Three-dimensional photonic crystal fluorinated tin oxide (FTO) electrodes: synthesis and optical and electrical properties

Photovoltaic (PV) schemes often encounter a pair of fundamentally opposing requirements on the thickness of semiconductor layer: a thicker PV semiconductor layer provides enhanced optical density, but inevitably increases the charge transport path length. An effective approach to solve this dilemma is to enhance the interface area between the terminal electrode, i.e., transparent conducting oxide (TCO) and the semiconductor layer. As such, we report a facile, template-assisted, and solution chemistry-based synthesis of 3-dimensional inverse opal fluorinated tin oxide (IO-FTO) electrodes. Synergistically, the photonic crystal structure possessed in the IO-FTO exhibits strong light trapping capability. Furthermore, the electrical properties of the IO-FTO electrodes are studied by Hall effect and sheet resistance measurement. Using atomic layer deposition method, an ultrathin TiO(2) layer is coated on all surfaces of the IO-FTO electrodes. Cyclic voltammetry study indicates that the resulting TiO(2)-coated IO-FTO shows excellent potentials as electrodes for electrolyte-based photoelectrochemical solar cells.     

TiO2-x抗凝材料中的电化学行为研究

通过对TiO2-x薄膜在模拟人体体液中的电化学行为研究,以确定薄膜和凝血因子之间的电荷传递信息.电化学阻抗图谱(EIS)结果表明,薄膜与添加凝血因子(纤维蛋白原)的溶液之间发生不同程度电荷转移.肖特基(Mott-schottky)图谱结果显示,在引入凝血因子后薄膜的栽流子浓度发生变化.     

Phase-junction Ag/TiO2 nanocomposite as photocathode for H2 generation

Developing anatase/rutile phase-junction in TiO2 to construct Z-scheme system is quite effective to improve its photoelectrochemical activity.In this work,the anatase/rutile phase-junction Ag/TiO2 nanocomposites are developed as photocathodes for hydrogen production.The optimized Ag/TiO2 nanocomposite achieves a high current density of 1.28 mA cm-2,an incident photon-to-current con-version efficiency(IPCE)of 10.8％,an applied bias photon-to-current efficiency(ABPE)of 0.32 at 390 nm and a charge carriers'lifetime up to 2000s.Such enhancement on photoelectrochemical activity can be attributed to:(i)the generated Z-scheme system in the anatase/rutile phase-junction Ag/TiO2 photocath-ode enhances the separation,diffusion and transformation of electron/hole pairs inside the structure,(ii)Ag nanodots modification in the anatase/rutile phases leading to the tuned band gap with enhanced light absorption and(iii)the formed Schottky barrier after Ag nanodots surface modification provides enough electron traps to avoid the recombination of photogenerated electrons and holes.Our results here sug-gest that developing phase-junction nanocomposite as photocathode will provide a new vision for their enhanced photoelectrochemical generation of hydrogen.     

Quantum rod-sensitized solar cell: nanocrystal shape effect on the photovoltaic properties

The effect of the shape of nanocrystal sensitizers in photoelectrochemical cells is reported. CdSe quantum rods of different dimensions were effectively deposited rapidly by electrophoresis onto mesoporous TiO(2) electrodes and compared with quantum dots. Photovoltaic efficiency values of up to 2.7% were measured for the QRSSC, notably high values for TiO(2) solar cells with ex situ synthesized nanoparticle sensitizers. The quantum rod-based solar cells exhibit a red shift of the electron injection onset and charge recombination is significantly suppressed compared to dot sensitizers. The improved photoelectrochemical characteristics of the quantum rods over the dots as sensitizers is assigned to the elongated shape, allowing the build-up of a dipole moment along the rod that leads to a downward shift of the TiO(2) energy bands relative to the quantum rods, leading to improved charge injection.     

Vertically aligned single crystal TiO2 nanowire arrays grown directly on transparent conducting oxide coated glass: synthesis details and applications

Single-crystal one-dimensional (1D) semiconductor architectures are important in materials-based applications requiring a large surface area, morphological control, and superior charge transport. Titania has widespread utility in applications including photocatalysis, photochromism, photovoltaics, and gas sensors. While considerable efforts have focused on the preparation of 1D TiO2, no methods have been available to grow crystalline nanowire arrays directly onto transparent conducting oxide (TCO) substrates, greatly limiting the performance of TiO2 photoelectrochemical devices. Herein, we present a straightforward low temperature method to prepare single crystal rutile TiO2 nanowire arrays up to 5 microm long on TCO glass via a non-polar solvent/hydrophilic substrate interfacial reaction under mild hydrothermal conditions. The as-prepared densely packed nanowires grow vertically oriented from the TCO glass substrate along the (110) crystal plane with a preferred (001) orientation. In a dye sensitized solar cell, N719 dye, using TiO2 nanowire arrays 2-3 microm long we achieve an AM 1.5 photoconversion efficiency of 5.02%.     

