Flexible, transferable, and thermal-durable dye-sensitized solar cell photoanode consisting of TiO₂ nanoparticles and electrospun TiO₂/SiO₂ nanofibers

Room-temperature preparation of TiO2/graphene composite photoanodes for efficient dye-sensitized solar cells

Semi-transparent TiO₂/graphene photoanodes are prepared at room temperature via an electrophoretic deposition method followed by compression and applied in dye-sensitized solar cells (DSSCs). Compression enhances the power conversion efficiency (PCE) of a DSSC, which constitutes up 18.4 times improvement compared to the uncompressed device. Incorporating graphene into the compressed film further improves the PCE by 28.8%. Simultaneously, compressing and graphene incorporating can greatly increase the film's transmittance at long wavelengths, benefiting to the use of DSSCs as front unit in tandem solar cells. Scanning electron microscopy, porosity measurements, electrochemical impedance spectroscopy and open circuit voltage decay are performed to investigate the mechanisms. It is demonstrated that compressing a film can reduce the porosity and improve the inter-particle connections, which accounts for the increased light transmittance and enhanced PCE. The incorporated graphene can provide extra charge carrier pathway due to its excellent charge transport properties, as well as protect TiO₂ nanostructure by preventing film cracking upon pressing due to its good flexibility, thus increases PCE to 6.75%, which, to our best knowledge, is the highest value among DSSCs with room-temperature prepared photoanodes.

Progress of electrospray and electrospinning in energy applications

In the promotion of energy strategies to address the global energy crisis, nanotechnology has been successfully used to generate novel energy materials with excellent characteristics, such as high specific surface area, good flexibility and large porosity. Among the various methods for fabricating nanoscale materials, electrospray and electrospinning technologies have unlocked low-cost, facile and industrial routes to nanotechnology over the past ten years. This review highlights research into the key parts and primary theory of these techniques and their application in preparing energy-related materials and devices: especially fuel cells, solar cells, lithium ion batteries, supercapacitors as well as hydrogen storage systems. The challenges and future prospects of the manufacturing technologies are also covered in this paper.

Structural and Optical Properties of Nanocrystalline TiO2 with Multiwalled Carbon Nanotubes and Its Photovoltaic Studies Using Ru(II) Sensitizers

In this study, the in situ sol-gel method has been deployed to prepare the titanium dioxide/multiwalled carbon nanotubes (TiO₂/MWCNTs) nanocomposite (NCs) powders with varying content of MWCNTs (0.01-1.0 wt %), to construct the dye-sensitized solar cells (DSSCs). First, binder-free NCs were deposited on a transparent-conducting F:SnO₂ (FTO) glass substrate by a doctor-blade technique and then anchored with Ru(II)-based dyes to either N719 or ruthenium phthalocyanine (RuPc). The structural and optical properties and interconnectivity of the materials within the composite are investigated thoroughly by various spectral techniques (XRD, XPS, Raman, FT-IR, and UV-vis), electron microscopy (HRTEM), and BET analysis. The experimental results suggest that the ratio of MWCNTs and TiO₂ in NCs, morphology, and their interconnectivity influenced their structural, optical, and photovoltaic properties significantly. Finally, the photovoltaic performances of the assembled DSSCs with different content of MWCNTs to TiO₂ films anchored with two different dyes were tested under one sun irradiation (100 mW/cm²). The measured current-voltage (IV) curve and incident photon-to-current conversion efficiency (IPCE) spectra of TiO₂/0.1 wt % MWCNTs (T@0.1 C) for N719 dye show three times more power conversion efficiency (η = 6.21%) which is opposed to an efficiency (η = 2.07%) of T@0.1 C for RuPc dye under the same operating conditions.

