Jiyuan Zhang

Solar hydrogen generation from seawater with a modified BiVO4 photoanode

Hydrogen is a very promising candidate as a future energy carrier. It is attractive to produce hydrogen from solar energy and seawater, the most abundant renewable energy source and the most abundant natural resource on the earth. To date, there is no report on a stable photoelectrode with a high incident photon conversion efficiency (IPCE) in seawater splitting under irradiation by visible light. Herein, we report an efficient and stable system for seawater splitting based on a multi-metal oxide BiVO4 after modification. The results indicated that modified BiVO4 had a photocurrent density of 2.16 mA cm−2 at 1.0 VRHE in natural seawater under AM 1.5G sunlight (1000 W m−2) and exhibited the highest IPCE at 1.0 VRHE in the visible light region of 440–480 nm among all known oxide photoanodes.

Visible‐Light‐Driven H2 Generation from Water and CO2 Conversion by Using a Zwitterionic Cyclometalated Iridium(III) Complex

Concerns over global warming and energy demand have motivated academic research towards the increasing utilization of solar energy. In practice, solar-energy conversion is yet a challenging and important subject for light-driven hydrogen evolution from water and reduction of carbon dioxide into chemical energy stored in the form of fuel. To achieve this long-standing goal, solar-light-driven, electrontransfer reactions, accomplished by means of molecularbased photosensitizers (PSs) and sensitizer-semiconductors, have displayed the most promising result, because of their importance for understanding of solar-energy harvesting and artificial photosynthesis. Even though photosynthetic systems with considerable activity have been created, the solarto-fuel conversion efficiency by visible light still remains a rather difficult challenge. In the search for better lightdriven systems, which usually suffer from a potential thermodynamic limit, the key component is photoactive materials that are capable of capturing photon energy and result in efficient generation of a long-lived, charge-separated state and facilitate the extremely complex multielectron reduction of substrates at low overpotentials. Then use of transitionmetal complexes is still an extremely attractive strategy, because tuning of the photophysical and electrochemical properties can be systematically achieved through synthetic modification. Most of the photochemical systems for hydrogen generation proceed efficiently in aqueous media with a high proportion of organic solvent, such as acetonitrile and acetone, as cosolvents, because of the complexity of the multielectron processes and the insolubility of PSs in water. An increasing amount of water in the water/organic solvent mixture causes a substantial decrease of catalytic activity for hydrogen formation. Organic solvents with high dielectric constants provide a higher solubility of the PS in homogeneous systems and also offers a beneficial action for reducing the internal charge of the PS. However, the use of aqueous media has been steadily gaining importance to minimize potential environmental impacts and simplify the systems. Despite improvements, water reduction by visible light is still less active in pure water without the organic cosolvents and only a few aqueous homogeneous systems have been described to date. These urgent aspects inspired us to focus on design and development of synthetic complexes with improved photophysical properties for a typical innovative process and a better understanding of the solar-light-driven reaction that is involved. Herein we describe the formation of a new heteroleptic iridium complex [IrACHTUNGTRENNUNG(4-CF3bt)2ACHTUNGTRENNUNG(Hbpdc)] (1) (where 4CF3bt= (4-trifluoromethyl)-2-phenylbenzothiazole and H2bpdc= 2,2’-bipyridine-4,4’-dicarboxylate) and its activity towards highly efficient H2 generation from water and clean conversion of CO2 under visible-light irradiation. By employing the ancillary H2bpdc (N^N) and 4-CF3bt (C^N) ligands, complex 1 was readily available in a satisfying yield by a general two-step, bridge-splitting pathway. The N^N ligand with carboxyl substituents may assist, not only in imparting water solubility of the complexes because of the presence of an acid–base equilibria, but also in anchoring on nanocrystalline TiO2 photoanodes for an efficient and directional electron transport. In addition, we chose 2-phenylbenzothiazole (bt) as a suitable subunit in the potential photoactive compounds based primarily on the attractive electron-demanding capabilities and photochemical stabilities. Modification of the bt species by attachment of a trifluoromethyl moiety often minimizes self-quenching and improve the charge-transfer properties of the corresponding complex; this is an essential prerequisite for certain photochemical applications. As a consequence, the assembly of the d-metal iridium ACHTUNGTRENNUNG(III) with the combination of the bt and bdpc species offers great potential for interesting light-harvesting processes in water. The desired zwitterionic complex 1 was fully characterized by conventional spectroscopic and analytical methods (see the Supporting Information). Furthermore, the molecular structure in the solid state was confirmed by a single-crystal X-ray study. A pair of cyclometalated C^N ligands and a chelating N^N ligand is oriented in a distorted octahedral coordination geometry around the central iridium atom, as shown in Figure 1. The trans-orientated Ir N ACHTUNGTRENNUNG(Hbpdc) distances of 2.1041(2) and 2.1267(2) are found to be significantly longer than those observed in the cyclometalated ligand [a] Y.-J. Yuan, Dr. Z.-T. Yu, X.-Y. Chen, J.-Y. Zhang, Prof. Dr. Z.-G. Zou Eco-Materials and Renewable Energy Research Center National Laboratory of Solid State Microstructures Department of Materials Science and Engineering Nanjing University, NO. 22, Hankou Road, Nanjing Jiangsu 210093 (P.R. China) Fax: (+86) 25-8368-6632 E-mail : yuzt@nju.edu.cn zgzou@nju.edu.cn Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201102147.

