Plasma assisted deposition of single and multistacked TiO2 hierarchical nanotubes photoanodes

We present herein an evolved methodology for the growth of nanocrystalline hierarchical nanotubes combining physical vapor deposition of organic nanowires (ONWs) and plasma enhanced chemical vacuum deposition of anatase TiO2 layers. The ONWs act as vacuum removable 1D and 3D templates, with the whole process occurring at temperatures ranging from RT to 250 °C. As a result, a high density of hierarchical nanotubes with tunable diameter, length and tailored wall microstructures are formed on a variety of processable substrates as metal and metal oxide films or nanoparticles including transparent conductive oxides. The reiteration of the process leads to the development of an unprecedented 3D nanoarchitecture formed by stacking the layers of hierarchical TiO2 nanotubes. As a proof of concept, we present the superior performance of the 3D nanoarchitecture as a photoanode within an excitonic solar cell with efficiencies as high as 4.69% for a nominal thickness of the anatase layer below 2.75 ¿m. Mechanical stability and straightforward implementation in devices are demonstrated at the same time. The process is extendable to other functional oxides fabricated by plasma-assisted methods with readily available applications in energy harvesting and storage, catalysis and nanosensingJunta de Andalucia(FQM 1851 and FQM-2310)España Mineco 201560E055 MAT2016-79866-R MAT2013-40852-R MAT2013-4MAT2013-47192-C3-3-R2900-

Investigating the Electron Transport and Light Scattering Enhancement in Radial Core-Shell Metal-Metal Oxide Novel 3D Nanoarchitectures for Dye Sensitized Solar Cells

Dye-sensitized solar cells (DSSCs) have attained considerable attention during the last decade because of the potential of becoming a low cost alternative to silicon based solar cells. Electron transport is one of the prominent processes in the cell and it is further a complex process because the transport medium is a mesoporous film. The gaps in the pores are completely filled by an electrolyte with high ionic strength, resulting in electron-ion interactions. Therefore, the electron transport in these so called state-of-the-art systems has a practical limit because of the low electron diffusion coefficient (Dn) in this mesoporous film photoanode. This work focuses on the influence of the advanced core-shell nanoarchitecture geometry on electron transport and also on the influence of electron-ion interactions. In order to achieve the proposed goals, DSSCs based on ordered, highly aligned, 3D radial core-shell Au-TiO2 hybrid nanowire arrays were fabricated, using three different approaches. J-V, IPCE, and EIS characteristics were studied. The efficiency, light scattering and charge transport properties of the core-shell nanowire based devices were compared to TiO2 nanotube as well as TiO2 mesoporous film based DSSCs. The Au nanowires inside the crystalline TiO2 anatase nanoshell provided a direct conduction path from the TiO2 shell to the TCO substrate and improved transport of electrons between the TiO2 and the TCO. The optical effects were studied by IPCE measurement which demonstrated that Au-TiO2 nanowires showed an improved light harvesting efficiency, including at longer wavelengths where the sensitizer has weak absorption. The metal nanostructures could enhance the absorption in DSSCs by either scattering light enabling a longer optical path-length, localized surface plasmon resonance (LSPR) or by near-field coupling between the surface plasmon polariton (SPP) and the dye excited state. Rapid, radial electron collection is of practical significance because it should allow alternate redox shuttles that show relatively fast electron-interception dynamics to be utilized without significant sacrifice of photocurrent. A combination of improved electron transport and enhanced light harvesting capability make Au-TiO2 core-shell nanowire arrays a promising photoanode nanoarchitecture for improving photovoltaic efficiency while minimizing costs by allowing thinner devices that use less material in their construction

Investigating the Electron Transport and Light Scattering Enhancement in Radial Core-Shell Metal-Metal Oxide Novel 3D Nanoarchitectures for Dye Sensitized Solar Cells

