Preparation of ultrathin TiO2 single-crystal nanowires for high performance dye sensitized solar cells

Ultrathin TiO₂ anatase nanowires (NWs) were successfully prepared via a rapid and facile hydrothermal route. Consequently, the TiO₂ NWs and TiO₂ nanoparticles (NPs) composites electrodes were prepared with different weight ratios (25, 50 and 100 %) for a dye sensitized solar cell, and the photoelectrical performance has been systematically studied. It is observed that although the amount of absorption dye decreases, the composite solar cells exhibit a higher power conversion efficiency compared to either pure TiO₂ NP or NW solar cells by rationally tuning the weight ratios. The behavior was attributed to a combination of the rapid carrier transport in NW framework and the high dye loading on P25 surface.     

Hybrid photoelectrode by using vertically aligned rutile TiO2 nanowires inlaid with anatase TiO2 nanoparticles for dye-sensitized solar cells

We report a hybrid photoelectrode fabricated by using single crystalline rutile TiO₂ nanowires (NWs) inlaid with anatase TiO₂ nanoparticles (NPs) for efficient dye-sensitized solar cells. For this purpose, ∼4-μm-thick vertically aligned NWs were synthesized on the FTO glass substrate through a solvothermal treatment. Then, as-prepared NW film was treated with the NP colloidal dispersion to construct the NW–NP film. In particular, the NWs offer a fast pathway for electron transport as well as light scattering effect. On the other hand, the inlaid NPs give an extra amount of space for the dye-uptake. Accordingly, the present NW–NP electrode exhibited 6.2% of the conversion efficiency, which corresponds to ∼48% improvement over the efficiency of the NP-DSC. We attribute this notable result to the synergetic effects of the enhanced light confinement, charge collection, and dye-loading.     

Growth of ZnO nanorods on TiO2 nanoparticles films and their application to the electrode of dye-sensitized solar cells

A ZnO nanorods (NRs)/TiO₂ nanoparticles (NPs) film has been prepared by electrochemical deposition of ZnO NRs growth on P25 TiO₂ NPs film surfaces. It was found that ZnO NRs/TiO₂ NPs could significantly improve the efficiency of dye-sensitized solar cells owing to its relatively enhanced light-scattering capability and efficient charge transport efficiency. The overall energy-conversion efficiency (η) of 3.48 % was achieved by the formation of ZnO NRs/TiO₂ NPs film, which is 33 % higher than that formed by TiO₂ NPs alone (η = 2.62 %). The charge recombination behavior of cells was investigated by electrochemical impedance spectra, and the results showed that ZnO NRs/TiO₂ NPs film has the longer electron lifetime than TiO₂ NPs alone, which could facilitate the reduction of recombination processes and thus would promote the photocatalysis and solar cell performance.     

Reduced graphene oxide-multiwalled carbon nanotubes hybrid film with low Pt loading as counter electrode for improved photovoltaic performance of dye-sensitised solar cells

In this work, the role of reduced graphene oxide (rGO) with hyperbranched surfactant and its hybridisation with multiwalled carbon nanotubes (MWCNTs) and platinum (Pt) nanoparticles (NPs) as counter electrode (CE) were investigated to determine the photovoltaic performance of dye-sensitised solar cells (DSSCs). Sodium 1,4-bis(neopentyloxy)-3-(neopentyloxycarbonyl)-1,4-dioxobutane-2-sulphonate (TC14) surfactant was utilised as dispersing and stabilising agent in electrochemical exfoliation to synthesise graphene oxide (GO) as initial solution for rGO production prior to its further hybridisation and fabrication as thin film. A chemical reduction process utilising hydrazine hydrate was conducted to produce rGO due to the low temperature process and water-based GO solution. Subsequently, hybrid solution was prepared by mixing 1 wt% MWCNTs into the produced rGO solution. TC14-rGO and TC14-rGO_MWCNTs hybrid solution were transferred into fluorine-doped tin oxide substrate to fabricate thin film by spraying deposition method. Finally, the CE films were prepared by coating with thin Pt NPs. Photoanode film was prepared by a two-step process: hydrothermal growth method to synthesise titanium dioxide nanowires (TiO₂ NWs) and subsequent squeegee method to apply TiO₂ NPs. According to solar simulator measurement, the highest energy conversion efficiency (η) was achieved by using CE-based TC14-rGO_MWCNTs/Pt (1.553%), with the highest short current density of 4.424 mA/cm². The highest η was due to the high conductivity of CE hybrid film and the morphology of fabricated TiO₂ NWs/TiO₂ NPs. Consequently, the dye adsorption was high, and the photovoltaic performance of DSSCs was increased. This result also showed that rGO and rGO_MWCNTs hybrid can be used as considerable potential candidate materials to replace Pt gradually.     