Soft Zr-doped TiO2 Nanofibrous Membranes with Enhanced Photocatalytic Activity for Water Purification

Self-standing photocatalytic membranes constructed from TiO₂ nanofibers hold great promise in environmental remediation; however, challenges still remained for the poor mechanical properties of polycrystalline TiO₂ nanofibers. Herein, soft Zr-doped TiO₂ (TZ) nanofibrous membranes with robust mechanical properties and enhanced photocatalytic activity were fabricated via electrospinning technique. The Zr⁴⁺ incorporation could effectively inhibit the grain growth and reduce the surface defects and breaking point of TiO₂ nanofiber. The as-prepared TZ membranes were composed of well-interconnected nanofibers with a high aspect ratios, small grain size and pore size, which exhibited good tensile strength (1.32 MPa) and showed no obvious damage after 200 cycles of bending to a radius of 2 mm. A plausible bending deformation mechanism of the soft TZ membranes was proposed from microscopic single nanofiber to macroscopical membranes. Moreover, the resultant TZ membranes displayed better photocatalytic performance for methylene blue degradation compared to a commercial catalyst (P25), including high degradation degree of 95.4% within 30 min, good reusability in 5 cycles, and easiness of recycling. The successful preparation of such fascinating materials may open up new avenues for the design and development of soft TiO₂-based membranes for various application.

SiO2/TiO2 Nanocomposite Films on Polystyrene for Light-Induced Cell Detachment Application

Light-induced cell detachment shows much potential in in vitro cell culture and calls for high-performance light-responsive films. In this study, a smooth and dense SiO₂/TiO₂ nanocomposite thin film with thickness of around 250 nm was first fabricated on H₂O₂ treated polystyrene (PS) substrate via a low-temperature sol-gel method. It was observed that the film could well-adhere on the PS surface and the bonding strength became increasingly high with the increase of SiO₂ content. The peeling strength and shear strength reached 3.05 and 30.02 MPa, respectively. It was observed the surface of the film could transform into superhydrophilic upon 20 min illumination of ultraviolet with a wavelength of 365 nm (UV365). In cell culture, cells, i.e., NIH3T3 and MC3T3-E1 cells, cultured on SiO₂/TiO₂ nanocomposite film were easily detached after 10 min of UV365 illumination; the detachment rates reached 90.8% and 88.6%, respectively. Correspondingly, continuous cell sheets with good viability were also easily obtained through the same way. The present work shows that SiO₂/TiO₂ nanocomposite thin film could be easily prepared on polymeric surface at low temperature. The corresponding film exhibits excellent biocompatibility, high bonding strength, and good light responses. It could be a good candidate for the surface of cell culture utensils with light-induced cell detachment property.

Tailored Fabrication of Transferable and Hollow Weblike Titanium Dioxide Structures

The preparation of weblike titanium dioxide thin films by atomic layer deposition on cellulose biotemplates is reported. The method produces a TiO₂ web, which is flexible and transferable from the deposition substrate to that of the end application. Removal of the cellulose template by calcination converts the amorphous titania to crystalline anatase and gives the structure a hollow morphology. The TiO₂ webs are thoroughly characterized using electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy to give new insight into manufacturing of porous titanium dioxide structures by means of template-based methods. Functionality and integrity of the TiO₂ hollow weblike thin films were successfully confirmed by applying them as electrodes in dye-sensitized solar cells.

Robotic Deposition of TiO2 Films on Flexible Substrates from Hybrid Inks: Investigation of Synthesis-Processing-Microstructure-Photocatalytic Relationships