Dye-sensitized solar cells (DSSCs) have attained considerable attention during the last decade because of the potential of becoming a low cost alternative to silicon based solar cells. Electron transport is one of the prominent processes in the cell and it is further a complex process because the transport medium is a mesoporous film. The gaps in the pores are completely filled by an electrolyte with high ionic strength, resulting in electron-ion interactions. Therefore, the electron transport in these so called state-of-the-art systems has a practical limit because of the low electron diffusion coefficient (Dn) in this mesoporous film photoanode. This work focuses on the influence of the advanced core-shell nanoarchitecture geometry on electron transport and also on the influence of electron-ion interactions. In order to achieve the proposed goals, DSSCs based on ordered, highly aligned, 3D radial core-shell Au-TiO2 hybrid nanowire arrays were fabricated, using three different approaches. J-V, IPCE, and EIS characteristics were studied. The efficiency, light scattering and charge transport properties of the core-shell nanowire based devices were compared to TiO2 nanotube as well as TiO2 mesoporous film based DSSCs. The Au nanowires inside the crystalline TiO2 anatase nanoshell provided a direct conduction path from the TiO2 shell to the TCO substrate and improved transport of electrons between the TiO2 and the TCO. The optical effects were studied by IPCE measurement which demonstrated that Au-TiO2 nanowires showed an improved light harvesting efficiency, including at longer wavelengths where the sensitizer has weak absorption. The metal nanostructures could enhance the absorption in DSSCs by either scattering light enabling a longer optical path-length, localized surface plasmon resonance (LSPR) or by near-field coupling between the surface plasmon polariton (SPP) and the dye excited state. Rapid, radial electron collection is of practical significance because it should allow alternate redox shuttles that show relatively fast electron-interception dynamics to be utilized without significant sacrifice of photocurrent. A combination of improved electron transport and enhanced light harvesting capability make Au-TiO2 core-shell nanowire arrays a promising photoanode nanoarchitecture for improving photovoltaic efficiency while minimizing costs by allowing thinner devices that use less material in their construction

Two novel hierarchical homogeneous nanoarchitectures of TiO2 nanorods branched and P25-coated TiO2 nanotube arrays and their photocurrent performances

We report here for the first time the synthesis of two novel hierarchical homogeneous nanoarchitectures of TiO2 nanorods branched TiO2 nanotube arrays (BTs) and P25-coated TiO2 nanotube arrays (PCTs) using two-step method including electrochemical anodization and hydrothermal modification process. Then the photocurrent densities versus applied potentials of BTs, PCTs, and pure TiO2 nanotube arrays (TNTAs) were investigated as well. Interestingly, at -0.11 V and under the same illumination condition, the photocurrent densities of BTs and PCTs show more than 1.5 and 1 times higher than that of pure TNTAs, respectively, which can be mainly attributed to significant improvement of the light-absorbing and charge-harvesting efficiency resulting from both larger and rougher surface areas of BTs and PCTs. Furthermore, these dramatic improvements suggest that BTs and PCTs will achieve better photoelectric conversion efficiency and become the promising candidates for applications in DSSCs, sensors, and photocatalysis

Efficient design principle for interfacial charge separation in hydrogen-intercalated nonstoichiometric oxides

Establishing effective strategies to boost the separation of interfacial charge carriers is necessary to address the limiting bottlenecks of photocatalysis. Although oxygen vacancy modulation has become the prevalent strategy to improve the photoactivity, controversy persists regarding the real role of these defects in charge separation. To exert the great potential of nonstoichiometric semiconductors, one needs not only to establish a full atomistic picture of oxygen vacancies, but also to deliberate their possible interactions with other interfacial structures (represented by the ubiquitous intercalated hydrogen). Herein, WO3 was used as a typical model to demonstrate the principle of hydrogen-intercalated nonstoichiometric oxides for photoelectrochemical water splitting. Both experimental characterizations and theoretical calculations evidenced the synergetic interactions between oxygen vacancies and intercalated hydrogen. The sequential formation of subsurface defect clusters and surface O–H bonds contributed significantly to the spatial separation of charge carriers and the impressive stability of nonstoichiometric photoanodes. Profiting from this principle, an unprecedented photocurrent of 2.94 mA cm−2 at 1.23 V vs. RHE was achieved, apart from a 100 mV cathodic shift in the onset potential. Our principle is applicable to several semiconductors, e.g. TiO2 and Fe2O3. Thus, it opens up a promising avenue into designing high-performance nonstoichiometric nanoarchitectures for a wide range of applications. The termination-dependent surface reactivity also provides new opportunities of reactive species modulation for high-performance photocatalysis

Study of the annealing conditions and photoelectrochemical characterization of a new iron oxide bi-layered nanostructure for water splitting