Fabrication of TiO2/Tin-doped indium oxide-based photoelectrode coated with overlayer materials and its photoelectrochemical behavior

The photoelectrochemical properties of TiO2-based photoelectrodes with metal oxide overlayers (e.g., ZnO, ZrO2, MgO, and Al2O3) were investigated. The metal oxides were deposited on TiO2/tin-doped indium oxide (ITO) films by spin-coating metal-alkoxide precursors. The formation of the overlayers was confirmed by energy dispersive X-ray spectroscopy (EDS) and high resolution transmission electron microscopy (HRTEM). Each overlayers were well-coated on the TiO2-based films and have approximately 2 nm thickness. The prepared films were used as photoanodes in a photoelectrochemical system with a Pt counter electrode to evaluate hydrogen production performance. Comparing with other overlayers, the ZnO-coated photoelectrode exhibits the highest rate of hydrogen evolution and which is better than the uncoated one. From the photoelectrochemical and spectroscopic study, the superior hydrogen production property of the ZnO-coated TiO2 photoelectrode was attributed to both the higher light absorbance of ZnO compared to TiO2 and the formation of hydroxyl groups at the ZnO surface.     

Visible light photoelectrochemical performance of W-loaded TiO2 nanotube arrays: structural properties

Well aligned TiO2 nanotubes were successfully synthesized by anodization of Ti foil at 60 V in a fluorinated bath comprised of ethylene glycol with 5 wt% of NH4F and 5 wt% of H2O2. In order to enhance the visible light absorption and photoelectrochemical response of pure TiO2 nanotube arrays, a mixed oxide system (W-TiO2) was investigated. W-TiO2 nanotube arrays were prepared using radio-frequency (RF) sputtering to incorporate the W into the lattice of TiO2 nanotube arrays. The W atoms occupy the substitutional position within the vacancies of TiO2 nanotube arrays. The as-anodized TiO2 is amorphous in nature while the annealed TiO2 is anatase phase. The mixed oxide (W-TiO2) system in suitable TiO2 phase plays important roles in efficient electron transfers due to the reduction in electron-hole recombination. In this article, the effect of the sputtered W into the as-anodized/annealed TiO2 nanotube arrays on the photoelectrochemical response was presented.     

Branched TiO₂ nanorods for photoelectrochemical hydrogen production

We report a hierarchically branched TiO(2) nanorod structure that serves as a model architecture for efficient photoelectrochemical devices as it simultaneously offers a large contact area with the electrolyte, excellent light-trapping characteristics, and a highly conductive pathway for charge carrier collection. Under Xenon lamp illumination (UV spectrum matched to AM 1.5G, 88 mW/cm(2) total power density), the branched TiO(2) nanorod array produces a photocurrent density of 0.83 mA/cm(2) at 0.8 V versus reversible hydrogen electrode (RHE). The incident photon-to-current conversion efficiency reaches 67% at 380 nm with an applied bias of 0.6 V versus RHE, nearly two times higher than the bare nanorods without branches. The branches improve efficiency by means of (i) improved charge separation and transport within the branches due to their small diameters, and (ii) a 4-fold increase in surface area which facilitates the hole transfer at the TiO(2)/electrolyte interface.     

In situ growth of single-crystal TiO2 nanorod arrays on Ti substrate: Controllable synthesis and photoelectrochemical water splitting

Despite one-dimensional (1D) semiconductor nanostructure arrays attracting increasing attention due to their many advantages,highly ordered TiO2 nanorod arrays (TiO2 NR) are rarely grown in situ on Ti substrates.Herein,a feasible method to fabricate TiO2 NRs on Ti substrates by using a through-mask anodization process is reported.Self-ordered anodic aluminum oxide (AAO) overlaid on Ti substrate was used as a nanotemplate to induce the growth of TiO2 NRs.The NR length and diameter could be controlled by adjusting anodization parameters such as electrochemical anodization voltage,anodization time and temperature,and electrolyte composition.Furthermore,according to the proposed NR formation mechanism,the anodized Ti ions migrate and deposit in the AAO nanochannels to form Ti(OH)4 or amorphous TiO2 NRs under electric field,owing to the confinement effect of the template.Photoelectrochemical tests indicated that,after hydrogenation,the TiO2 NRs presented higher photocurrent density under simulated sunlight and visible light illuminations,suggesting their potential use in photoelectrochemical water splitting,photocatalysis,solar cells,and sensors.     

