Rutile Nanowires Research Articles

Enhanced n-Type conductivity of rutile TiO2 nanowires

This paper reports a solvothermal process to synthesize the rutile TiO2 assisted by the hydrogen reducing atmosphere and vanadium doping technique. The synthesized TiO2 nanowires have a rutile structure with a diameter of about 100 nm and a single crystal growth direction of [01–1]. Moreover, TiO2 nanowires exhibit n-type conductivity with a carrier concentration of 1.87 × 1018 cm−3 and mobility of 1 cm2V−1S−1. The research results show that the n-type conductivity of rutile TiO2 nanowires can be enhanced under the synergistic effect of reducing atmosphere and V doping. The rutile nanowires with good n-type conductivity have bright application in future nano-electronic devices and nano-photoelectric devices.

Sawtooth Faceting in Rutile Nanowires.

Sawtooth faceting, with a diameter-dependent period, is pervasively observed in many Si, III–V, and II–VI nanowires during vapor-phase growth. This can be linked to an oscillation in surface energies, which are influenced by many factors such as crystal anisotropy, surface decoration, and twinning. Without the presence of surface decoration and planar defects, TiO2 rutile nanowires, axially oriented along a low-symmetry axis of ⟨110⟩, are promising to decouple the effect of crystal anisotropy from other factors. In this work, we synthesized ⟨110⟩ rutile nanowires, which exhibit complex periodic faceting consisting of {101} and {11̅0} facets. In addition to the expected linear width dependence, rutile nanowires, with the same width but different outward-inclined shapes, maintain the same period of their sawtooth faceting, as measured from TEM micrographs. In spite of different surface energy oscillations caused by different shapes, identical nucleation sites, which become energetically favorable during sawtooth growth, are predicted using thermodynamic models for nanowires with different shapes. This finding provides new insights into the morphological control of nanowires.

Atomic Layer Deposition Seeded Growth of Rutile SnO2 Nanowires on Versatile Conducting Substrates.

Extended and oriented rutile nanowires (NWs) hold great promise for numerous applications because of their various tunable physicochemical properties in air and/or solution media, but their direct synthesis on a wide range of conducting substrates remains a significant challenge. Their device performance is governed by relevant NW geometries that cannot be fully controlled to date by varying bulk synthetic conditions. Herein, orientation engineering of rutile SnO2 NWs on a variety of conducting substrates by atomic layer deposition (ALD) seeding has been investigated. The seeded growth controls the nucleation event of the NW, and thicknesses and crystallographic properties of seed layers are the key parameters toward tuning the NW characteristics. The seed layers on carbon cloth produce NWs with highly enhanced electrochemically active surface area, which would show efficient electrochemical CO2 reduction. In addition, the hierarchical architecture resulted from the seeded growth of NWs on SnO2 nanosheets allows thin layers of BiVO4, forming a heterojunction photoanode, which shows a record charge separation efficiency of 96.6% and a charge-transfer efficiency of 90.2% at 1.23 V versus the reversible hydrogen electrode among, to date, the reported BiVO4-based photoanodes for water oxidation. Our study illustrates that such a versatile interfacial engineering effort by the ALD technique would be promising for further wide range of practical applications.

Probing Thermally Activated Atomic and Nanocrystalline Defect Motion Through Noise Processes in RuO2 Nanowires

The present-day nanodevice dimensions continuously shrink, with the aim to prolong Moore's law. As downsizing meticulously persists, undesirable dynamic defects, which cause low-frequency noise and structural instability, play detrimental roles on limiting the ultimate performance and reliability of miniaturized devices. A good understanding and a meaningful control of the defect kinetics then become fundamental and urgent issues. Here we report observations of thermally activated atomic defect motion as well as nanocrystalline defect motion through electrical noise processes in metallic RuO$_2$ rutile nanowires around room temperature. First, we extract the energy distribution function and the number density of mobile atomic defects (oxygen vacancies). Second, we obtain the geometrical size, grain-boundary bonding strength, and relaxation times of dynamic nanocrystallites. Our results show clearly a powerful probe for effective and noninvasive characterizations of nanostructures and nanomaterials for which quantitative information about mechanical hardness, breakdown current density, and/or resistance noise is essential.