Iron oxide nanostructures have emerged as promising materials for being used as photocatalysts for hydrogen production due to their advantageous properties. However, their low carrier mobility and short hole diffusion length limit their efficiency in water splitting. To overcome these drawbacks, in the present study, we synthetized a new hematite (α-Fe2O3) bi-layered nanostructure consisting of a top nanosphere layer and a nanotubular underneath one by electrochemical anodization. Annealing parameters such as temperature, heating rate and atmosphere were studied in detail in order to determine the optimum annealing conditions for the synthetized nanostructure. The obtained new bi-layered nanostructure was characterized by Field Emission Scanning Electron Microscopy, Raman Spectroscopy, Mott-Schottky analysis and Electrochemical Impedance Spectroscopy. The results show the best water splitting performance for the bi-layered nanostructure annealed in argon atmosphere at 500 °C at a heating rate of 15 °C min−1 achieving a photocurrent density of ~0.143 mA cm−2 at 1.54 V (vs. RHE). The results indicate that the bi-layered nanostructure is an efficient photocatalyst for applications such as water splitting

Metal Oxide Nanostructures for Energy Conversion and Storage

With the depletion of non-renewable resources and the related problems such as environmental pollutions and global warming effect, the society urgently needs more efficient and controllable energy conversion and storage systems. Three-dimensional ZnO nanoforests were prepared and morphology-tunable, which benefits both of energy conversion and storage systems. Willow-like ZnO nanoforests led the highest conversion efficiency of photoelectrochemical water splitting in reported pure ZnO nanostructures. Compared with nanowire arrays, ZnO@MnO2 nanoforests delivered 5 times higher specific capacitances within the same footprint area. V2O5@PPy exhibited higher specific capacitance and better rate stability than pure V2O5 nanofibers. This work lays the foundation for outperformed hybrid electrode materials applied in energy conversion and storage

ZnO Nanowires for Dye Sensitized Solar Cells

This chapter provides a broad review of the latest research activities focused on the synthesis and application of ZnO nanowires (NWs) for dye‐sensitized solar cells (DSCs) and composed of three main sections. The first section briefly introduces DSC‐working principles and ZnO NW application advantages and stability issues. The next section reviews ZnO NW synthesis methods, demonstrating approaches for controlled synthesis of different ZnO NW morphology and discussing how this effects the overall efficiency of the DSC. In the last section, the methods for ZnO NW interface modification with various materials are discussed, which include ZnO core‐shell structures with semiconductive or protective layers, ZnO NW hybrid structures with other materials, such as nanoparticles, quantum dots and carbon nanomaterials and their benefit for charge and light transport in DSCs. The review is concluded with some perspectives and outlook on the future developments in the ZnO nanowire application for DSCs

Homogeneous photosensitization of complex TiO 2 nanostructures for efficient solar energy conversion

TiO 2 nanostructures-based photoelectrochemical (PEC) cells are under worldwide attentions as the method to generate clean energy. For these devices, narrow-bandgap semiconductor photosensitizers such as CdS and CdSe are commonly used to couple with TiO 2 in order to harvest the visible sunlight and to enhance the conversion efficiency. Conventional methods for depositing the photosensitizers on TiO 2 such as dip coating, electrochemical deposition and chemical-vapor-deposition suffer from poor control in thickness and uniformity, and correspond to low photocurrent levels. Here we demonstrate a new method based on atomic layer deposition and ion exchange reaction (ALDIER) to achieve a highly controllable and homogeneous coating of sensitizer particles on arbitrary TiO 2 substrates. PEC tests made to CdSe-sensitized TiO 2 inverse opal photoanodes result in a drastically improved photocurrent level, up to ∼15.7 mA/cm 2 at zero bias (vs Ag/AgCl), more than double that by conventional techniques such as successive ionic layer adsorption and reaction

In situ growth of ultrathin Co-MOF nanosheets on Α-Fe2O3 hematite nanorods for efficient photoelectrochemical water oxidation

Efficient charge transport is an important factor in photoelectrochemical (PEC) water splitting. The charge transfer at the semiconductor/electrolyte interface is of great importance, especially for the complex water oxidation reaction. In this study, we explored the feasibility of improving charge transfer efficiency at the interface of semiconductor/electrolyte by in situ growth of Co based Metal-Organic Frame work (Co-MOF) through a facile ion-exchanging method. Under optimized conditions, the Co-MOF nanosheet-modified hematite gave a photocurrent density of 2.0 mA cm−2 (200% improvement) at 1.23 VRHE with a cathodic shift of 180 mV in the photocurrent onset potential, in comparison to bare α-Fe2O3 (0.71 mA cm−[email protected] VRHE). To elucidate the role of Co-MOF, X-ray photoelectron spectroscopy, electrochemical impedance spectroscopy and Mott-Schottky measurements were carried out. It was found that the atomically distributed Co2+ in Co-MOF possessed excellent hole storage capability and charge transfer efficiency, as evidenced by the high surface capacitance and extremely low surface charge transfer resistance