Wenzhang Li

Hydrothermal synthesis and photoelectrochemical properties of vertically aligned tungsten trioxide (hydrate) plate-like arrays fabricated directly on FTO substrates

Without the assistance of a WO3 seed layer, a uniform tungsten trioxide hydrate (WO3·H2O) plate-like array film was grown directly on bare fluorine-doped tin oxide (FTO) glass by a simple hydrothermal method at mild temperature using ammonium oxalate ((NH4)2C2O4) as a structure-directing agent. The dependence of the crystal structure and morphology on the growth temperature and growth time in the as-prepared samples was studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The XRD studies showed that the as-prepared thin films obtained below 150 °C were comprised of orthorhombic WO3·H2O and were completely converted to monoclinic WO3 at 180 °C. SEM analysis revealed that the thickness of the WO3·H2O nanoplates increased with the increase of growth temperature as well as growth time. Moreover, the formation mechanism of the WO3·H2O plate-like arrays was discussed. It was found that the (NH4)2C2O4 played an important role in the formation of vertically aligned plate-like arrays. The thin films calcined at 450 °C for 1 h showed fine photocatalytic activities. The plate-like arrays grown on the bare FTO substrate synthesized at 120 °C for 12 h exhibited the best photocatalytic activity, which generated an anodic photocurrent of 4.13 mA cm−2 at 1.6 V (vs. Ag/AgCl) under illumination of a 500 W Xe lamp in 0.5 M H2SO4 electrolyte.

In situ synthesis of g-C3N4/WO3 heterojunction plates array films with enhanced photoelectrochemical performance

g-C3N4/WO3 heterojunction plate array films with enhanced photoelectrochemical (PEC) performance were successfully synthesized through a combination of hydrothermal and dipping-annealing methods. Urea aqueous solutions were prepared as the precursors to in situ synthesize g-C3N4 nanoparticles on the surface of WO3 platelets. The PEC performances of the photoanodes were investigated by the photocurrent density and incident photon-to-current conversion efficiency (IPCE). As-prepared g-C3N4/WO3 heterojunction films achieved a maximum photocurrent density of 2.10 mA cm−2 at +2.0 V (vs. RHE), which was almost 3-fold higher than that of the pure WO3 film (0.78 mA cm−2) under illumination. And the highest IPCE value increased from 25.1% to 53.1% after the g-C3N4 nanoparticles deposition. The enhanced PEC performance was attributed to the increased carriers density, better electron transport properties, longer electron lifetime and effective charge separation at the interface of heterojunction, which were confirmed by Mott–Schottky and electrochemical impedance spectroscopy (EIS). This study demonstrates that the low-dimensional morphological structure and in situ formation of heterojunction structure are expected to provide a promising photoanode for photoelectric catalysis.

In situ synthesis of Bi2S3 sensitized WO3 nanoplate arrays with less interfacial defects and enhanced photoelectrochemical performance

In this study, Bi2S3 sensitive layer has been grown on the surface of WO3 nanoplate arrays via an in situ approach. The characterization of samples were carried out using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and ultraviolet–visible absorption spectroscopy (UV-vis). The results show that the Bi2S3 layer is uniformly formed on the surface of WO3 nanoplates and less interfacial defects were observed in the interface between the Bi2S3 and WO3. More importantly, the Bi2S3/WO3 films as photoanodes for photoelectrochemical (PEC) cells display the enhanced PEC performance compared with the Bi2S3/WO3 films prepared by a sequential ionic layer adsorption reaction (SILAR) method. In order to understand the reason for the enhanced PEC properties, the electron transport properties of the photoelectrodes were studied by using the transient photocurrent spectroscopy and intensity modulated photocurrent spectroscopy (IMPS). The Bi2S3/WO3 films prepared via an in situ approach have a greater transient time constant and higher electron transit rate. This is most likely due to less interfacial defects for the Bi2S3/WO3 films prepared via an in situ approach, resulting in a lower resistance and faster carrier transport in the interface between WO3 and Bi2S3.