Nanostructuring Confinement for Controllable Interfacial Charge Transfer

Carbon nanostructures supported semiconductors are common in photocatalytic and photoelectrochemical applications, as it is expected that the nanoconductors can improve the spatial separation and transport of photogenerated charge carriers. Transfer of charge carriers through the carbon‐semiconductor interface is the key electronic process, which determines the role of charge separation channels, and is sensitively influenced by band structures of the semiconductor near the contacts. Usually, this electronic process suffers from excessive energy dissipation by thermionic emission, which will undesirably prevent the interfacial charge transfer and eventually aggravate the recombination of photogenerated charge carriers. Unfortunately, this critical issue has hardly been consciously considered. Here, ultrathin dopant‐free tunneling interlayers coated on the surface of graphene and sandwiched between the carbon sheets and the semiconductor nanostructures are adopted as a model system to demonstrate energy saving for the interfacial charge transfer. The nanostructuring confinement of band bending within the ultrathin interlayers in contact with the graphene sheets effectively narrows the width of the potential barriers, which enables tunneling of a substantial number of photogenerated electrons to the co‐catalysts without unduly consuming energy. Besides, the dopant‐free tunneling interlayers simultaneously block the transferred electrons in the sandwiched graphene sheets from leakage.     

Controlled Sn-doping in TiO2 nanowire photoanodes with enhanced photoelectrochemical conversion

We demonstrate for the first time the controlled Sn-doping in TiO(2) nanowire (NW) arrays for photoelectrochemical (PEC) water splitting. Because of the low lattice mismatch between SnO(2) and TiO(2), Sn dopants are incorporated into TiO(2) NWs by a one-pot hydrothermal synthesis with different ratios of SnCl(4) and tetrabutyl titanate, and a high acidity of the reactant solution is critical to control the SnCl(4) hydrolysis rate. The obtained Sn-doped TiO(2) (Sn/TiO(2)) NWs are single crystalline with a rutile structure, and the incorporation of Sn in TiO(2) NWs is well controlled at a low level, that is, 1-2% of Sn/Ti ratio, to avoid phase separation or interface scattering. PEC measurement on Sn/TiO(2) NW photoanodes with different Sn doping ratios shows that the photocurrent increases first with increased Sn doping level to >2.0 mA/cm(2) at 0 V vs Ag/AgCl under 100 mW/cm(2) simulated sunlight illumination up to ~100% enhancement compared to our best pristine TiO(2) NW photoanodes and then decreases at higher Sn doping levels. Subsequent annealing of Sn/TiO(2) NWs in H(2) further improves their photoactivity with an optimized photoconversion efficiency of ~1.2%. The incident-photon-to-current conversion efficiency shows that the photocurrent increase is mainly ascribed to the enhancement of photoactivity in the UV region, and the electrochemical impedance measurement reveals that the density of n-type charge carriers can be significantly increased by the Sn doping. These Sn/TiO(2) NW photoanodes are highly stable in PEC conversion and thus can serve as a potential candidate for pure TiO(2) materials in a variety of solar energy driven applications.     

Quantitative photoelectrochemical detection of biological affinity reaction: biotin-avidin interaction

Quantitative detection of a biological affinity reaction, the biotin/avidin recognition, was achieved using our newly developed photoelectrochemical analytical system. The system is based on the operation mechanism of the well-developed dye-sensitized photoelectrochemical solar cells and comprises a ruthenium tris(2,2'-bipyridine) (Ru-bipy) derivative as the photoelectrochemical signal-generating molecule, oxalate as the sacrificial electron donor, and tin oxide nanoparticle as the semiconductor electrode material. To perform the affinity reaction, avidin was immobilized on SnO(2) electrode by passive adsorption. Biotin-linked bovine serum albumin (BSA) was labeled with an NHS-ester derivative of Ru-bipy. After binding of BSA to the surface-immobilized avidin through biotin, photoelectrochemical measurement was carried out in the presence of oxalate. Anodic photocurrent was turned on and off repeatedly by control of incidental light. The action spectrum of the photocurrent resembled the absorption spectrum of Ru-bipy, proving the photocurrent was generated from the metal complex. A linear relationship between photocurrent and BSA concentration was obtained in the range of 1-100 microg/mL. This is the first case of quantitative photoelectrochemical detection of a biological affinity interaction.     