Ultrathin TiO2 rutile nanowires enable reversible Mg-ion intercalation

• Novel structures for multi-valent ion battery. • The hydrothermal production of ultrathin TiO 2 nanowires. • Preferable intercalation of Mg 2+ ion into TiO 2 rutile nanowires. • The importance of nanostructure engineering to energy storage. Crystalline ultrathin TiO 2 nanowires with oriented configurations are promising for high-performance energy storage and conversion devices. In this work, we use hierarchical microspheres constructed from radially oriented single-crystalline TiO 2 nanowires as electrode materials for reversible Mg-ion intercalation. The obtained mesoporous TiO 2 nanowires deliver a capacity of 118 mA h g −1 at 20 mA/g current rate. The ultra small size of TiO 2 nanocrystalline is beneficial for reaction kinetics, enhance the bulk intercalation of Mg ions, which remarkably increases storage capacity of TiO 2 . The discharge capacity over 100 cycles remains 105.2 mA h g −1 , demonstrating the superiority of this material structure and its potential in magnesium ion batteries.

Kinetically Favorable Vapor–Adsorbate–Solid Growth of Rutile Nanowires

AbstractThe vapor‐based synthesis of nanowires, particularly vapor–liquid–solid growth and its variants, is an undoubtedly promising method for fabricating high‐quality silicon‐based and group III–V semiconductors. However, the assembly of oxide nanowires has limited success while adopting these methods. Herein, a simple and scalable approach is developed to synthesize single‐crystal oxide nanowires with controllable morphology. Using titanium oxide as the model system for validation, this approach emphasizes the essential role of the surface characteristic of the seed for the space‐confined growth of nanowires. The shape and growth directions of nanowires can be additionally tailored by using bimetallic seeds, with compositional segregations and dissimilar surface characteristics. Since the source material has the same thermodynamic conditions as the produced nanowires within a closed vessel, the results suggest that the growth of nanowires can be dominated by the kinetic enrichment at seed surfaces or referred to as kinetically favorable vapor–adsorbate–solid growth. While identifying the key thermodynamic and kinetic parameters during growth, this approach is applicable for a wide range of materials.

Microwave-assistant hydrothermal synthesis of SnO2@ZnO hierarchical nanostructures enhanced photocatalytic performance under visible light irradiation

In this paper, for the first time, SnO2@ZnO hierarchical nanostructures (HNSs) were synthesized by a simple two-step microwave-assistant hydrothermal method. Systematical characterization of the SnO2@ZnO HNSs revealed that the rutile SnO2 nanowires (NWs) were uniformly and tightly grown on the entire surface of each ZnO nanoflowers (NFs). The photocatalytic performance of the SnO2@ZnO HNSs showed a remarkably enhanced photocatalytic performance under visible light irradiation (400 nm≤λ ≤ 800 nm). The photoluminescence (PL) spectrum results indicated that the defect related energy level is the main mechanism of the photocatalytic performance under the visible light irradiation. The unique morphology and heterojunction structures in SnO2@ZnO HNSs were the core enhanced mechanism in photodegradation process. The advantage endowed with the microwave-assistant hydrothermal method can result in the efficient spatial charge separation between ZnO nanorods (NRs) and SnO2 NWs and reduce the charge recombination along the heterojunction, which makes SnO2@ZnO HNSs a highly promising for photocatalytic applications.

Synthesis and First-principle Calculation of TiO2 Rutile Nanowire Electrodes for Dye-sensitized Solar Cells

In this paper, a TiO2 nanowire film synthesized via a hydrothermal method was prepared as a photoanode for dye-sensitized solar cells (DSSCs). The synthesized TiO2 nanowires were characterized by t ...

