Single-crystal TiO2 Nanowire Research Articles

Dialysate regeneration via urea photodecomposition with TiO2 nanowires at therapeutic rates.

The standard weekly treatment for end-stage renal disease patients is three 4-h-long hemodialysis sessions with each session c'onsuming over 120 L of clean dialysate, which prevents the development of portable or continuous ambulatory dialysis treatments. The regeneration of a small (~1L) amount of dialysate would enable treatments that give conditions close to continuous hemostasis and improve patient quality of life through mobility. Small-scale studies have shown that nanowires of TiO2 are highly efficient at photodecomposing urea into CO2 and N2 when using an applied bias and an air permeable cathode. To enable the demonstration of a dialysate regeneration system at therapeutically useful rates, a scalable microwave hydrothermal synthesis of single crystal TiO2 nanowires grown directly from conductive substrates was developed. These were incorporated into 1810 cm2 flow channel arrays. The regenerated dialysate samples were treated with activated carbon (2min at 0.2g/mL). The photodecomposition system achieved the therapeutic target of 14.2g urea removal in 24 h. TiO2 electrode had a high urea removal photocurrent efficiency of 91%, with less than 1% of the decomposed urea generating NH4 + (1.04 μg/h/cm2 ), 3% generating NO3 - and 0.5% generating chlorine species. Activated carbon treatment could reduce total chlorine concentration from 0.15 to <0.02 mg/L. The regenerated dialysate showed significant cytotoxicity which could be removed by treatment with activated carbon. Additionally, a forward osmosis membrane with sufficient urea flux can cut off the mass transfer of the by-products back into the dialysate. Urea could be removed from spent dialysate at a therapeutic rate using a TiO2 based photooxidation unit, which can enable portable dialysis systems.

Carbon quantum dots induced one-dimensional ordered growth of single crystal TiO2 nanowires while boosting photoelectrochemistry properties

TiO2 nanowire arrays (TNWAs) thin film of flexible substrates has great application potential in photoelectrochemical (PEC) hydrogen evolution. Moreover, the photoquantum efficiency of TNWAs can be effectively enhanced by reducing the nanoscale as much as possible or combining with other co-catalysts. This work presents a simple and effective strategy for the synthesis of ultrafine TiO2 nanowire arrays by a one-step hydrothermal method using CQDs as an inducer for single crystal growth. Meanwhile, CQDs recombine on the surface of TNWAs to form CQDs/TNWAs heterojunction during the synthesis process. The synthesized TNWAs have well ordered separability, whose average diameter is regulated at 11.21 nm with an average aspect ratio of 1097. PEC analysis showed that the performance of TNWAs was significantly improved compared with initial TiO2 with unregulated morphology. The charge separation efficiency increased by 120%, applied bias photon-to-current efficiency (ABPE) reached 0.37%(0.68 v vs RHE), and incident photon-to-current conversion efficiency (IPCE) reached 50.31% (380 nm). The influence of TiO2 nano-morphology optimization and formation of CQDs/TiO2 heterojunction on the PEC performance were discussed in detail.

Self-rectifying and forming-free nonvolatile memory behavior in single-crystal TiO2 nanowire memory device

Vertically aligned single-crystal TiO2 nanowire with preferred orientation along the [001] direction has been directly synthesized using a simple low-temperature hydrothermal process. A stable self-rectifying and forming-free nonvolatile memory behavior with a high resistance ratio of more than two orders of magnitude has been achieved in the TiO2 nanowire memory device, which can be remained for over 103 s and without any obvious decline. Furthermore, the memory behavior in the low resistance state is attributed to the Ohmic conduction mechanism, while the high resistance state is governed by the trap-regulated SCLC mechanism. In addition, the metallic conductive filament model adjusted by oxygen vacancies has been proposed, in which the self-rectifying and forming-free nonvolatile memory behavior could be ascribed to the formation and erasure of the Magnéli phase conductive filament in the TiO2 nanowire memory device. This work suggests the wide prospects of the TiO2 nanowire memory device for next-generation nonvolatile memory applications.

Length-independent charge transport of well-separated single-crystal TiO2 long nanowire arrays.

Long, well-separated single crystal TiO2 nanowire (NW) arrays with rapid charge transport properties hold great promise in photoelectrochemical and energy storage devices. Synthesis variations to increase the NWs length generally result in the widening of the NWs and fusion at their roots which, in turn, increases the structural disorder and slows charge transport. As such, well-separated single-crystal TiO2 NW arrays with rapid charge transport properties have been limited to lengths of about 3-4 μm. In this work, by adjusting the HCl/DI-water ratio and adding specific organic ligands to the reaction solution that slow the lateral growth rate we achieve well-separated single-crystal rutile TiO2 NW arrays with a length of ∼10 μm and an aspect ratio of approximately 100. The charge transport is 100 times faster than that of nanoparticle films and remarkably exhibits length-independence, a behavior that can be attributed to the well-separated architecture. The synthesis strategy can be extended to the fabrication of other well-separated metal oxide NW arrays and represents an important tool in achieving high performance photoelectrochemical and electrical energy storage devices.