Fabrication of PbS nanoparticle-sensitized TiO₂ nanotube arrays and their photoelectrochemical properties

TiO(2) nanotube arrays (NTAs) are modified with PbS nanoparticles by successive ionic layer adsorption and reaction (SILAR) or electrodeposition, with an aim towards tuning the photoelectrochemical cell to the visible region. The PbS modification of the TiO(2) NTAs results in an increase in the visible light adsorption, however the increase in photocurrent is dependent on the modification method. PbS/TiO(2) NTAs prepared by SILAR and electrodeposition show, respectively, photocurrents of 11.02 and 5.72 mA/cm(2). The increase in photocurrent is attributed to enhanced charge separation efficiency and improved electron transport.     

Initial formation behaviour of polypyrrole on single crystal TiO2 through photo-electrochemical reaction

Hybrid materials of the organic and inorganic semiconductors have a potential to show the better performance in the charge separation at the junction upon the photovoltaic action by the presence of the space charge layer in the inorganic semiconductor. In this study, the photo-anodic polymerization was selected as a fabrication method for the hybrid materials composed of TiO2 and polypyrrole on the basis of some advantages of this method. For the process control of the photo-anodic polymerization, it is important to elucidate the formation and growth mechanisms of the organic polymer. In this study, a flat sheet of single-crystal TiO2 was used as a well-defined surface for preparation of the organic polymer of pyrrole. Photo-anodic polarization behaviour was clarified and polypyrrole was prepared on TiO2. The formation process, especially the initial step was revealed by observation of polypyrrole with atomic force microscope (AFM) and statistical interpretation of the morphology of polypyrrole in the nano-scopic level. The formation process of polypyrrole on the TiO2 surface was summarized; (1) adsorption of precursors, (2) localized formation and growth of polypyrrole under the photo-illumination, and (3) homogenous growth of polypyrrole with the external current application under the photo-illumination.     

The Development of Cocatalysts for Photoelectrochemical CO2 Reduction

The ever‐increasing anthropogenic consumption of fossil fuels and the resulting large emission of CO₂ have led to a severe energy crisis and climate change. Photocatalytic reduction of CO₂ into fuels using solar energy is considered as a promising way to address these two problems. In particular, photoelectrochemical (PEC) reduction of CO₂ can integrate and optimize the advantages of both photocatalysis and electrocatalysis for improved conversion efficiency and selectivity. In addition to the charge generation and separation, the efficient reduction of CO₂ on the surface of a semiconductor‐based photoelectrode remains a scientifically critical challenge, which can be greatly enhanced by the surface modification of cocatalysts. Herein, the recent developments of cocatalysts in PEC CO₂ reduction over semiconductor‐based photoelectrodes are described, and the basic principles of PEC CO₂ reduction and the function of the cocatalyst in photoelectrocatalysis are discussed. The structure optimization between the photoelectrodes and the cocatalysts is also summarized since the loading of cocatalyst may shield the incident light and hinder charge transfer between them. Furthermore, the challenges and perspectives for PEC reduction of CO₂ are also presented.     

Poly(3-hexylthiophene)/TiO2 nanoparticle-functionalized electrodes for visible light and low potential photoelectrochemical sensing of organophosphorus pesticide chlopyrifos