Vertically building Zn2SnO4 nanowire arrays on stainless steel mesh toward fabrication of large-area, flexible dye-sensitized solar cells

Zn(2)SnO(4) nanowire arrays were for the first time grown onto a stainless steel mesh (SSM) in a binary ethylenediamine (En)/water solvent system using a solvothermal route. The morphology evolution following this reaction was carefully followed to understand the formation mechanism. The SSM-supported Zn(2)SnO(4) nanowire was utilized as a photoanode for fabrication of large-area (10 cm × 5 cm size as a typical sample), flexible dye-sensitized solar cells (DSSCs). The synthesized Zn(2)SnO(4) nanowires exhibit great bendability and flexibility, proving potential advantage over other metal oxide nanowires such as TiO(2), ZnO, and SnO(2) for application in flexible solar cells. Relative to the analogous Zn(2)SnO(4) nanoparticle-based flexible DSSCs, the nanowire geometry proves to enhance solar energy conversion efficiency through enhancement of electron transport. The bendable nature of the DSSCs without obvious degradation of efficiency and facile scale up gives the as-made flexible solar cell device potential for practical application.

Effective electron collection in highly (110)-oriented ZnO porous nanosheet framework photoanode

A highly (110)-oriented ZnO porous nanosheet framework is designed as the photoanode in photoelectrochemical systems, by virtue of its anisotropic electronic properties. It can be facilely prepared in large scale via a hydrothermal method. X-ray diffraction (XRD) analyses show that the orientation index of the (110) diffraction plane is 3.54, indicating the films possess (110) preferred orientation. Field-emission scanning electron microscope (FE-SEM) images exhibit that most of the nanosheets stand nearly perpendicularly on the substrate. The {002} lattice planes work just like conducting wires and induce the electrons to transport to the substrate. Chronoamperometry measurement demonstrates an effective electron collection. When the nanostructured photoanode is introduced to dye-sensitized solar cells, a conversion efficiency of 3.7% is obtained. The photoanode also has potential application in the other photoelectrochemical systems, such as photocatalytical splitting of water.

Multilayer structure with gradual increasing porosity for dye-sensitized solar cells

Multilayer structure was prepared in a dye-sensitized solar cell work electrode by enlarging the porosity in each layer being coated on the fluorine-doped tin oxide transparent conducting glass from bottom to top. The multilayer structure exhibits an improved light scattering character, which resulted in better light harvesting of the cell. An obvious improvement in short circuit current is obtained. I-V characteristic measurement indicates an improved efficiency by 13% as compared to homogeneous pore-size samples. Diffuse reflectance spectra, scanning electron microscope images, and porosity measurements demonstrate that larger porosity is the cause of enhanced light scattering.