Correction: Single-crystal TiO2 nanowires by seed assisted thermal oxidation of Ti foil: synthesis and photocatalytic properties

Correction for ‘Single-crystal TiO2 nanowires by seed assisted thermal oxidation of Ti foil: synthesis and photocatalytic properties’ by E. Arcadipane et al., RSC Adv., 2016, 6, 55490–55498.

Understanding and removing surface states limiting charge transport in TiO2 nanowire arrays for enhanced optoelectronic device performance.

Charge transport within electrode materials plays a key role in determining the optoelectronic device performance. Aligned single-crystal TiO2 nanowire arrays offer an ideal electron transport path and are expected to have higher electron mobility. Unfortunately, their transport is found not to be superior to that in nanoparticle films. Here we show that the low electron transport in rutile TiO2 nanowires is mainly caused by surface traps in relatively deep energy levels, which cannot be removed by conventional approaches, such as oxygen annealing treatment. Moreover, we demonstrate an effective wet-chemistry approach to minimize these trap states, leading to over 20-fold enhancement in electron diffusion coefficient and 62% improvement in solar cell performance. On the basis of our results, the potential of TiO2 NWs can be developed and well-utilized, which is significantly important for their practical applications.

Confined-space synthesis of single crystal TiO₂ nanowires in atmospheric vessel at low temperature: a generalized approach.

Extensive efforts have been devoted to develop innovative synthesis strategies for nanomaterials in order to exploit the true potential of nanotechnology. However, most approaches require high temperature or high pressure to favor crystallization. Here we highlight an unconventional approach for the confined-space synthesis of the single crystal TiO2 nanowires in the atmospheric vessel at low temperature by cleverly manipulating the unique physical properties of straight-chain saturated fatty acids. Our method also applys to icosane due to its straight-chain saturated hydrocarbon structure and similar physical properties to the saturated fatty acids. Interestingly, we also found that the unsaturated fatty acids can facilitate the crystal growth, but their bent chains lead to the formation of TiO2 particle aggregates. In addition, we demonstrate the growth of TiO2 nanowires on arbitrary substrates, which are of great importance for their wider applications. We thus anticipate our presented method to be a possible starting point for non-classical crystallization strategies and be easily adapted for the fabrication of all other inorganic materials.

Investigation of photoinduced electron transfer on TiO2 nanowire arrays/porphyrin composite via scanning electrochemical microscopy

A photo-induced electron transfer (PET) system was constructed by the combination of vertically aligned single-crystal TiO2 nanowire arrays and porphyrin to investigate the mechanism of the charge transfer process in artificial photosynthesis.

Facile one-pot controlled synthesis of Sn and C codoped single crystal TiO2 nanowire arrays for highly efficient photoelectrochemical water splitting

Here we report a (Sn, C) cation–anion codoped single crystal TiO2 nanowire (NW) arrays as a highly efficient solar water splitting photoelectrode, able to be fabricated using a facile one pot hydrothermal synthesis method. The synergetic effects of codoping on the photoelectrochemical activity of the photoanode were investigated and compared to undoped and monodoped photoelectrodes. The (Sn, C) codoped TiO2 NW photoanode generated the highest saturated photocurrent density i.e. 2.8mA/cm2 at 1.23V vs RHE while yielding a maximum solar energy conversion efficiency of 1.32% at a potential of 0.55V vs RHE – representing 60%, 94%, and 100% efficiency improvements compared to undoped, Sn doped, and C doped TiO2 NW respectively at same potential. This improvement is attributed to the synergetic effects of Sn and C codopants to lower recombination and enhance life time of photogenerated charge-separated carriers on the surface states that lead to efficient hole transfer at the photoelectrode/electrolyte interface. In addition, an increased charge carrier density and conductivity (as evidenced from electrochemical impedance spectroscopy) and the enhanced incorporation of dopants in the codoped system, compared to monodoped system as quantified by XPS, highlights the importance of codoping. The nanomaterial was characterized by XRD, TEM, SEM and Raman UV–vis measurements. This study will guide improvements in the efficiency of TiO2 for PEC water splitting using optimized codopant pairs.

Synthesis of Single-Crystal TiO2 Nanowire Using Titanium Monoxide Powder by Thermal Evaporation

TiO2 nanowires were synthesized successfully in a large quantity by thermal evaporation using titanium monoxide powder as precursor. X-ray diffraction results showed that all the products were pure rutile phase of TiO2. According to microstructural observations, the nanowires have two typical morphologies, a long straight type and a short tortuous type. The straight nanowires were obtained at a wide temperature range of 900–1050 °C, while the tortuous ones were formed below 900 °C. Transmission electron microscopy characterization revealed that both the straight and the tortuous nanowires are single-crystal rutile TiO2. The preferential growth direction of the nanowires was determined as [110] orientation according to electron diffraction and high-resolution image analyses. The morphological change of TiO2 nanowires was discussed by considering the different atomic diffusion rates of Ti atoms caused by the phase transformation in Ti substrate at around 900 °C.