Enhanced humidity sensing performance using Sn-Doped ZnO nanorod Array/SnO2 nanowire heteronetwork fabricated via two-step solution immersion

A unique heteronetwork consisting of crystalline tin-doped zinc oxide nanorod arrays and rutile tin oxide nanowires (SZO/SnO2) was fabricated for the first time using a two-step solution immersion method at a low temperature and immersion time (2h) for humidity sensing applications. The average diameter and thickness of the SZO are approximately 95 and 730nm, respectively. The diameter of the SnO2 nanowires ranges from 60nm to 120nm, and the length of the nanowire is approximately 10µm. The SZO/SnO2-based humidity sensor exhibits high stability and superior humidity sensing performance compared with the SZO-, SnO2-, and ZnO/SnO2-based sensors with a sensitivity of 67.8. Results demonstrate that the unique architecture of SZO/SnO2 heteronetwork can enhance the humidity sensing capabilities of the nanowires.

Quantifying and Elucidating Thermally Enhanced Minority Carrier Diffusion Length Using Radius-Controlled Rutile Nanowires.

The minority carrier diffusion length (LD) is a crucial property that determines the performance of light absorbers in photoelectrochemical (PEC) cells. Many transition-metal oxides are stable photoanodes for solar water splitting but exhibit a small to moderate LD, ranging from a few nanometers (such as α-Fe2O3 and TiO2) to a few tens of nanometers (such as BiVO4). Under operating conditions, the temperature of PEC cells can deviate substantially from ambient, yet the temperature dependence of LD has not been quantified. In this work, we show that measuring the photocurrent as a function of both temperature and absorber dimensions provides a quantitative method for evaluating the temperature-dependent minority carrier transport. By measuring photocurrents of nonstoichiometric rutile TiO2-x nanowires as a function of wire radius (19-75 nm) and temperature (10-70 °C), we extract the minority carrier diffusion length along with its activation energy. The minority carrier diffusion length in TiO2-x increases from 5 nm at 25 °C to 10 nm at 70 °C, implying that enhanced carrier mobility outweighs the increase in the recombination rate with temperature. Additionally, by comparing the temperature-dependent photocurrent in BiVO4, TiO2, and α-Fe2O3, we conclude that the ratio of the minority carrier diffusion length to the depletion layer width determines the extent of temperature enhancement, and reconcile the widespread temperature coefficients, which ranged from 0.6 to 1.7% K-1. This insight provides a general design rule to select light absorbers for large thermally activated photocurrents and to predict PEC cell characteristics at a range of temperatures encountered during realistic device operation.

Methanol Photo-Oxidation on Rutile TiO2 Nanowires: Probing Reaction Pathways on Complex Materials

Understanding photochemical processes on nanomaterials is key to developing effective photocatalysts. Herein, methanol oxidation and reduction is used to probe the thermal and photochemical properties of rutile titania nanowires grown using a hydrothermal method. The presence of oxygen vacancy defects leads to methoxy formation and subsequent disproportionation to formaldehyde and methanol at 700 K. Methane and dimethyl ether are also produced in minor quantities. Oxygen adatoms enhance the formation of methoxy, which led to an increase in the disproportionation products and dimethyl ether at high temperature and a decreased amount of methane. The thermal reactivity of the nanowires parallels that of rutile TiO2(110) single crystals. Photo-oxidation of methoxy using UV light produced formaldehyde and methyl formate. These product yields were enhanced on nanowires with oxygen adatoms, but a majority of methoxy (∼70%) is not photoactive. In contrast, all methoxy is photo-oxidized on rutile TiO2(110) when O-adatoms are present. This difference indicates that holes created in the nanowires during UV excitation do not migrate to most of the methoxy—a required step for methoxy photo-oxidation. This lack of activity could be due to either trapping of holes in the material or different binding of the inactive methoxy. These studies demonstrate that while charge carriers can be efficiently created in nanowires differences in chemical properties can suppress photo-oxidation.

