Snow-Ice-Inspired Approach for Growth of Amorphous Silicon Nanotips

The growth of one-dimensional nanostructures without a metal catalyst via a simple solution method is of considerable interest due to its practical applications. In this study, the growth of amorphous silicon (a-Si) nanotips was investigated using an aqueous solution dropped onto the Si substrate, followed by drying at room temperature or below for 24 h, resulting in the formation of a-Si nanotips on the Si substrate. Typically, the a-Si nanotips were up to 1.6 μm long, with average top and middle diameters of 30 and 80 nm, respectively, and contained no metal catalyst in their structure. The growth of a-Si nanotips can be explained in terms of the liquid-solid mechanism, where the supercritical Si solution (liquid) generated on the Si substrate (after reaction with the aqueous solution) promotes the nucleation of solid Si (acting as seeds) on the roughened surface, followed by surface diffusion of Si atoms along the side wall of the Si seeds. This is very similar to the phenomenon observed in the growth of snow ice crystals in nature. When photoexcited at 265 nm, the a-Si nanotips showed blue luminescence at around 435 nm (2.85 eV), indicating feasible applicability of the nanotips in optoelectronic functional devices.

Rational Design of Photoelectrodes with Rapid Charge Transport for Photoelectrochemical Applications

Photoelectrode materials are the heart of photoelectrochemical (PEC) cells, which hold great promise to address global energy and environmental issues by converting solar energy into electricity or chemical fuels. In recent decades, significant research efforts have been devoted to the design and construction of photoelectrodes for the efficient generation and utilization of charge carriers to boost PEC performance. Herein, insights from a literature study on the relationship between the architecture and charge dynamics of photoelectrodes are presented. After briefly introducing the fundamental theories of charge dynamics in nanostructured photoelectrodes, the development of photoelectrode design in 1D polycrystalline nanotube arrays, 1D single-crystalline nanowire arrays, and hierarchical and mesoporous nanowire arrays is reviewed with a focus on the interplay between architecture and charge transport properties. For each design, commonly used synthetic approaches and the corresponding charge transport properties are discussed. Subsequently, the applications of these photoelectrodes in PEC systems are summarized. In conclusion, future challenges in the rational design of photoelectrode architecture are presented. The basic relationships between the architectures and charge dynamics of photoelectrode materials discussed here are expected to provide pertinent guidance and a reference for future advanced material design targeting improved light energy conversion systems.

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.

Enhanced photoelectrochemical degradation of Ibuprofen and generation of hydrogen via BiOI-deposited TiO2 nanotube arrays

This study employed BiOI-deposited TiO₂ nanotube arrays (BiOI-TNTAs) electrode in a photoelectrochemical (PEC) system to oxidize Ibuprofen and generate hydrogen in the anodic and cathodic chamber, respectively. FESEM results revealed the diameter of TiO₂ nanotubes was 90-110nm. According to the XRD analysis, the BiOI-TNTAs were dominated by the anatase phase and tetragonal structure of BiOI. XPS results confirmed the coexistence of BiOI in the BiOI-TNTAs associated with Bi (33.76%) and I (8.81%). UV-vis absorption spectra illustrated BiOI-TNTAs exhibit strong absorptions in the visible light region. The PEC method showed the best degradation efficiency for Ibuprofen is a rate constant of 3.21×10^-2min^-1. The results of the Nyquist plot revealed the recombination of photogenerated electron-hole pairs was inhibited as the bias potential was applied. Furthermore, the Bode plot demonstrated the lifetime (τ_el) of photoexcited electrons of BiOI-TNTAs was 1.8 and 4.1 times longer than that of BiOI-Ti and TNTAs, respectively. In the cathodic chamber, the amount of hydrogen generation reached 219.94μM/cm² after 3h of reaction time.