TiO2 is an important material widely used in optoelectronic devices due to its semiconducting and photocatalytic properties, nontoxicity, and chemically inert nature. Some indicative applications include water purification systems and energy harvesting. The use of solution, water-based inks for the direct writing of TiO2 on flexible substrates is of paramount importance since it enables low-cost and low-energy intensive large-area manufacturing, compatible with roll-to-roll processing. In this work we study the effect of crystalline TiO2 and polymer addition on the rheological and direct writing properties of Ti-organic/TiO2 inks. We also report on the bridging crystallite formation from the Ti-organic precursor into the TiO2 crystalline phase, under ultraviolet (UV) exposure or mild heat treatments up to 150 °C. Such crystallite formation is found to be enhanced by polymers with strong polarity and pKα such as polyacrylic acid (PAA). X-ray diffraction (XRD) coupled with Raman and X-ray photoelectron (XPS) spectroscopy are used to investigate the crystalline-phase transformation dependence based on the initial TiO2 crystalline-phase concentration and polymer addition. Transmission electron microscopy imaging and selected area electron diffraction patterns confirm the crystalline nature of such bridging printed structures. The obtained inks are patterned on flexible substrates using nozzle-based robotic deposition, a lithography-free, additive manufacturing technique that allows the direct writing of material in specific, digitally predefined, substrate locations. Photocatalytic degradation of methylene blue solutions highlights the potential of the studied films for chemical degradation applications, from low-cost environmentally friendly materials systems.

Facile fabrication of organic/inorganic nanotube heterojunction arrays for enhanced photoelectrochemical water splitting

Organic/inorganic heterojunction photoanodes are appealing for making concurrent use of the highly photoactive organic semiconductors, and the efficient dielectric screening provided by their inorganic counterparts. In the present work, organic/inorganic nanotube heterojunction arrays composed of TiO2 nanotube arrays and a semiconducting N,N-(dicyclohexyl) perylene-3,4,9,10-tetracarboxylic diimide (PDi) layer were fabricated for photoelectrochemical water splitting. In this arrayed architecture, a PDi layer with a tunable thickness was coated on anodic TiO2 nanotube arrays by physical vapor deposition, which is advantageous for the formation of a uniform layer and an adequate interface contact between PDi and TiO2. The obtained PDi/TiO2 junction exhibited broadened visible light absorption, and an effective interface for enhanced photogenerated electron-hole separation, which is supported by the reduced charge transfer resistance and prolonged excitation lifetime via impedance spectroscopy analysis and fluorescence emission decay investigations. Consequently, such a heterojunction photoanode was photoresponsive to a wide visible light region of 400-600 nm, and thus demonstrated a highly enhanced photocurrent density at 1.23 V vs. a reversible hydrogen electrode. Additionally, the durability of such a photoanode can be guaranteed after long-time illumination because of the geometrical restraint imposed by the PDi aggregates. These results pave the way to discover new organic/inorganic assemblies for high-performance photoelectric applications and device integration.

High Efficiency Dye-sensitized Solar Cells Constructed with Composites of TiO2 and the Hot-bubbling Synthesized Ultra-Small SnO2 Nanocrystals

An efficient photo-anode for the dye-sensitized solar cells (DSSCs) should have features of high loading of dye molecules, favorable band alignments and good efficiency in electron transport. Herein, the 3.4 nm-sized SnO₂ nanocrystals (NCs) of high crystallinity, synthesized via the hot-bubbling method, were incorporated with the commercial TiO₂ (P25) particles to fabricate the photo-anodes. The optimal percentage of the doped SnO₂ NCs was found at ~7.5% (SnO₂/TiO₂, w/w), and the fabricated DSSC delivers a power conversion efficiency up to 6.7%, which is 1.52 times of the P25 based DSSCs. The ultra-small SnO₂ NCs offer three benefits, (1) the incorporation of SnO₂ NCs enlarges surface areas of the photo-anode films, and higher dye-loading amounts were achieved; (2) the high charge mobility provided by SnO₂ was confirmed to accelerate the electron transport, and the photo-electron recombination was suppressed by the highly-crystallized NCs; (3) the conduction band minimum (CBM) of the SnO₂ NCs was uplifted due to the quantum size effects, and this was found to alleviate the decrement in the open-circuit voltage. This work highlights great contributions of the SnO₂ NCs to the improvement of the photovoltaic performances in the DSSCs.

Fabrication and photoactivity of short rod-shaped mesoporous SiO2@TiO2 composites with TiO2 shell

Short rod-shaped mesoporous SiO₂@TiO₂ composites containing TiO₂ shell were prepared using short rod-shaped mesoporous SiO₂–PGMA–PEGMA as template and TBT as titanium source.