Efficient Planar Perovskite Solar Cells with Reduced Hysteresis and Enhanced Open Circuit Voltage by Using PW12–TiO2 as Electron Transport Layer

An electron transport layer is essential for effective operation of planar perovskite solar cells. In this Article, PW12-TiO2 composite was used as the electron transport layer for the planar perovskite solar cell in the device structure of fluorine-doped tin oxide (FTO)-glass/PW12-TiO2/perovskite/spiro-OMeTAD/Au. A proper downward shift of the conduction band minimum (CBM) enhanced electron extraction from the perovskite layer to the PW12-TiO2 composite layer. Consequently, the common hysteresis effect in TiO2-based planar perovskite solar cells was significantly reduced and the open circuit voltage was greatly increased to about 1.1 V. Perovskite solar cells using the PW12-TiO2 compact layer showed an efficiency of 15.45%. This work can contribute to the studies on the electron transport layer and interface engineering for the further development of perovskite solar cells.

Enhancing photoelectrochemical activity of CdS quantum dots sensitized WO3 photoelectrodes by Mn doping

Mn-doped CdS quantum dots sensitized WO3 photoelectrodes were successfully synthesized by a combination of hydrothermal and chemical bath deposition (CBD) methods. To improve the stability of the photoelectrodes in an alkaline environment, the electrodes were treated with TiCl4 to form a nano-TiO2 buffer layer on the WO3 plate surface before depositing CdS quantum dots (QDs). The resulting electrodes were applied as photoanodes in the photoelectrochemical cell for water splitting. The photoelectrochemical (PEC) properties were investigated by the photocurrent density curves and incident photon-to-current conversion efficiency (IPCE). The as-prepared Mn-CdS QDs sensitized WO3 plate-like photoelectrodes exhibit a significant improvement in their photoelectrochemical performance compared with undoped photoelectrodes. To better understand the enhanced PEC properties, the electron transport properties and efficient electron lifetime were studied in detail using electrochemical impedance spectroscopy (EIS), transient photocurrent spectroscopy and intensity modulated photocurrent spectroscopy (IMPS). The results show that the Mn-CdS QDs/TiO2/WO3 photoelectrodes exhibit a higher electron transit rate and a longer electron lifetime. This is most likely due to the existence of electronic states in the mid-gap region of the Mn-CdS QD.

Enhancing photoelectrochemical performance with a bilayer-structured film consisting of graphene–WO3 nanocrystals and WO3 vertically plate-like arrays as photoanodes

A bilayer-structured film photoelectrode, containing an under-layer of reduced graphene oxide–WO3 nanocrystals (G-WNC) or WO3 nanocrystals (WNC) and a top layer of WO3 vertically plate-like arrays (WP), was designed and fabricated by solution-based and hydrothermal methods. The morphologies, structural and optical properties of as-prepared composites were analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and ultraviolet visible (UV-Vis) spectrometry. Moreover, the photoelectrochemical properties were studied through linear scan voltammetry, electrochemical impedance spectroscopy (EIS) and incident photon to current conversion efficiency (IPCE). It was found that the photocurrents of WP, WNC/WP and G-WNC/WP films were 0.68, 0.80 and 1.23 mA cm−2 at 1.2 V (vs. Ag/AgCl) under AM 1.5G illumination, respectively, whereas the IPCE values were 28.12%, 31.33% and 39.52% successively at the wavelength of 355 nm. EIS data illustrated that the charge transfer resistance was clearly reduced by introducing graphene in the underlayer of the composite film. The enhancement of G-WNC/WP film could be ascribed to the synergic effects of the bilayer structure and the limitation of charge recombination.

Epitaxial growth of Bi2S3 nanowires on BiVO4 nanostructures for enhancing photoelectrochemical performance

In this paper, a novel Bi2S3/BiVO4 heterojunction film was prepared by a facile drop-casting and hydrothermal method for the first time. The as-prepared films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and ultraviolet visible spectrometry (UV-Vis). Interestingly, the heterojunction film was formed by epitaxial growth of Bi2S3 nanowires on BiVO4 nanostructures and exhibited a good visible light absorption performance. Photoelectrochemical (PEC) hydrogen generation was demonstrated using the prepared films as photoanodes. The heterojunction photoelectrode showed an excellent PEC activity and generated a photocurrent density of 7.81 mA cm−2 at 0.9761 V vs. RHE (0.1 V vs. Ag/AgCl) in the electrolyte solution containing 0.35 M Na2SO3 and 0.25 M Na2S. The present study provides new insight into the design of highly efficient heterojunction photoelectrodes for hydrogen generation.