Construction of hierarchical TiO<Subscript>2</Subscript> nanorod array/graphene/ZnO nanocomposites for high-performance photocatalysis

The photocatalytic performance of heterostructure photocatalysts is limited in practical use due to the charge accumulation at the interface and its low efficiency in utilizing solar energy during photocatalytic process. In this work, a ternary hierarchical TiO₂ nanorod arrays/graphene/ZnO nanocomposite is prepared by using graphene sheets as bridge between TiO₂ nanorod arrays (NRAs) and ZnO nanoparticles (NPs) via a facile combination of spin-coating and chemical vapor deposition techniques. The experimental study reveals that the graphene sheets provide a barrier-free access to transport photo-excited electrons from rutile TiO₂ NRAs and ZnO NPs. In addition, there generates an interface scattering effect of visible light as the graphene sheets provide appreciable nucleation sites for ZnO NPs. This synergistic effect in the ternary nanocomposite gives rise to a largely enhanced photocurrent density and visible light-driven photocatalytic activity, which is 2.6 times higher than that of regular TiO₂ NRAs/ZnO NPs heterostructure. It is expected that this hierarchical nanocomposite will be a promising candidate for applications in environmental remediation and energy fields.     

Heterojunction solar cells with integrated Si and ZnO nanowires and a chalcopyrite thin film

ZnO nanowires (NWs) have been successfully synthesized using a hydrothermal technique on both glass and silicon substrates initially coated with a sputtered ZnO thin film layer. Varying ZnO seed layer thicknesses were deposited to determine the effect of seed layer thickness on the quality of ZnO NW growth. The effect of growth time on the formation of ZnO NWs was also studied. Experimental results show that these two parameters have an important effect on formation, homogeneity and vertical orientation of ZnO NWs. Silicon nanowires were synthesized by a Ag-assisted electroless etching technique on an n-type Si (100) wafer. SEM observations have revealed the formation of vertically-aligned Si NWs with etching depth of ∼700 nm distributed over the surface of the Si. An electron-beam evaporated chalcopyrite thin film consisting of p-type AgGa_0.5In_0.5Se₂ with ∼800 nm thickness was deposited on the n-type ZnO and Si NWs for the construction of nanowire based heterojunction solar cells. For the Si NW based solar cell, from a partially illuminated area of the solar cell, the open-circuit voltage, short-circuit current density, fill factor and power conversion efficiency were 0.34 V, 25.38 mA cm⁻², 63% and 5.50%, respectively. On the other hand, these respective parameters were 0.26 V, 3.18 mA cm⁻², 35% and 0.37% for the ZnO NW solar cell.     

Sonocatalytic damage of composite TiO<Subscript>2</Subscript>/ZnO powder to bovine serum albumin under ultrasonic irradiation

The composite TiO₂/ZnO was prepared by mixing nano-sized TiO₂ and ZnO powders directly, in the molar proportion of 4:6, followed by heat-treatment at 500°C for 40 min. The products were charactered by powder X-ray diffraction and transmission electron microscopy. The test of sonocatalytic activities of the composite TiO₂/ZnO powders was carried out through the damage of bovine serum albumin (BSA). Otherwise, the effects of several factors on the damage of BSA molecules were evaluated by means of UV-vis and fluorescence spectra. It was found that the damage degree was aggravated with the increase of ultrasonic irradiation time and composite TiO₂/ZnO addition amount. These research results were of great significance for driving sonocatalytic method to treat tumor in clinic application.     

Highly efficient visible light photocatalysis of novel CuS/ZnO heterostructure nanowire arrays

Here, a facile approach for the fabrication of CuS nanoparticle (NP)/ZnO nanowire (NW) heterostructures on a mesh substrate through a simple two-step solution method is demonstrated. Successive ionic layer adsorption and reaction (SILAR) was employed to uniformly deposit CuS NPs on the hydrothermally grown ZnO NW array. The synthesized CuS/ZnO heterostructure NWs exhibited superior photocatalytic activity under visible light compared to bare ZnO NWs. This strong photocatalytic activity under visible light is due to the interfacial charge transfer (IFCT) from the valence band of the ZnO NW to the CuS NP, which reduces CuS to Cu(2)S. After repeated cycles of photodecolorization of Acid Orange 7 (AO7), the photocatalytic behavior of CuS/ZnO heterostructure NWs exhibited no significant loss of activity. Furthermore, our CuS/ZnO NWs/mesh photocatalyst floats in solution via partial superhydrophobic modification of the NWs.     