Study on dynamic properties of the photoexcited charge carriers at anatase TiO2 nanowires/fluorine doped tin oxide interface

The photoexcited electrons transfer dynamics at the TiO₂ film/fluorine doped tin oxide (FTO) interface of anatase TiO₂ nanowire arrays (NWAs) and QD-sensitized TiO₂ NWAs films have been studied by using surface photovoltage (SPV) and transient photovoltage (TPV) techniques. Various SPV and TPV responses were obtained when the laser beam was incident from the front side illumination and back side illumination. Based on the work function values of anatase TiO₂ NWAs and FTO, the results indicate that diffusion is the major way for the separation and transfer of the photoexcited charge in the both anatase TiO₂ NWAs and QD-sensitized TiO₂ NWAs films under front side illumination. And the photoexcited charge were separated by drift under the built-in electric field at the TiO₂ film/FTO interface for anatase TiO₂ NWAs and QD-sensitized TiO₂ NWAs films under back side illumination. In addition, under back side illumination the built-in electric field and band structure of CdS/CdSe QDs and anatase TiO₂ NWAs lead to the separation and transfer of the photoexcited charge for CdS/CdSe QDs sensitized TiO₂ NWAs/FTO film. As the intensity of illumination increases, the effect of built-in electric field on the separation and transfer of the photoexcited charge in the QD-sensitized TiO₂ NWAs film decreases.

Photoelectrochemical oxidation of ibuprofen via Cu2O-doped TiO2 nanotube arrays

A p-n junction based Cu2O-doped TiO2 nanotube arrays (Cu2O-TNAs) were synthesized and used as a working anode in a photoelectrochemical (PEC) system. The results revealed that the Cu2O-TNAs were dominated by the anatase phase and responded significantly to visible light. XPS analyses indicated that with an amount of 24.79% Cu doping into the structure, the band gap of Cu2O-TNAs was greatly reduced. SEM images revealed that the supported TiO2 nanotubes had diameters of approximately 80nm and lengths of about 2.63μm. Upon doping with Cu2O, the TiO2 nanotubes maintained their structural integrity, exhibiting no significant morphological change, favoring PEC applications. Under illumination, the photocurrent from Cu2O/TNAs was 2.4 times larger than that from TNAs, implying that doping with Cu2O significantly improved electron mobility by reducing the rate of recombination of electron-hole pairs. The EIS and Bode plot revealed that the estimated electron lifetimes, τel, of TNAs and Cu2O/TNAs were 6.91 and 26.26ms, respectively. The efficiencies of degradation of Ibuprofen by photoelectrochemical, photocatalytic (PC), electrochemical (EC) and photolytic (P) methods were measured.

Computational Analysis of the Optical and Charge Transport Properties of Ultrasonic Spray Pyrolysis-Grown Zinc Oxide/Graphene Hybrid Structures

We demonstrate a systematic computational analysis of the measured optical and charge transport properties of the spray pyrolysis-grown ZnO nanostructures, i.e. nanosphere clusters (NSCs), nanorods (NRs) and nanowires (NWs) for the first time. The calculated absorbance spectra based on the time-dependent density functional theory (TD-DFT) shows very close similarity with the measured behaviours under UV light. The atomic models and energy level diagrams for the grown nanostructures were developed and discussed to explain the structural defects and band gap. The induced stresses in the lattices of ZnO NSCs that formed during the pyrolysis process seem to cause the narrowing of the gap between the energy levels. ZnO NWs and NRs show homogeneous distribution of the LUMO and HOMO orbitals all over the entire heterostructure. Such distribution contributes to the reduction of the band gap down to 2.8 eV, which has been confirmed to be in a good agreement with the experimental results. ZnO NWs and NRs exhibited better emission behaviours under the UV excitation as compared to ZnO NSCs and thin film as their visible range emissions are strongly quenched. Based on the electrochemical impedance measurement, the electrical models and electrostatic potential maps were developed to calculate the electron lifetime and to explain the mobility or diffusion behaviours in the grown nanostructure, respectively.

Ultralong Rutile TiO2 Nanowire Arrays for Highly Efficient Dye-Sensitized Solar Cells

Vertically aligned rutile TiO2 nanowire arrays (NWAs) with lengths of ∼44 μm have been successfully synthesized on transparent, conductive fluorine-doped tin oxide (FTO) glass by a facile one-step solvothermal method. The length and wire-to-wire distance of NWAs can be controlled by adjusting the ethanol content in the reaction solution. By employing optimized rutile TiO2 NWAs for dye-sensitized solar cells (DSCs), a remarkable power conversion efficiency (PCE) of 8.9% is achieved. Moreover, in combination with a light-scattering layer, the performance of a rutile TiO2 NWAs based DSC can be further enhanced, reaching an impressive PCE of 9.6%, which is the highest efficiency for rutile TiO2 NWA based DSCs so far.