100-fold improvement in carrier drift mobilities in alkanephosphonate-passivated monocrystalline TiO2 nanowire arrays

Single crystal rutile titania nanowires grown by solvothermal synthesis are actively being researched for use as electron transporting scaffolds in perovskite solar cells, in low detection limit ultraviolet photodetectors, in photoelectrochemical water-splitting, and in chemiresistive and electrochemical sensing. The electron drift mobility (μn) in solution-grown TiO2 nanowires is very low due to a high density of deep traps, and reduces performance in optoelectronic devices. In this study, the effects of molecular passivation of the nanowire surface by octadecylphosphonic acid (ODPA), on carrier transport in TiO2 nanowire ensembles, were investigated using transient space charge limited current measurements. Infrared spectroscopy indicated the formation of a highly ordered phosphonate monolayer with a high likelihood of bidentate binding of ODPA to the rutile surface. We report the hole drift mobility (μp) for the first time in unpassivated solvothermal rutile nanowires to be 8.2 × 10−5 cm2 V−1 s−1 and the use of ODPA passivation resulted in μp improving by nearly two orders of magnitude to 7.1 × 10−3 cm2 V−1 s−1. Likewise, ODPA passivation produced between a 2 and 3 order of magnitude improvement in μn from ∼10−5–10−6 cm2 V−1 s−1 to ∼10−3 cm2 V−1 s−1. The bias dependence of the post-transit photocurrent decays in ODPA-passivated nanowires indicated that minority carriers were lost to trapping and/or monomolecular recombination for small values of bias (<5 V). Bimolecular recombination was indicated to be the dominant recombination mechanism at higher bias values.

Magnetic, specific heat and electrical transport properties of oxygen-deficient nanosized rutile TiO2−δ

An oxygen-deficient nanosized , sample was synthesized by a solvothermal method, and was characterized to have both ∼3 nm amorphous solid and ∼36–46 nm diameter rutile nanowires. Physical properties of the sample were investigated by measuring magnetic, specific heat, electrical resistance and magnetoresitance properties. DC magnetization M(H) data confirm ferromagnetic behavior previously reported for undoped TiO2. Furthermore, M(T) dependence follows the power-law relation in the near-critical regime, yielding Curie temperature K and critical exponent β = 0.2. Moreover, our results of AC magnetic susceptibility measurements suggest an additional phase transition at K, presumably due to spin orientation. The metallic-like electrical resistance exhibits a distinct drop below with a strong thermal hysteresis in the temperature range 225–275 K. Specific heat in the temperature range 20–300 K is well described by the sum of contributions from acoustic phonons with Debye temperature 605 K and optical phonons with Einstein temperature 113 K. Below 10 K the specific heat divulges a large excess, which can be interpreted as an additional contribution originating from soft potentials.

Graphene-TiO2 mesoporous spheres assembled by anatase and rutile nanowires for efficient NO photooxidation

We design and synthesize a novel graphene embedded orient-assembled TiO2 mesoporous spheres composite (OATMS/GP) evidenced by SEM, TEM and optical spectra. Mesoporous structure, characteristics of single-crystal-like TiO2, mixed phases of anatase and rutile and embedding of graphene have been integrated into a composite system to investigate the structure-property relationship for enhancing photocatalytic activity. Three-dimensional (3D) open mesoporous TiO2 microspheres were well-constructed by the radially oriented self-assembly of single-crystal-like TiO2 nanowires. The aggregation of nanowires leads to the formation of 3D hierarchical microsphere including mesopores that is convenient for light harvesting and adsorption of reaction molecules. Especially, the phase ratio of anatase and rutile in the composite is readily tunable by the change of reaction agents. The incorporation of graphene accelerates the electron transfer and further prolongs the lifetime of the photoinduced electron–hole pairs. This unique structure makes the composite ideal be an efficient photocatalyst for the removal of pollutants. The OATMS/GP composite for degradation of Nitric Oxide (NO) shows an excellently photocatalytic activity and stability during photocatalytic reaction. The results presented here are expected to make a contribution to preparing delicately photocatalytic nanocomposites toward the promising applications for air purification and solar energy utilization.