Electrospun titania fibers by incorporating graphene/Ag hybrids for the improved visible-light photocatalysis

A novel graphene/Ag nanoparticles (NPs) hybrid (prepared by a physical method (PM)) was incorporated into electrospun TiO₂ fibers to improve visible-lightdriven photocatalytic properties. The experimental study revealed that the graphene/Ag NPs (PM) hybrid not only decreased the bandgap energy of TiO₂, but also enhanced its light response in the visible region due to the surface plasmon resonance (SPR) effect. In addition, compared with those of TiO₂ fibers incorporating the graphene/Ag NPs hybrid (prepared by a chemical method (CM)), TiO₂-graphene/Ag NPs (PM) fibers exhibited a higher surface photocurrent density and superior photocatalytic performance, i.e., the visible-light-driven photocatalytic activity was enhanced by 2 times. The main reasons include a lower surface defect density of the graphene/Ag NPs (PM) hybrid, a smaller particle size (10 nm) and a higher dispersity of Ag NPs, which promote the rapid transfer of photoexcited charge carriers and inhibit the recombination of photogenerated electrons and holes. It is expected that this kind of ternary electrospun fibers will be a promising candidate for applications in water splitting, solar cells, CO₂ conversion and optoelectronic devices, etc.     

Novel Structure for High Performance UV Photodetector Based on BiOCl/ZnO Hybrid Film

A novel type of high performance ultraviolet (UV) photodetector (PD) based on a ZnO film has been prepared by incorporating a BiOCl nanostructure into the film. The responsivity of the BiOCl/ZnO hybrid film PD in UV region can reach 182.87 mA W^?1, which is about 2.72 and 6.87 times for that of TiO₂/ZnO hybrid film PD and pure ZnO film PD. The rise/decay time of BiOCl/ZnO hybrid film PD is 25.83/11.25 s, which is much shorter than that of TiO₂/ZnO hybrid film PD (51.94/26.05 s) and pure ZnO film PD (69.34/>120 s). The BiOCl nanostructure can inject photogenerated electrons into the ZnO film under UV light illumination, leading to the increase of photocurrent, and forms barriers to block the straight transmission of electrons between electrodes, resulting in the decrease of decay time. The results of control experiment show that the transfer path of photogenerated electrons formed by p–n junction will be cut off after depositing gold nanoparticles on the film surface, which means this hybrid film is a unique and novel structure to improve the optoelectronic performance of photodetectors. This novel BiOCl/ZnO hybrid structure paves new route for the development of film PDs based on ZnO film.     

Facile route to prepare film of poly(3,4-ethylene dioxythiophene)-TiO2 nanohybrid for solar cell application

Composite using poly(3,4-ethylenedioxythiophene) (PEDOT) as electronic conducting polymer and nanocrystalline titanium dioxide (TiO₂) as host matrix were prepared by the template method. We applied an original in situ photopolymerization technique to synthesize PEDOT inside the TiO₂ pore and characterized the polymer and pore filling by different analysis (cyclic voltammetry, atomic force microscopy, spectroscopy and thermogravimetric measurements). Results were compared with those obtained on PEDOT films synthezised by monomer oxidization in the presence of FeCl₃. In situ generation of PEDOT by photopolymerization was observed to be higher and self-limiting after 22% filling of the mesoporous TiO₂ network. Hybrid materials were used to fabricate an indium-tin oxide/nano-crystalline TiO₂/PEDOT/Au device. The current-voltage characteristics indicate that a built-in electrical field has been created at the nano-crystalline TiO₂/PEDOT interface with energy conversion efficiency of 0.09% without dye.     

CdSe quantum dot-sensitized Au/TiO2 hybrid mesoporous films and their enhanced photoelectrochemical performance

Novel CdSe quantum dot (QD)-sensitized Au/TiO₂ hybrid mesoporous films have been designed, fabricated, and evaluated for photoelectrochemical (PEC) applications. The Au/TiO₂ hybrid structures were made by assembly of Au and TiO₂ nanoparticles (NPs). A chemical bath deposition method was applied to deposit CdSe QDs on TiO₂ NP films with and without Au NPs embedded. We observed significant enhancements in photocurrent for the film with Au NPs, in the entire spectral region we studied (350–600 nm). Incident-photon-to-current efficiency (IPCE) data revealed an average enhancement of 50%, and the enhancement was more significant at short wavelength. This substantially improved PEC performance is tentatively attributed to the increased light absorption of CdSe QDs due to light scattering by Au NPs. Interestingly, without QD sensitization, the Au NPs quenched the photocurrent of TiO₂ films, due to the dominance of electron trapping over light scattering by Au NPs. The results suggest that metal NPs are potentially useful for improving the photoresponse in PEC cells and possibly in other devices such as solar cells based on QD-sensitized metal oxide nanostructured films. This work demonstrates that metal NPs can serve as light scattering centers, besides functioning as photo-sensitizers and electron traps. The function of metal NPs in a particular nanocomposite film is strongly dependent on their structure and morphology.      

