux of O2 is over 6sccm, the density and length of nanowires decrease under the combined effect of the increasing lateral surface diffusion and longitudinal growth. The formation of TiO2 nanowires comes along with the consumption of TiSi2, TiO2 nanowires grow along the [001] direction of the tetragonal rutile TiO2 crystal from the bottom, with the tip being pushed upwards. The growth process is proposed which is consistent with our experiment results.

Hierarchical TiO2 nanowire/graphite fiber photoelectrocatalysis setup powered by a wind-driven nanogenerator: A highly efficient photoelectrocatalytic device entirely based on renewable energy

Photoelectrocatalysis is an efficient approach for the degradation of organic pollutants as well as for water splitting. However, an external power source supplying a direct current (DC) is essential to enhance the separation of photo-induced carriers. In this paper, a fully-functional photoelectrocatalysis device was constructed by connecting single crystalline TiO2 nanowires assembled on graphite microfibers (TiO2 nanowire/graphite fiber, TNGF) to a wind-driven triboelectric nanogenerator (WDTENG). The excellent photocatalytic and photoelectrocatalytic properties of TNGF originate from the ability of graphite fibers to transport rapidly the charge carriers, the high photocatalytic activity of TiO2 nanowires and the photo-induced carrier separation enhancement created by the zero band gap of graphite. When this system is used for hydrogen generation via photoelectrocatalytic water splitting, the hydrogen evolution of the TNGF is significantly increased under assistance of the WDTENG. Photoelectrochemical analysis demonstrates that the separation and recombination of photo-induced charge carriers in the 7TNGF composite is dependent on the applied voltage bias. Thus, the wind driven generator provides large enough voltage bias for efficient charge separation, leading to a highly enhanced photocatalytic performance. This work is the first instance of high performance photoelectrocatalysis device aimed either at depollution or at hydrogen production which is entirely based on renewable energy.

Bio-inspired porous antenna-like nanocube/nanowire heterostructure as ultra-sensitive cellular interfaces

In this study, an unconventional antenna-like heterostructure comprised of arrays of nanoporous Prussian blue (PB) nanocube heads/TiO2 nanowire (NW) arms (PB-TiO2) is developed for efficient three-dimensional interfacial sensing of small molecules and cellular activities. Inspired by insect tentacles, which are comprised of both target recognition and signal transduction units, one-dimensional TiO2 NW arrays are grown, followed by selective growth of nanoporous PB nanocubes on the tips of the NW arrays. Due to their high selectivity and bioaffinity toward cells, long biostability under a cell culture adhesion condition (up to 108 h) is obtained, and with its inherent bio-mimetic enzymatic activity, the obtained nanoporous PB nanocubes (head segment) serve as robust substrates for site-selective cell adhesion and culture, which allows for sensitive detection of H2O2. Simultaneously, the single-crystalline TiO2 NWs (arm segment) provide efficient charge transport for electrode substrates. Compared with PB-functionalized planar electrochemical interfaces, the PB-TiO2 antenna NW biointerfaces exhibit a substantial enhancement in electrocatalytic activity and sensitivity for H2O2, which includes a low detection limit (∼20 nM), broad detection range (10−8 to 10−5 M), short response time (∼5 s) and long-term biocatalytic activity (up to 6 months). The direct cultivation of HeLa cells is demonstrated on the PB-TiO2 antenna NW arrays, which are capable of sensitive electrochemical recording of cellular activity in real time, where the results suggest the uniqueness of the biomimic PB-TiO2 antenna NWs for efficient cellular interfacing and molecular recognition.

High-efficiency dye-sensitized solar cells based on ultra-long single crystalline titanium dioxide nanowires

High-efficiency dye-sensitized solar cells (DSCs) based on 3D networks of ultra-long single crystalline TiO2 nanowires are fabricated. By hydrothermal reaction of Ti foils in alkali aqueous solution, following ion exchange and high temperature sintering processes, the ultra-long single crystalline TiO2 nanowires can be prepared. Due to long enough of the TiO2 nanowires, they not only form 1D arrays perpendicular to Ti foils, but also extend to form 3D network structure on the top-side of the arrays. By optimizing the hydrothermal duration at 24 h, the formed TiO2 nanowire membrane based front-side illuminated photoanode shows enhanced dye-loading and light scattering abilities and excellent charge transport properties. So the DSC with this photoanode can attain to the highest power conversion efficiency about 8.05%.

A tri-functional TiO2photoelectrode: single crystalline nanowires directly grown on nanoparticles for dye-sensitized solar cells

We prepared a hybrid photoelectrode with single crystalline TiO2 nanowires (NWs) directly grown on TiO2 nanoparticles (NPs) for efficient dye-sensitized solar cells (DSCs). For this purpose, the NWs were grown on the commercial NPs, P25 Degussa through use of a solvothermal treatment. The NWs offer the dual functions of optical confinement and rapid electron transport, while the incorporated NPs provide a high surface area for dye loading. As a result, the NP–NW hybrid electrode showed 6.6% of the conversion efficiency under AM 1.5 illumination of 100 mW cm−2, which corresponds to a noticeable enhancement over the efficiency of the reference device with the NPs. Thus, we expect that the hybridization of two distinct nanomaterials will provide an effective strategy in designing the photoelectrode of DSCs.