Synthesis of long TiO2 nanowire arrays with high surface areas via synergistic assembly route for highly efficient dye-sensitized solar cells

Vertically aligned single crystal TiO2 nanowire arrays grown on transparent conductive substrates are of considerable interest for use as photoanodes in dye-sensitized solar cells because they can provide direct pathways that ensure the rapid collection of charge carriers generated throughout the cells. However, growth of TiO2 nanowire arrays on conductive glass with the combined characteristics of a large surface area and lengths up to tens of micrometers is still challenging and desirable for highly efficient solar cells. Here, we reported a mild hydrothermal approach for growing vertically aligned TiO2 nanowire arrays directly on conductive glass via the synergistic interaction of the octanoic acid and titanium trichloride. The resulting single crystal TiO2 nanowire arrays possess a large surface area of 95 m2 g−1 and a controlled length in the range of 6–46 μm. By applying 9.6 μm-long nanowire arrays in dye-sensitized solar cells, an overall photoconversion efficiency of 5.13% is achieved under a white light illumination of 100 mW cm−2.

Formation of Bi4Ti3O12 One-Dimensional Structures by Solid-State Reactive Diffusion. From Core−Shell Templates to Nanorods and Nanotubes

One-dimensional nanostructures are usually fabricated using liquid-phase chemical methods or growth from the vapor phase. A different approach is proposed in this investigation. Using two different kinds of reactive templates with core−shell structure obtained using colloidal chemistry methods, we fabricated for the first time Bi4Ti3O12 nanorods and nanotubes by means of solid-state reactions. The intimate contact of the parent compounds in the core−shell template resulted in low reaction temperatures (500−600 °C), overcoming the morphological instability problem of one-dimensional (1D) templates. As the formation of bismuth titanate is dominated by the coupled ambipolar diffusion of Bi3+ and O2− ions through the titanate layer, the final morphology, nanorod or nanotube, is determined by the relative arrangement of the parent compounds in the core−shell template. When the bismuth precursor is located in the shell region and the core is a single-crystal TiO2 nanowire, formation of a single-crystal Bi4Ti3O12 nanorod occurs by a topochemical reaction. Opposite, when the shell corresponds to nanocrystalline TiO2 and the core to the bismuth precursor, the reaction leads to a porous nanorod or a nanotube depending on the template thickness. The development of the central cavity is ascribed to the coalescence of Kirkendall porosity and the effect of mechanical stresses accumulated during the reaction. The present results indicate that a careful control of the solid-state reactions at the nanoscale can be achieved by means of reactive templates with core−shell structure thus providing an effective tool for the design and the large-scale fabrication of a variety of nano-objects with different dimensionalities.

TiO2 Nanowire Growth Driven by Phosphorus-Doped Nanocatalysis

Ni-catalyzed single-crystal TiO2 nanowire growth was observed to occur well below the bulk Ni melting point when a small amount of P was present. TEM, SAED, EDXS, and EELS analyses as well as consideration of the Ni/P phase diagram point to a previously unreported mechanism of nanowire growth catalyzed by a liquid Ni/P eutectic shell surrounding a solid Ni core. High-resolution elemental analysis supports the conclusion that the active catalyst is the outer liquid Ni/P layer with P present at a 3−8% level surrounding a solid Ni core. Growth proceeds by precursor adsorption onto the liquid layer followed by diffusion to the growing surface of the nanowire. This catalyst system produces rutile TiO2 nanowires efficiently. We believe that the technique is generalizable to other metal/dopant systems which could lead to the synthesis of other hard-to-synthesize nanowires.

Oriented single crystalline titanium dioxide nanowires

We report the synthesis of oriented single crystalline titanium dioxide(TiO2)nanowire arrays on titanium foil. The synthesis method relies on the ability to grow single crystal sodiumtitanate (Na2Ti2O5·H2O) nanowires on titanium foil through a novel alkali hydrothermal growth process. Following growth,the Na2Ti2O5·H2O nanowires are converted to protonated bititanate(H2Ti2O5·H2O) nanowires through an ion-exchange reaction without changing their morphology or crystalstructure. Finally, the protonated bititanate nanowires are converted to single crystalline anataseTiO2 nanowires through a topotactic transformation by calcination. These three sequential steps yield a carpetof 2–50 µm long single crystalline nanowires oriented in the [100] direction and primarilynormal to the titanium foil. Even longer nanowires can be grown. The single crystalTiO2 nanowire arrays on flexible titanium substrate may be used in photocatalytic andphotovoltaic devices such as dye-sensitized solar cells and may enhance theirperformance by providing fast electron transport. The nanowires can also be used astemplates for producing hierarchical nanostructures such as nanowires decorated withnanoparticles on their periphery or nanotubes with walls made of nanoparticles.