Interfacial modification of photoelectrode in ZnO-based dye -sensitized solar cells and its efficiency improvement mechanism

A ZnO compact layer prepared by a sol–gel method was introduced into a photoelectrode at the interface between fluorine-doped tin oxide (FTO) substrate and a mesoporous ZnO layer in ZnO-based dye-sensitized solar cells (DSSCs). The ZnO compact layer was characterized by field-emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM) and UV-Vis spectroscopy. The compact layer increased the photoelectric conversion efficiency of ZnO-based DSSCs by 20%. An electrochemical impedance spectroscopy (EIS) study demonstrated that the compact layer strikingly reduced the interfacial resistance in the device by enhancing the conductive contact between nanocrystalline ZnO and FTO substrate. The photocurrent density–voltage characteristics in the dark suggests that the compact ZnO layer also plays the role of a blocking layer suppressing the charge recombination, which is illustrated by the suppression of dark current density. The two effects effectively elevate the short circuit current density (JSC) and open circuit voltage (VOC), and finally improve the overall conversion efficiency of ZnO-based DSSCs.

Understanding of the chopping frequency effect on IPCE measurements for dye-sensitized solar cells: from the viewpoint of electron transport and extinction spectrum

The incident monochromatic photon to electron conversion efficiency (IPCE) is an essential characterization method for the photoelectrical performance of solar cells. An IPCE measurement apparatus involving alternating current (ac) and direct current (dc) methods was set up. A chopping frequency effect on IPCE measurements was found for dye-sensitized solar cells (DSSCs), that is, with the increase in chopping frequency, the IPCE spectrum decreased significantly, and the different bands of the IPCE spectrum declined to different degrees. The chopping frequency effect was studied in detail by measuring the short-circuit current waveform, the extinction spectrum of the dye-coated TiO2 photoelectrode film and electrochemical impedance spectroscopy. The mechanism of the chopping frequency effect was investigated from the electron transport and extinction spectrum. The electron transport properties of the TiO2 photoelectrode film determined the slow response of DSSCs. From the extinction spectrum, the transport distance of electrons in the TiO2 film varied under the illumination of different monochromatic light. For DSSCs, the ac method was remarkably influenced by the trap states of electrons and the optical penetration depth, while the dc method was a steady-state measurement avoiding the impact of these two factors. Thus, the dc method is more suitable than the ac method for IPCE measurements of DSSCs.

The effects of nitridation on the permeability-frequency spectra in nanocrystalline Fe88Zr7B4Cu alloys

The effects of nitridation on the permeability-frequency spectra in Nanoperm alloys were studied. In the initial stage of nitridation, the nitrogen atom diffusion into Nanoperm alloys may make the extent of increase of the relaxation frequency f0 exceed the extent of decrease of initial permeability μi. However, as the materials are nitrided for 200 min in a nitrogen atmosphere of 0.03 MPa at 400°C, μif0 shows a tendency to be constant and the increase of the strength of domain wall pinning with nitriding time becomes the main factor affecting the dynamic magnetization process. Nitridation can improve the soft magnetic properties of nanocrystalline soft magnetic materials in the higher frequency range.

Quasi-Topotactic Transformation of FeOOH Nanorods to Robust Fe2O3 Porous Nanopillars Triggered with a Facile Rapid Dehydration Strategy for Efficient Photoelectrochemical Water Splitting

A facile rapid dehydration (RD) strategy is explored for quasi-topotactic transformation of FeOOH nanorods to robust Fe2O3 porous nanopillars, avoiding collapse, shrink, and coalescence, and compared with a conventional treatment route. Additionally, the so-called RD process is capable of generating a beneficial porous structure for photoelectrochemical water oxidation. The obtained RD-Fe2O3 photoanode exhibits a photocurrent density as high as 2.0 mA cm-2 at 1.23 V versus reversible hydrogen electrode (RHE) and a saturated photocurrent density of 3.5 mA cm-2 at 1.71 V versus RHE without any cocatalysts, which is about 270% improved photocurrent density over Fe2O3 with the conventional temperature-rising route (0.75 mA cm-2 at 1.23 V vs RHE and 1.48 mA cm-2 at 1.71 V vs RHE, respectively). The enhanced photocurrent on RD-Fe2O3 is attributed to a synergistic effect of the following factors: (i) preservation of single crystalline nanopillars decreases the charge-carrier recombination; (ii) formation of long nanopillars enhances light harvesting; and (iii) the porous structure shortens the hole transport distance from the bulk material to the electrode-electrolyte interface.