Sensitive enzymatic glucose detection by TiO2nanowire photoelectrochemical biosensors

Photoelectrochemical detection represents a unique signal transducing modality, where the photogenerated charge carriers are modulated by the redox reactions of molecular targets over the electrode surface. In this work, we report a novel glucose sensor based on hydrothermally-grown, single-crystalline TiO2 nanowires with surface-functionalized glucose oxidase (GOx), which can oxidize glucose to gluconic acid. The key feature of this glucose sensor design is that the photogenerated holes over the TiO2 anode are utilized to form O2, which directly serves as an efficient electron shuttling mediator between the enzymatic redox center of GOx and the sensor surface, thus leading to an increase of the photocurrent. As a proof-of-concept, the GOx-functionalized TiO2 (TiO2–GOx) nanowires exhibit a high sensitivity of ∼0.9 nM in the detection of glucose in buffer. The capability of detecting glucose in mouse serum is also demonstrated. Our work suggests the potential of developing these nanowire-based photoelectrochemical biosensors for convenient, efficient and low-cost biomarker detection and disease diagnosis.

Photoelectrochemical Detection of Glutathione by IrO2–Hemin–TiO2 Nanowire Arrays

We have developed sensitive detection of glutathione using the IrO2-hemin-TiO2 nanowire arrays. Single-crystalline TiO2 nanowires are synthesized by a hydrothermal reaction, followed by surface functionalization of ~3 nm thick hemin and ~1-2 nm diameter IrO2 nanoparticles. The IrO2-hemin-TiO2 nanowire arrays offer much enhanced photocurrent with ∼100% increase compared to the pristine TiO2 nanowires and allow for label-free, real-time, sensitive photoelectrochemical detection of glutathione. The sensitivity achieved is ~10 nM in buffer, comparable to or better than most of the existing glutathione detection methods. Furthermore, cell extracts containing glutathione are robustly detected, with ~8000 cells/mL for HeLa cells and ~5000 cells/mL for human embryonic kidney 293T cells. This nanowire PEC sensor assay exhibits excellent selectivity and stability, suggesting a potential detection platform for analyzing the glutathione level in biosamples.

Large-Scale Synthesis of Transition-Metal-Doped TiO2 Nanowires with Controllable Overpotential

Practical implementation of one-dimensional semiconductors into devices capable of exploiting their novel properties is often hindered by low product yields, poor material quality, high production cost, or overall lack of synthetic control. Here, we show that a molten-salt flux scheme can be used to synthesize large quantities of high-quality, single-crystalline TiO2 nanowires with controllable dimensions. Furthermore, in situ dopant incorporation of various transition metals allows for the tuning of optical, electrical, and catalytic properties. With this combination of control, robustness, and scalability, the molten-salt flux scheme can provide high-quality TiO2 nanowires to satisfy a broad range of application needs from photovoltaics to photocatalysis.

Improved photo-catalytic activity of single-crystalline TiO2 nanowires surrounded by Pt cube nanoparticles

We report improved photo-catalytic properties of single-crystalline TiO2 nanowires surrounded by Pt cubes for methanol electrooxidation under UV illumination. The TiO2–Pt consists of single-crystalline TiO2 nanowires grown along [001] direction and Pt cube nanoparticles with dominantly exposed {100} facets confirmed by transmission electron microscopy images and fast Fourier transform patterns. The nanostructure electrode consisting of TiO2 nanowires and Pt cubes exhibits a remarkably improved performance for methanol electrooxidation in acid solution as compared to that of TiO2 nanowires.

Large-scale growth of single-crystalline TiO<SUB align="right">2 nanowires and their visible-light photocatalytic activity

Single–crystalline TiO2 nanowires were successfully grown by oxidation of as–sputtered Ti films with ethanol. X–ray diffractometer (XRD) and high resolution transmission electron microscopy (HRTEM) studies revealed that the nanowires had a single–crystalline structure and grew along the [110] axis. The formation of the TiO2 thin film was found to play an important role in the nanowire growth due to the ability to reduce the diffusion rates of Ti atoms from Ti films for the limited nucleation by thermally activated. The growth of TiO2 nanowires was determined by the reaction of ethanol with Ti diffusion which was supplied from the Ti films. The TiO2 nanowires showed an efficient visible–light photocatalytic activity. Moreover, after thermal annealing, both UV and visible–light photocatalytic activities of TiO2 nanowires were enhanced significantly due to the increase in the surface defects on the nanowires.

Growth of TiO2 nanowire bundle arrays and their application in dye-sensitized solar cells

Highly oriented single-crystalline TiO2 nanowire bundle arrays on transparent conductive fluorine-doped tin oxide substrates are prepared by hydrothermal method using the precursors of titanium butoxide, deionized water and hydrochloric acid. The structure and morphology characteristics of all the samples have been analyzed by X-ray diffraction (XRD), scanning electron microscopy and transmission electron microscopy. Results show that the diameter, length, and density of the nanowire bundle arrays can be varied by changing the growth parameters, such as growth time, initial reactant concentration and acidity. The enhanced (002) peak in XRD patterns indicate that the nanowire is well crystallized and grow perpendicular to the substrate. The high resolution transmission electron microscope images and selected-area electron diffraction patterns confirm that there are approximately 10–30 nanowires in each bundle. The nanowire is single crystalline. Dye-sensitized solar cells assembled from oriented TiO2 nanowire bundle arrays as the photoanode are studied. The light-to-electricity conversion efficiency is about 2.17 %.