Enhanced Photoelectrochemical Performance from Rationally Designed Anatase/Rutile TiO2 Heterostructures.

In a photoelectrochemical (PEC) cell for water splitting, the critical issue is charge separation and transport, which is usually completed by designing semiconductor heterojunctions. TiO2 anatase-rutile mixed junctions could largely improve photocatalytic properties, but impairs PEC water splitting performance. We designed and prepared two types of TiO2 heterostructures with the anatase thin film and rutile nanowire phases organized in different sequences. The two types of heterostructures were used as PEC photoanodes for water splitting and demonstrated completely opposite results. Rutile nanowires on anatase film demonstrated enhanced photocurrent density and onset potential, whereas strong negative performance was obtained from anatase film on rutile nanowire structures. The mechanism was investigated by photoresponse, light absorption and reflectance, and electrochemical impedance spectra. This work revealed the significant role of phase sequence in performance gain of anatase-rutile TiO2 heterostructured PEC photoanodes.

Photocatalytic Hydrogen Production Via Water Splitting Using Different Gold-Based Catalysts

In order to avoid serious environmental and economic damages from energy use, humans must stop using fossil fuels altogether, as soon as possible. One possible strategy to cut the dependence of fossil fuels and, at the same time create a new economic force, is to develop a hydrogen based energy economy. A potentially viable way forward is to produce H2 from water by combining solar energy and heterogeneous photocatalysts. For these reasons the objectives of this investigation were: 1) synthesize a high surface area TiO2 nanowires (NWs) catalyst in the rutile phase, 2) incorporate different amount of gold nanoparticles into the as-synthesized catalyst and into the commercial form of TiO2 (Degussa P-25) using a chemical reduction method, 3) produce hydrogen via water splitting using visible and ultraviolet light. The hypotheses of the study were: a) The catalyst with the biggest surface are will produce the biggest amount of hydrogen, and b) The gold nanoparticles will enhance the hydrogen production and will allow the use of visible light, diminishing the semiconductor’s band gap and working as an electron trap. Interestingly, the incorporation of gold nanoparticles into the titania surface enhanced the surface area in both P25 and TiO2 NWs. The hydrogen production obtained by using Au/P25 catalysts was measured to be 800 μmolg-1h-1 under irradiation at 400 nm and 1,436 μmolg-1h-1 using Au/TiO2 NWs at the same wavelength. Both of the hypotheses were correct and all the objectives achieved. The characterization of the synthetized compounds were performed with: 1) X-ray diffraction (XRD) to confirm the crystalline form of the TiO2 synthetized nanowires. 2) Field emission Scanning Electron Microscopy (FESEM), to confirm the morphology of the synthetized compound. 3) Brunauer, Emmett and Teller (BET) instrument to measure the surface area of the gold based nanowires and the commercial TiO2 form P-25. 4) The hydrogen production was measured using a Gas Chromatographer with a Thermal Conductive detector (GC-TCD) attached to a solar simulator. 5) Ultra Violet Visible spectroscopy (UV-VIS) was used to obtain the absorption spectrums of the synthetized TiO2 rutile phase nanowires before and after the gold deposition. Finding renewable sources of energy is one of the biggest challenges of the 21st century and this investigation contributes into that.