Microwave assisted hydrothermal synthesis and characterization of ZnO–TNT composites

TiO₂ nanotubes with different contents of ZnO (3–40 wt.% ZnO) have been successfully synthesized by microwave assisted hydrothermal process by using commercial TiO₂-P25 as a precursor. The phase and crystallinity of the obtained ZnO–TNT were analyzed by X-ray Diffraction (XRD). The surface area of the ZnO–TNT was determined by BET method. The effect of the different contents of ZnO on morphology of TiO₂ nanotubes was investigated by SEM and TEM. Optical properties and band gap energy of ZnO–TNT were calculated by using UV–vis DRS spectroscopy and modified Kubelka–Munk equation. Photocatalytic performance of ZnO–TNT was investigated by degradation of rhodamine B (RhB) dye under UV and visible light irradiation. Increasing ZnO content in TNT gradually decreased the diameter and length of nanotubes. Furthermore, addition of 40 wt.% ZnO into the TNT exceeded the saturation limit of ion exchangeability of Zn²⁺ and Na⁺ ions and aggregation of finely dispersed ZnO particles on the surface of TNT were observed. The ZnO–TNT has shown relatively larger band gap energies than that of TiO₂-P25. However, ZnO–TNT has shown considerable increase in photo-activity for degradation of RhB dye in visible light as compared to UV light irradiation.     

Hybrid nanocomposite of Fe nanoparticles on SiO2 nanowires by sublimation route

A simple route was developed to synthesize the hybrid nanocomposite with Fe nanoparticles (NPs) dispersed on the surface of SiO₂ nanowires (NWs), where SiO₂ NWs with the diameter of 20-40 nm were produced by heating single-crystal silicon wafer, and Fe NPs in the size range of 3-20 nm were generated by heating Fe powders. The nucleation and growth of Fe NPs follows the solid-vapor-solid (S-V-S) mechanism, namely, Fe powders firstly sublime and then Fe atoms deposit on SiO₂ NWs to form Fe NPs.     

Dielectric investigations of pristine and modified ZnO nanoparticles for energy storage devices

Pristine ZnO, Al-doped ZnO, and TiO₂ coated ZnO nanoparticles (NPs) were synthesized by the wet chemical precipitation technique. All the synthesized NPs were characterized using X-ray diffraction (XRD), Field emission scanning electron microscopy (FESEM), Transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy. XRD analysis of pristine ZnO and Al-doped ZnO NPs revealed the hexagonal wurtzite structure with P6₃mc space group with no secondary phases and impurities. FESEM micrographs also depicted hexagonal grains with well-defined grain boundaries. TEM images showed hexagonal polyhedral shape for pure ZnO NPs and spherical shape dominating polyhedral particle for Al-doped ZnO NPs, and pseudospherical particles for TiO₂ coated ZnO NPs. Energy-dispersive X-ray spectroscopy of Al-doped ZnO indicates the eminent exchange of dopant in the lattice site of Zn. Dielectric Studies reveal the highest value of the dielectric constant and lowest value of dielectric loss for Al-doped ZnO as compared to pure and TiO₂-coated ZnO NPs. Suggesting Al-doped ZnO to be used as a dielectric material that can serve as a basic building block of the energy storage devices such as dielectric capacitor. TiO₂-coated ZnO NPs demonstrated higher AC conductivity in comparison to pure ZnO and Al-doped ZnO NPs suggesting their use as a conductive nanofiller materials in a polymer-based nanocomposite to achieve higher energy density.

     

Hierarchical Metal/Semiconductor Nanostructure for Efficient Water Splitting

A hierarchically patterned metal/semiconductor (gold nanoparticles/ZnO nanowires) nanostructure with maximized photon trapping effects is fabricated via interference lithography (IL) for plasmon enhanced photo‐electrochemical water splitting in the visible region of light. Compared with unpatterned (plain) gold nanoparticles‐coated ZnO NWs (Au NPs/ZnO NWs), the hierarchically patterned Au NPs/ZnO NWs hybrid structures demonstrate higher and wider absorption bands of light leading to increased surface enhanced Raman scattering due to the light trapping effects achieved by the combination of two different nanostructure dimensions; furthermore, pronounced plasmonic enhancement of water splitting is verified in the hierarchically patterned Au NPs/ZnO NWs structures in the visible region. The excellent performance of the hierarchically patterned Au NPs/ZnO NWs indicates that the combination of pre‐determined two different dimensions has great potential for application in solar energy conversion, light emitting diodes, as well as SERS substrates and photoelectrodes for water splitting.     