Cu2O loaded titanate nanotube arrays for simultaneously photoelectrochemical ibuprofen oxidation and hydrogen generation

A p-n junction Cu2O doped TiO2 nanotube arrays (Cu2O/TNAs) were synthesized by square wave voltammetry electrochemical (SWVE) deposition method and employed as the working anode. The crystalline, optical properties, surface morphology, and structure of the Cu2O/TNAs were characterized by XRD, UV-vis absorbance edges, SEM, and XPS. Results showed that the Cu2O/TNAs were dominated by anatase phase after sintering at 450 °C with significant visible light response. XPS finding confirmed XRD results that the copper element in Cu2O/TNAs was Cu (I) instead of Cu (II). SEM images illustrated the diameter and the length of supported TiO2 nanotubes was approximately 100 nm and 2.75-4.34 μm, respectively. After Cu2O doping, the nano-tubular structure of TiO2 nanotube kept its integrity with no significant morphological change, which was beneficial for PEC applications. The photocurrent of Cu2O/TNAs was 1.45 times larger than that of TNAs, implying that Cu2O doping significantly enhanced electron mobility by reducing the recombination of electron-hole pairs. In addition, electrochemical impedance spectroscopy (EIS) measurements revealed that the recombination of photogenerated electron-hole pairs was inhibited as the bias potential was applied. Results of Bode plot further demonstrated that the electron lifetime τel of Cu2O/TNAs-20 (30.79 ms), under 0.5 V bias potential, was about 2.23 times higher than that of pure TNAs (13.82 ms). Results of electron spin resonance (ESR) analyses demonstrate that the hydroxyl radicals (OH) are responsible for the PEC decomposition of Ibuprofen.

Efficiency enhanced rutile TiO2 nanowire solar cells based on an Sb2S3 absorber and a CuI hole conductor

The spray technique is introduced for CuI deposition on Sb₂S₃-sensitized TiO₂ nanowire solar cells, which enhances the photovoltaic performance of the device.

Bilayer TiO2photoanode consisting of a nanowire–nanoparticle bottom layer and a spherical voids scattering layer for dye-sensitized solar cells

A bilayer TiO₂photoanode prepared by a one-time spray technique on a TiO₂NW array shows significantly enhanced photovoltaic performance in DSSCs.

Concordantly fabricated heterojunction ZnO–TiO2 nanocomposite electrodes via a co-precipitation method for efficient stable quasi-solid-state dye-sensitized solar cells

ZnO/TiO₂ nanocomposites supported on an FTO substrate are used as the photoanode electrode for quasi-solid-state dye-sensitized solar cells.

Titania nanobundle networks as dye-sensitized solar cell photoanodes

Quasi-one-dimensional (1D) titania nanobundles were synthesized via a hydrothermal method and used to print random network nanostructured films. These films are shown to be ideally suited for application as photoanodes in dye-sensitized solar cells (DSCs) as they have a higher porosity compared to the traditional 1D nanostructured TiO2 materials. Devices constructed using the N719 dye and iodide/triiodide as the redox mediator in the electrolyte yielded energy conversion efficiencies (η = 6.1 ± 0.2%), which were marginally lower than for devices made with the commonly used P25 titania films (η = 6.3 ± 0.1%) under one sun simulated solar radiation. Application of an electrolyte based on the [Co(bpy)3](2+/3+) redox couple and the MK2 organic sensitizer resulted in higher efficiencies (η = 7.70 ± 0.1%) than for the P25 devices (η = 6.3 ± 0.3%). Each performance parameter (short circuit current density, open circuit voltage and fill factor) was higher for the TiO2 nanobundle devices than those for the P25-based devices. The results of electrochemical impedance spectroscopy (EIS), intensity-modulated photovoltage spectroscopy (IMVS), and dye-loading measurements indicated that the better performance of TiO2 nanobundle devices with cobalt electrolytes correlates with higher porosity, relatively fast electron transport and more efficient suppression of electron recombination. A faster rate of diffusion of the cobalt complexes through the highly porous TiO2 nanobundle network is proposed to contribute to the enhanced device efficiency.

A facile solution route to deposit TiO2 nanowire arrays on arbitrary substrates

A facile solution-based technique was developed to grow vertically aligned TiO2 nanowires with predominantly anatase phase on arbitrary substrates of stainless steel, glass, silicon wafer and carbon cloth at the low temperature of 80 °C and in an open atmosphere.