Spectroelectrochemical Photoluminescence of Trap States in H-Treated Rutile TiO2 Nanowires: Implications for Photooxidation of Water

Photoluminescence (PL) spectroscopy of intrabandgap trap states is used to investigate the mechanism of photocatalytic water oxidation in pristine and hydrogen-treated rutile TiO2 nanowires. H-treated nanowires annealed at 350 °C were previously shown to be superior to pristine nanowires for the photoelectrochemical evolution of oxygen from water, owing to more efficient charge separation and carrier transport. The present work examines the near-IR PL of rutile nanowires under applied electrochemical potential in order to address the possible influence of hydrogen treatment on electron or hole traps and their role in water oxidation. It is concluded that the near-IR PL of rutile nanowires, identical to that of bulk and conventional nanocrystalline rutile, arises from the radiative recombination of trapped electrons with valence band holes and that valence band holes, rather than trapped holes, are more likely responsible for water oxidation. The shape and intensity of the near-IR PL is found to be invaria...

Hydrothermal synthesis and photoluminescence properties of SnO2 nanowire array and pinecone-like nanoparticles on ITO substrate

The SnO2 nanowire array and pinecone-like nanoparticles were synthesized on the indium tin oxide (ITO) substrate via a simple hydrothermal process. The microstructure and morphology of the samples indicated that both nanowire array and pinecone-like nanoparticles were composed of rutile SnO2 nanowires. Furthermore, a reasonable growth mechanism of SnO2 nanowire array and pinecone-like nanoparticles was proposed, and the molar concentration ratio of NaOH to SnCl4 of the precursor solution played a key role in the morphological evolution. Finally, the photoluminescence properties and the ultraviolet–visible absorbance spectrum were investigated. The band gap of SnO2 nanowire array was about 3.43eV, which was consistent with the emission peak of PL spectrum.

Photohole Induced Corrosion of Titanium Dioxide: Mechanism and Solutions.

Titanium dioxide (TiO2) has been extensively investigated as photoanode for water oxidation, as it is believed to be one of the most stable photoanode materials. Yet, we surprisingly found that TiO2 photoanodes (rutile nanowire, anatase nanotube, and P25 nanoparticle film) suffered from substantial photocurrent decay in neutral (Na2SO4) as well as basic (KOH) electrolyte solution. Photoelectrochemical measurements togehter with electron microscopy studies performed on rutile TiO2 nanowire photoanode show that the photocurrent decay is due to photohole induced corrosion, which competes with water oxidation reaction. Further studies reveal that photocurrent decay profile in neutral and basic solutions are fundamentally different. Notably, the structural reconstruction of nanowire surface occurs simultaneously with the corrosion of TiO2 in KOH solution resulting in the formation of an amorphous layer of titanium hydroxide, which slows down the photocorrosion. Based on this discovery, we demonstrate that the photoelectrochemical stability of TiO2 photoanode can be significantly improved by intentionally coating an amorphous layer of titanium hydroxide on the nanowire surface. The pretreated TiO2 photaonode exhibits an excellent photocurrent retention rate of 97% after testing in KOH solution for 72 h, while in comparison the untreated sample lost 10-20% of photocurrent in 12 h under the same measurement conditions. This work provides new insights in understanding of the photoelectrochemical stability of bare TiO2 photoanodes.

Facile conversion of rutile titanium dioxide nanowires to nanotubes for enhancing the performance of dye-sensitized solar cells

It has been proven that growing longer nanowires (NWs) with larger surface areas is not always a feasible way to increase the conversion efficiency of dye-sensitized solar cells (DSSCs) because the photogenerated electrons and holes may have higher recombination rates as the electron travel distance increases. In this work, we employed a convenient hydrothermal selective etching method to convert single-crystalline rutile TiO2 NW arrays to nanotube (NT) arrays. The resulting NTs retain many characteristics of the initial unetched NWs, such as the orientation, crystal structure, and geometry size (in terms of length and outer diameter). When used as photoanode materials in DSSCs, the as-prepared TiO2 NTs have achieved 112.5% enhancement in the overall power conversion efficiency compared to the initial unetched NWs. The significantly improved performance is credited to 1) the newly created inner surfaces, 2) the increased surface roughness, and 3) the unchanged electron travel distance. Our findings shed light on a new, general way to enhance the performance of the current NW-based DSSCs by simply converting initial NWs to NTs.