Preparation of DyVO4/WO3 heterojunction plate array films with enhanced photoelectrochemical activity

In this work, DyVO4/WO3 heterojunction plate arrays were first fabricated on FTO using a hydrothermal method for WO3 vertical plate arrays and a dipping–annealing process for the deposition of DyVO4 nanoparticles. The samples were characterized by various techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Photoelectrochemical activities were investigated by linear sweep voltammetry (LSV) and incident photon to current conversion efficiency (IPCE). The DyVO4/WO3 heterojunction electrode exhibited a maximum photocurrent density of 0.78 mA cm−2 at +1.2 V (vs. Ag/AgCl), while the photocurrent density of pure WO3 was just 0.49 mA cm−2 under illumination. And the highest IPCE value increased from 27.4% to 54.1% after the DyVO4 nanoparticle deposition. The enhanced PEC performance was attributed to the longer electron lifetime, increased carrier density and high charge separation efficiency at the interface of heterojunction, which were confirmed by electrochemical impedance spectroscopy (EIS) and Mott–Schottky analysis. The study demonstrates that metal orthovanadates may be good heterojunction candidates to couple with WO3 to provide promising photoanodes for water splitting.

ZnO nanoparticle-functionalized WO3 plates with enhanced photoelectrochemical properties

In this work, ZnO NPs-functionalized WO3 vertical plate-like arrays were first fabricated on FTO with a hydrothermal process for WO3 vertical plate-like arrays and an electrodeposition process for the functionalization of ZnO. The ZnO nanoparticles are preferentially loaded on the active points of WO3 in the shape of a sphere about 10 nm. The samples were characterized by various techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Photoelectrochemical properties were investigated by photoelectrochemical measures, such as linear sweep voltammograms, electrochemical impedance spectroscopy (EIS), intensity-modulated photocurrent spectroscopy (IMPS) and incident photon to current conversion efficiency (IPCE). The results show that the photocurrent of WO3 increases from 0.88 to 1.23 mA cm−2 at 1.2 V (vs. Ag/AgCl) after functionalized with ZnO. Furthermore, the lifetime of the electron–hole has been prolonged from 6.44 to 8.56 ms, but there is no decrease in the electron transport time. In this case, the enhancement of the photoelectrochemical performance is attributed to effective transfer of photo-generated holes so as to retard the recombination of electrons and holes.

Co Nanoparticles Confined in 3D Nitrogen‐Doped Porous Carbon Foams as Bifunctional Electrocatalysts for Long‐Life Rechargeable Zn–Air Batteries

Proper design and simple preparation of nonnoble bifunctional electrocatalysts with high cost performance and strong durability for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) is highly demanded but still full of enormous challenges. In this work, a spontaneous gas-foaming strategy is presented to synthesize cobalt nanoparticles confined in 3D nitrogen-doped porous carbon foams (CoNCF) by simply carbonizing the mixture of citric acid, NH4 Cl, and Co(NO3 )2 ·6H2 O. Thanks to its particular 3D porous foam architecture, ultrahigh specific surface area (1641 m2 g-1 ), and homogeneous distribution of active sites (C-N, Co-Nx , and Co-O moieties), the optimized CoNCF-1000-80 (carbonized at 1000 °C, containing 80 mg Co(NO3 )2 ·6H2 O in precursors) catalyst exhibits a remarkable bifunctional activity and long-term durability toward both ORR and OER. Its bifunctional activity parameter (ΔE) is as low as 0.84 V, which is much smaller than that of noble metal catalyst and comparable to state-of-the-art bifunctional catalysts. When worked as an air electrode catalyst in rechargeable Zn-air batteries, a high energy density (797 Wh kg-1 ), a low charge/discharge voltage gap (0.75 V), and a long-term cycle stability (over 166 h) are achieved at 10 mA cm-2 .