A dramatic visible light photoelectrochemical sensing platform for the detection of pesticide molecules at zero potential (versus saturated calomel electrode) was first constructed using poly(3-hexylthiophene)-functionalized TiO(2) nanoparticles. Poly(3-hexylthiophene) (P3HT) was synthesized via chemical oxidative polymerization with anhydrous FeCl(3) as the oxidant, 3-hexylthiophene as the monomer, and chloroform as the solvent, and the functional TiO(2) nanoparticles were facilely prepared by blending TiO(2) nanoparticles and P3HT in chloroform solution. The resulting photoelectrocatalysts were characterized by scanning electron microscopy, Raman spectroscopy, and X-ray diffractometry. Under visible light irradiation, P3HT generated the transition from the valence band to the conduction band, delivering the excited electrons into the conduction band of TiO(2) and then to the glassy carbon electrode. Simultaneously, a positive charged hole (h(+)) of TiO(2) may form and migrate to the valence band of P3HT, which can react with H(2)O to generate (?)OH, and then it converted chlopyrifos into chlopyrifos(?) that promoted the amplifying photocurrent response. On the basis of the proposed photoelectrochemical mechanism, a methodology for sensitive photoelectrochemical sensing for chlopyrifos at zero potential was thus developed. Under optimal conditions, the proposed photoelectrochemical method could detect chlopyrifos ranging from 0.2 to 16 μmol L(-1) with a detection limit of 0.01 μmol L(-1) at a signal-to-noise ratio of 3. The photoelectrochemical sensor had an excellent specificity against the other pesticides and could be successfully applied to the detection of reduced chlopyrifos in green vegetables, showing a promising application in photoelectrochemical sensing.     

掺锡TiO2复合薄膜的制备和光催化性能的研究

通过溶胶-凝胶工艺在玻璃表面制备了均匀透明的掺锡锐钛矿型TiO₂光催化复合薄膜,用SEM、XRD、XPS等对薄膜进行了表征。薄膜中除含有Ti,O, Sn等元素外,还存在一定量的来自有机前驱物未完全燃烧的C元素和从玻璃表面扩散到薄膜中的Na和Ca元素。甲基橙水溶液的光催化降解实验表明:掺锡TiO₂复合薄膜的表观降解速率常数明显高于未掺锡TiO₂薄膜的表观降解速率常数,这为进一步提高TiO₂薄膜的光催化活性开辟了新的途径。      

半导体耦合法改性纳米TiO2薄膜在DSSC中的应用

染料敏化纳米晶TiO2薄膜太阳能电池具有成本低廉、制作简单和环保等优点,吸引了国内外研究者的广泛关注。综述了半导体复合TiO2薄膜在DSSC中应用的研究进展,重点阐述了半导体复合TiO2薄膜的机理,总结了半导体复合TiO2薄膜的种类及制备方法,并分析了其对太阳能电池光电性能的影响。     

Nanorods and nanotubes for solar cells

Nanorods and nanotubes as photoactive materials as well as electrodes in photovoltaic cells have been launched a few years ago, and the literature in this field started to appear only recently. The first steps have shown both advantages and disadvantages of their application, and the main expectation associated with their effective charge transport has not been realized completely. This article aims to review both the first and the recent tendencies in the development and application of nanorod and nanotube materials in photovoltaic cells. Two basic techniques of synthesis of crystalline nanorod structures are described, the top-down and bottom-up approaches, respectively. Design and photovoltaic performance of solar cells based on various semiconductor nanorod materials, such as TiO2, ZnO, CdS, CdSe, CdTe, CuO, Si are presented and compared with respective solar cells based on semiconductor nanoparticles. Specific of synthesis and application of carbon nanotubes in photovoltaic devices is also reviewed.     

Visible light active ZnIn2S4/g-C3N4 heterostructure nanocomposite photoelectrode for efficient photoelectrochemical water splitting activity

Hexagonal zinc indium sulfide coupled g-C₃N₄ (H-ZnIn₂S₄/g-C₃N₄) nanocomposites were synthesized using chemisorption method and its performance towards photoelectrochemical water splitting activity was studied. The H-ZnIn₂S₄/g-C₃N₄ (H-ZIS/CN) nanocomposites exhibited ∼ 1.9 times enhanced photoelectrochemical performance as compared to the H-ZnIn₂S₄. The enhancement in the PEC water splitting activity of H-ZIS/CN nanocomposite is ascribed to the formation of type-II heterojunction which resulted in improved separation of photogenerated charge carriers and faster transfer of charges at the photoelectrode/electrolyte interface. The electrochemical impedance study and Mott-Schottky supported these results. Moreover, during photoelectrochemical reactions, H-ZIS/CN nanocomposites showed tremendous stability under visible light. A potential mechanism of the enhanced photoelectrochemical activity of H-ZIS/CN nanocomposites was proposed and endorsed by the PEC results. This study demonstrates that establishing a heterostructure system by coupling a ternary chalcogenide semiconductor with a conducting polymer is an effective strategy for PEC water splitting applications.