Multiple step growth of single crystalline rutile nanorods with the assistance of self-assembled monolayer for dye sensitized solar cells

A novel multiple step growth (MSG) process has been developed to synthesize rutile nanorods (NRs) on fluorine-doped tin oxide (FTO) glass with the assistance of a self-assembled monolayer (SAM) aiming to increase the internal surface area of the 1D materials for dye sensitized solar cell (DSSC) applications. The experimental result reveals that the SAM layer can be selectively decomposed at the tip of the nanorod, namely the rutile (001) surface, due to the anisotropic photocatalytic property of the rutile. The remaining SAM layer on the side-wall of the NRs remains intact and serves as water repellent which prevents the radial growth of the NRs during the next step hydrothermal synthesis; therefore, the spacing between the NRs and the porosity of the NR array can be retained after additional growth cycles. On the other hand, introduction of a middle layer formed via TiCl4 solution treatment before the next growth cycle is found to be an effective way to control the diameters of the newly grown NRs. The performance of DSSC made from the rutile NRs grown using the MSG technique has been examined, and it is significantly affected by the internal surfaces of the NRs. Furthermore, the MSG combined with NR etching treatment by acid at low temperature (150 °C) leads to a significant enhancement in the solar cell performance. The gigantic wettability difference of the NRs before and after the SAM treatment as well as the MSG method could be adapted to prepare superhydrophobic and superhydrophilic nanostructured patterns for other applications.

Hydrothermal fabrication of hierarchically anatase TiO2 nanowire arrays on FTO glass for dye-sensitized solar cells

Hierarchical anatase TiO₂ nano-architecture arrays consisting of long TiO₂ nanowire trunk and numerous short TiO₂ nanorod branches on transparent conductive fluorine-doped tin oxide glass are successfully synthesized for the first time through a facile one-step hydrothermal route without any surfactant and template. Dye-sensitized solar cells based on the hierarchical anatase TiO₂ nano-architecture array photoelectrode of 18 μm in length shows a power conversion efficiency of 7.34% because of its higher specific surface area for adsorbing more dye molecules and superior light scattering capacity for boosting the light-harvesting efficiency. The present photovoltaic performance is the highest value for the reported TiO₂ nanowires array photoelectrode.

One-step synthesis of vertically aligned anatase thornbush-like TiO2 nanowire arrays on transparent conducting oxides for solid-state dye-sensitized solar cells

Herein, we report a facile synthesis of high-density anatase-phase vertically aligned thornbush-like TiO2 nanowires (TBWs) on transparent conducting oxide glasses. Morphologically controllable TBW arrays of 9 μm in length are generated through a one-step hydrothermal reaction at 200 °C over 11 h using potassium titanium oxide oxalate dehydrate, diethylene glycol (DEG), and water. The TBWs consist of a large number of nanoplates or nanorods, as confirmed by SEM and TEM imaging. The morphologies of TBWs are controllable by adjusting DEG/water ratios. TBW diameters gradually decrease from 600 (TBW600) to 400 (TBW400) to 200 nm (TBW200) and morphologies change from nanoplates to nanorods with an increase in DEG content. TBWs are utilized as photoanodes for quasi-solid-state dye-sensitized solar cells (qssDSSCs) and solid-state DSSCs (ssDSSCs). The energy-conversion efficiency of qssDSSCs is in the order: TBW200 (5.2%)>TBW400 (4.5%)>TBW600 (3.4%). These results can be attributed to the different surface areas, light-scattering effects, and charge transport rates, as confirmed by dye-loading measurements, reflectance spectroscopy, and incident photon-to-electron conversion efficiency and intensity-modulated photovoltage spectroscopy/intensity-modulated photocurrent spectroscopy analyses. TBW200 is further treated with a graft-copolymer-directed organized mesoporous TiO2 to increase the surface area and interconnectivity of TBWs. As a result, the energy-conversion efficiency of the ssDSSC increases to 6.7% at 100 mW cm(-2) , which is among the highest values for N719-dye-based ssDSSCs.

Site-specific nucleation and controlled growth of a vertical tellurium nanowire array for high performance field emitters

We report the controlled growth of highly ordered and well aligned one-dimensional tellurium nanostructure arrays via a one-step catalyst-free physical vapor deposition method. The density, size and fine structures of tellurium nanowires are systematically studied and optimized. Field emission measurement was performed to display notable dependence on nanostructure morphologies. The ordered nanowire array based field emitter has a turn-on field as low as 3.27 V μm(-1) and a higher field enhancement factor of 3270. Our finding offers the possibility of controlling the growth of tellurium nanowire arrays and opens up new means for their potential applications in electronic devices and displays.