Optical and field emission characteristics of anodic aluminium oxide/ZnO hybrid nanostructure

Arrays of ZnO nanowires (NWs) were fabricated within the well-distributed pores of anodic aluminium oxide (AAO) template by a simple chemical method. The photoluminescence (PL) and field emission (FE) properties of the AAO/ZnO NWs hybrid structure were investigated in detail. The hybrid nanostructure exhibits interesting PL characteristics. ZnO NWs exhibit UV emission at 378 nm and two prominent blue-green emissions at about 462 and 508 nm. Intense blue emission from the AAO template itself was observed at around 430 nm. Herein, for the first time we report the FE characteristics of the ZnO/AAO hybrid structure to show the influence of the AAO template on the FE property of the hybrid structure. It is found that the turn-on electric field of the vertically grown and aligned ZnO NWs within the pores of AAO template is lower than the entangled unaligned ZnO NWs extracted from the template. Although the AAO template exhibits no FE current but it helps to achieve better FE property of the ZnO NWs through better alignment. The turn-on electric field of aligned NWs was found to be 3 V μm⁻¹ at a current of 0.1 μA. Results indicate that the AAO embedded ZnO NW hybrid structure may find useful applications in luminescent and field emission display devices.     

Partial conversion reaction of ZnO nanowires to ZnSe by a simple selenization method and their photocatalytic activities

A simple novel synthetic method for preparing ZnSe/ZnO heterostructured nanowire (NW) arrays via the selenization of ZnO NWs is reported. A hydrothermally grown ZnO NWs array on a glass substrate was reacted with selenium vapor to generate a 20–30 nm of zincblend ZnSe nanoparticles (NPs) on wurtzite ZnO NWs. A growth mechanism was proposed based on SEM, XRD, and TEM analysis to explain the partial chemical conversion of ZnO NW surfaces into ZnSe NPs. This mechanism is applicable to the synthesis of other chalcogenide compounds. The as-synthesized ZnSe/ZnO heterojunctions showed enhanced UV–visible light absorption properties. The materials exhibited excellent photocatalytic activity toward the decomposition of an organic dye compared to the bare ZnO due to enhanced light absorption and the type-II cascade band structure.     

Fabrication of TiO<Subscript>2</Subscript>/ZnO composite nanofibers with enhanced photocatalytic activity

TiO₂/ZnO composite nanofibers have been successfully prepared by electrospinning technique. X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, transmission electron microscopy, Raman spectrum, X-ray photoelectron spectroscopy and UV–Vis diffuse reflectance spectroscopy, were used to characterize the as-synthesized nanofibers. The photocatalytic studies revealed that the TiO₂/ZnO nanofibers exhibited enhanced photocatalytic efficiency of photodegradation. Additionally, the recycling experiment of TiO₂/ZnO nanofibers had been done, demonstrating that TiO₂/ZnO nanofibers have high efficiency and stability.     

Enhanced photovoltaic performance of a dye sensitized solar cell with Cu/N Co-doped TiO<Subscript>2</Subscript> nanoparticles

Pure and Copper/Nitrogen (Cu/N)-codoped TiO₂ photoanodes with various Cu concentrations are prepared via sol–gel route for the photoanode application in dye-sensitized solar cells (DSSCs). All the prepared samples are characterized by X-Ray Diffraction (XRD), X-Ray Photoelectron Spectroscopy (XPS), Scanning Electron Microscope (SEM), Transmission Electron Microscopy (TEM), UV–Vis spectroscopy (UV–VIS) and Electrochemical Impedance Spectroscopy (EIS). Addition of suitable amount of Cu and N content in TiO₂ can alter its optical and electrical properties by extending absorption in the visible region and band gap reduction. The results show that some of the Ti sites are replaced by Cu atoms while O sites are occupied by N atoms. Upon adequate addition of Cu/N could lead to smaller particle size, higher specific surface area, increased dye adsorption and retarded charge carrier recombination. A significant improvement in the power conversion efficiency is observed in case of optimized 0.3 mol% Cu/N-doped TiO₂ nanoparticles (NPs) based DSSC. This optimized 0.3 mol% Cu/N-doped photoanode accomplished a best power conversion efficiency of 11.70% with a short circuit current density of 23.41 mA cm^?2 which is 41% higher than that of the pure TiO₂ photoanode based DSSC (6.82%).